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Peer-reviewed

Research Article

Analysis of elite soccer players’ performance before and after signing a new contract

Contributed equally to this work with: Miguel-Ángel Gómez, Carlos Lago, María-Teresa Gómez, Philip Furley

Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

Affiliation Faculty of Physical Activity and Sports Sciences, Technical University of Madrid, Madrid, Spain

Roles Conceptualization, Data curation, Funding acquisition, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing

Affiliation Faculty of Education and Sport Sciences, University of Vigo, Pontevedra, Spain

Roles Conceptualization, Data curation, Formal analysis, Methodology, Resources, Validation, Writing – review & editing

Roles Conceptualization, Formal analysis, Investigation, Methodology, Resources, Validation, Writing – original draft, Writing – review & editing

Affiliation Institute of Cognitive and Team/Racket Sport Research, German Sport University Cologne, Cologne, Germany

• Miguel-Ángel Gómez, 
• Carlos Lago, 
• María-Teresa Gómez, 
• Philip Furley

• Published: January 25, 2019
• https://doi.org/10.1371/journal.pone.0211058
• Reader Comments

The aim of the current study was to analyse performance differences of football players 2-years prior and the year after signing a new contract (the following year) while taking playing position, nationality, player’s role, team ability, and age into account. The sample was comprised of 249 players (n = 109 defenders, n = 113 midfielders; and n = 27 forwards) from four of the major European Leagues (Bundesliga, English FA Premier League, Ligue 1, and La Liga) during the seasons 2008 to 2015. The dependent variables studied were: shooting accuracy, defense (the sum of defensive actions, tackles, blocks, and interceptions), yellow cards, red cards, passing accuracy, tackle success, and minutes played per match. Two-step cluster analysis allowed classifying the sample into three groups of defenders (national important, foreign important, and less important players) and four groups of midfielders and forwards (national important, foreign important, national less important, and foreign less important players). Magnitude Based Inference (MBI) was used to test the differences between player’s performances during the years of analyses. The main results (very likely and most likely effects) showed better performance in the year prior to signing a new contract than the previous year for foreign important defenders (decreased number of red cards), national important midfielders (increased number of minutes played), foreign important forwards (increased minutes played and defense), and national important forwards (increased minutes played). In addition, performance was lower the year after signing the contract compared to the previous one for less important defenders (decreasing defense), national less important midfielders (decreased minutes played), and foreign less important forwards (decreased defense). On the other hand, the players showed better performance in defense and more minutes played the year after signing the contract for less important defenders, national less important midfielders, and foreign less important forwards. These results may assist coaches to decide on when a new contract should be signed or the duration of the contract.

Citation: Gómez M-Á, Lago C, Gómez M-T, Furley P (2019) Analysis of elite soccer players’ performance before and after signing a new contract. PLoS ONE 14(1): e0211058. https://doi.org/10.1371/journal.pone.0211058

Editor: Jaime Sampaio, Universidade de Tras-os-Montes e Alto Douro, PORTUGAL

Received: July 15, 2018; Accepted: January 7, 2019; Published: January 25, 2019

Copyright: © 2019 Gómez et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: Due to the personal data protection law, data will be shared publicly without the name of the players.

Funding: The present study was supported by the Ministry of Economy and competitiveness of Spain with the project “Diseño y desarrollo de un software para el análisis del rendimiento en fútbol” (DEP2016-75785-R). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Incentives are known to play a substantial role in a person’s performance. An important incentive in professional football is signing a lucrative contract. While performance analysis in football has focused on numerous variables in explaining performance, there is limited knowledge on the impact of the time of signing a contract in football. Previous research on performance analysis in football has investigated the players’ performance from an individual point of view attending to physical demands [ 1 ] and technical and tactical indicators [ 2 ] according to different performance levels [ 3 ], playing positions [ 4 – 5 ], players’ role as starters or non-starters [ 6 ], the evolution of physical and technical parameters [ 7 – 8 ] or performance variability [ 9 ]. However, a limitation of this previous research has been to neglect a long-term analysis of a player’s individual performance [ 10 ]. Of further relevance to the present research, Mackenzie and Cushion [ 11 ] have argued that research on performance analysis should identify long term constraints on an individual’s performance which can be used to improve a player’s recruitment policies and to control for social-cultural (e.g., foreign/ national, top elite leagues) influences that impact players’ performance during their careers.

Pertinent to the present research, football players have different fixed contracts that have been argued to affect their efforts with cycles of performance variation at different moments of their career [ 12 ]. Research within sport management has shown that this is particularly evident just before a player signed a new contract (i.e., better performance) and after a lucrative contract was secured (i.e., maintenance or reduction of performance) [ 12 – 13 ]. Frick [ 12 ] studied 1,993 players from the German Bundesliga during the seasons 1995–1996 to 2007–2008 (13 seasons). The results showed that career matches played, matches played during the last season of contract, goals scored, yellow cards, playing position, and region of birth varied depending on signing a new contract as this seemed to have affected the player’s effort and motivation. Della Torre et al. [ 13 ] analysed 275 football players who played at least two consecutive seasons (year before and after signing a contract) in the Italian Serie A during the seasons 2012–2013 and 2013–2014. Their results showed that players perform better during the last year of their contract which is argued to be caused by top elite teams rewarding current/past performances (i.e., subsequent contracts are specially affected by the immediate past performance). Their results showed a pay-performance (current salary) relationship that reinforces good performances. However, this factor has disparities when controlling for player’s origin (national or foreign player) and their level of performance based on performance indicators (important or less important players). In fact, their results showed that domestic players performed better than foreign players due to the better knowledge of the culture of the country, the league, and the club. Accordingly, as football players sign new contracts of about three years of duration or renegotiate their contract with one or two years remaining, the performance analysis of players using key performance indicators is of great relevance to understand the cycle of efforts and motivation based on current contracts, renegotiation, and salary [ 13 ].

Of further relevance to the present research, player-related factors obviously impact on players’ performance. For example, the nationality of players has been argued to be a moderator on the performance effects of signing a new contract in elite football and has been studied from social and management perspectives [ 14 ]. More recently, Della Torre et al. [ 13 ] found that the individual performance during consecutive seasons was stronger for domestic players than foreign players when the end of the contract is near. In addition, a player’s age, his role in the team (categorized based on minutes played as important or less important), the strength of the team in which they play (UEFA ranking) or the evolution of their technical-tactical performances (such as passing, shooting, tackling, or defending behaviours) during the previous and posterior performances after signing or renegotiating a contract may affect the player’s performance from a long-term approach [ 2 – 3 , 6 , 8 – 9 , 12 – 14 ].

In summary, research has indicated that a player is willing to invest more effort when approaching the renegotiation of the same or a new contract. Data shows that this shows in a consecutive increase of his performances mainly during the previous year(s) of his contract [ 12 ]. Presumably this gradual process to deliver better performance allows reaching a better bargaining position for the new contract during the last season before his contract expires or to renegotiate the current contract with one or two years remaining.

In the present paper, we investigate player’s performance variations using technical and tactical performance indicators that arguably reflect the evolution of his efforts in different areas of playing (defense, attack or minutes played). Further, due to the limit of 2-year analyses of previous studies, we will analyse players’ efforts during three consecutive seasons as a novel approach to gaining a better understanding of the signing/ renegotiating of a contract from a longer-term perspective. This analysis will allow illuminating players’ tendencies of adjusting their efforts depending on the remaining years of their contract [ 12 ]. These analyses are likely to be of interest for stakeholders (i.e., coaches, players, managers, and media) in elite football [ 13 ]. Therefore, the current study tries to address the limitations of previous studies (i.e. only using 2-years period of analyses, not using minutes played by the players and key performance indicators, or the omission of some player-related factors that have the potential to affect long-term performance). Thus, the aim of the current study was to analyze differences in performances of individual football players according to the previous (2-years) and the later year after signing/ renegotiating a new contract while taking player-related characteristics into account (age, role in the team as important or less important, nationality, and team’s ability). We hypothesized that performance during the previous season is better (as indicated by the following performance indicators: shooting accuracy, defense, yellow cards, red cards, passing accuracy, tackle success, and minutes played per match) than performance immediately after signing/ renegotiating the new contract. Additionally, we assumed that this should be more pronounced in domestic than foreign players.

Materials and methods

The sample was comprised of 249 players (n = 109 defenders, n = 113 midfielders; and n = 27 forwards) from the French, German, Italian and Spanish professional leagues during the seasons 2008 to 2015. The distribution of players and total match observations for each league and playing position are presented in Table 1 . The use of the sample from 4 of the major leagues of Europe allows increasing the number of observations of Elite athletes that compete at the same level of performance (European professional leagues). This approach follows from previous research, while further reducing the omitted variables bias (less statistical risks) when increasing sample sizes of the same performance characteristics [ 15 – 17 ].

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https://doi.org/10.1371/journal.pone.0211058.t001

The players were selected if they played at least 20 matches (with more than 20 minutes per match) per season and completed a consecutive 3-years period that includes the year two seasons before the end of their contract (year -1), the last year of the contract (year 0) and the year immediately after signing a new contract (year 1). The year when players signed the new contract (considering either when it was renegotiated or when moving to another club) was considered as the reference year (year 0) in order to establish 2 years prior to sign (year -1), and the year after signing the contract (year 1). This allows to compare potential performance improvement due to the new contract with previous years.

The data included the mean performance of 249 players during three seasons (n = 747 mean individual observations). The following player-related characteristics that may affect performance depending on playing position (established by the official webpage of Opta Sports Company considering: defender, midfielder and forward) were: (i) age (year old of player during the year 0), (ii) team ability (UEFA ranking of the team where the player play during the year 0), (iii) player nationality (classified as national or domestic players of their respective leagues), and (iv) player’s role (classified by a k -means cluster using minutes played per match during the year 0 as important: 80.32±6.2 minutes; and less important players: 52.55±12.0 minutes).

The data observations were provided by OPTA Sportsdata Spain Company ( S1 Dataset ). The tracking system of this private company was previously tested by Liu, Hopkins, Gómez and Molinuevo [ 18 ] with acceptable inter-operator reliability. The study does not present the name of the players in order to keep the anonymity following the Company Ethics guidelines, the European Data Protection Law and the approval of the Institutional Review Board (Technical University of Madrid).

The variables (7 variables) were selected because they are arguably the most used performance indicators in the previous literature [ 2 , 18 ]. They have an established impact on individual performance and they can be used numerically to compare footballers over consecutive seasons [ 2 , 18 ]. The seven variables were defined as follows ( https://www.optasports.com/news/optas-event-definitions/ ) [ 18 ]:

• Shooting accuracy (%): shots on target divided by all shots (including blocked attempts).
• Defense : the sum of defensive actions including tackles (“where a player connects with the ball in a ground challenge where he successfully takes the ball away from the player in possession”), blocks (This variable includes blocked passes : “when a player tries to cut out an opposition pass by any means. Similar to an interception except there is much less reading of the pass”; and blocks : “where a player blocks a shot on target from an opposing player”), and interceptions (“where a player reads an opponent’s pass and intercepts the ball by moving into the line of the intended pass”).
• Yellow cards : The yellow cards booked by the referee due to rule violations.
• Red cards : the red cards booked by the referee due to rule violations.
• Passing accuracy (%): successful passes divided by total attempted passes (considering all the types and zones of passes, excluding crosses).
• Tackles success (%): successful tackles divided by total attempted tackles.
• Minutes per match : the number of minutes played during a season divided by the number of matches played during the season.

Statistical analysis

First, the k -means cluster for quantitative variables was used in order to establish cut-off point values for the variable minutes played per match. Then, two cluster were identified for this variable establishing important (n = 188) and less important (n = 61) players. Specifically, this model allows to divide n players’ observations into k clusters (groups) where each observation gets allocated to the cluster with the closest mean value.

Second, a two-step cluster analysis was used to classify the players into different categories based on player-related characteristics (age, team ability, player’s nationality and player’s role). The model allows the inclusion of categorical and continuous variables in order to find the best clustering solution. Then, this statistical analysis automatically determines the "optimal" number of clusters (player’s groups) using the Schwartz’s Bayesian Information criterion (Silhouette measure of clusters cohesion and separation and the variables importance). In addition, the log-likelihood distance measure was used to compute the similarity between clusters. Due to the non-significant effect when classifying the groups depending on team’s ability (variable importance = 0.0), the model was run without this variable (Silhouette measure indicated good results of 0.75, 0.70 and 0.68 for defenders, midfielders and forwards, respectively). Then, the sample was split into three groups of defenders (national important, foreign important, and less important players) and four groups for midfielders and forwards (national important, foreign important, national less important and foreign less important players). Table 2 shows the results (distribution of players) of this two-step cluster analysis for all the playing positions.

https://doi.org/10.1371/journal.pone.0211058.t002

Third, these groups were considered as the independent variable when comparing the performance indicators (dependent variable). Then, the player’s performance during the years (-1, 0 and 1) was analyzed using repeated measures ANOVA for normally distributed variables and the Friedman test for non-normally distributed variables. In addition, the pairwise comparisons (years -1, 0 and 1) were tested using the magnitude-based inference (MBI) method with the Hopkins’ spreadsheet [ 19 – 20 ]. This method uses the log-transformation of data in order to reduce bias due to non-uniformity error. The effect size (Cohen's d units at 90% CI) was estimated using pooled standard deviation for comparisons with the following magnitude ranges: 0–0.2 trivial; >0.2–0.6 small; >0.6–1.2 moderate; >1.2–2 large; >2 very large. The MBI analyses were assessed using the smallest worthwhile difference (0.2 times the standardization), estimated from the between-subjects standard deviation. The differences are defined as unclear if the confidence intervals (CI) for the difference in the means included substantial positive and negative values (±0.2*standardization) simultaneously. In order to control for differences between pairs of comparisons (years), the magnitude of a clear difference was assessed as follows: >0.25 trivial; 0.25%–75% possibly, 75%–95% likely, 95%- 99% very likely, and >99% most likely. The magnitude is considered unclear if the CI overlaps the positive and negative thresholds [ 19 – 20 ].

The descriptive results, repeated measures ANOVA, Friedman test and Two-step cluster analyses were performed using the statistical software IBM SPSS for Windows, version 22.0 (Armonk, NY: IBM Corp.).

The descriptive results (means and standard deviations), the repeated measures ANOVA, and Friedman tests results of the variables studied for defenders according to the player’s characteristics during the years prior to the end of their contract and immediately after signing a new contract are presented in Table 3 . The repeated measures tests showed significant differences (p<0.05) of red cards for important national defenders, yellow cards and minutes played for less important defenders, and defense for foreign important defenders.

https://doi.org/10.1371/journal.pone.0211058.t003

The results of MBI (see Fig 1 ) showed that foreign important players decreased the number of red cards (very likely effect) from year -1 to year 0. The results for less important players showed a decreased performance in defense (very likely effect) from year -1 to year 0 and they increased their performance in the number of minutes played (most likely effect) and in defense (very likely effect) from year 0 to year 1.

Asterisks indicate the likelihood of MBI effects as follows: *possibly, **likely, ***very likely, **** most likely.

https://doi.org/10.1371/journal.pone.0211058.g001

The descriptive results (means and standard deviations), the repeated measures ANOVA, and Friedman tests results for midfielder according to the group of players during the years studied are presented in Table 4 . The results showed statistically significant differences (p<0.05) of defense and minutes played for national less important players and minutes played for national important midfielders.

https://doi.org/10.1371/journal.pone.0211058.t004

The MBI results for midfielders (see Fig 2 ) showed that national less important players decreased the minutes played (very likely effect) from year -1 to year 0. Additionally, the players increased the minutes played (very likely effect) and defense from year 0 to year 1 (very likely effect). The results for national important players showed an increase in the number of minutes played from year -1 to year 0 (very likely effect). The foreign important players increased the minutes played and defense performance (very likely effect) from year -1 to year 0.

https://doi.org/10.1371/journal.pone.0211058.g002

The descriptive results (means and standard deviations) and the Friedman tests results for forwards according to the group of players during the years studied are presented in Table 5 . Statistical significant results (p<0.05) were identified in minutes played for foreign and national important players, for passing accuracy in national less important forwards, and in defense performance for foreign less important forwards.

https://doi.org/10.1371/journal.pone.0211058.t005

The MBI results for forward playing position (see Fig 3 ) showed that foreign less important players decrease defense performance (very likely effect) from year -1 to 0. Moreover, these players increased minutes played (very likely effect) and defense performance (most likely effect) from year 0 to year 1. Lastly, national important players increased the minutes played from year -1 to 0 (very likely effect).

Asterisks indicate the likelihood of MBI effects as follows: *possibly, **likely, ***very likely.

https://doi.org/10.1371/journal.pone.0211058.g003

The aim of the present study was to analyse differences in performance of individual football players depending on the previous (2-years) and the later year after signing or renegotiating a new contract, while taking player-related characteristics into account (age, role in the team as important or less important, nationality, and team’s ability). This approach enabled to gain a better understanding of the effects of signing a new contract over consecutive seasons. In contrast to the common perception among sports fans that players become lazy and expend less effort once they have signed a long-term contract [ 18 ], or conversely start to deliver better performances in order to reach a promising bargaining position in the last season before the contract expires, the present results do not provide clear support for this hypothesis. In this respect the present findings are not supportive of previous studies reporting evidence for a relationship between performance and contract duration in sports in general [ 21 – 23 ] and specifically in soccer [ 13 , 14 , 24 – 27 ].

Overall, our results do not demonstrate a clear association between performance and contract duration. While the minutes played and defense showed significant differences for years -1 and 1 in national less important defenders and midfielders and foreign less important forwards, with better values during the year after the new signing of a contract, no differences were found for the rest of the variables or other groups of players. Hence, the results do not support our hypothesis that performance during the previous season is better than the performances immediately after signing the new contract. On the contrary, the current results showed, for example, that performance was worse for the less important defenders (defense variable), national important midfielders (minutes played per match variable), and foreign less important forwards (defense and minutes played variables) during the previous season of signing a new contract (see Fig 4 ).

https://doi.org/10.1371/journal.pone.0211058.g004

Previous studies suggest that the nationality of footballers may affect the relationship between performance and signing a new contract. Hence, the nationality of players can be considered as a moderator on the performance effects of signing a new contract. For example, Della Torre et al. [ 13 ] found that the individual performance during consecutive seasons is stronger for domestic players than for foreign players when the end of the contract is near. The findings of the current study do not support this evidence. There are no substantial differences between foreign and national players (see Fig 4 ). For example, national and foreign important midfielders increased the minutes played from year -1 to year 0. When the year after signing a contract is considered, no differences were found between national and foreign important players in any playing position. More studies are needed in order to clarify this conflicting pattern of results in the literature [ 13 ].

To the best of our knowledge, this is the first study analyzing the association between performance and contract duration depending on playing position. The current findings do not support previous studies [ 12 – 13 ]. According to our results, players did not perform better during the last year of their contract suggesting that maybe signing a new contract has no clear impact on the player’s motivation as it has been proposed previously. This may be due to the limit of 2-years analysis adopted in previous studies or the different performance indicators used in the current study. Future studies should scrutinize these findings.

However, managers, supporters and players should be careful when interpreting these results. Given the complexities of soccer, the match-performance trends of the players in the current study could also reflect the actions of the opposing team and teammates. Thus, individual performances could be influenced by collective strategies and tactics potentially disguising small effects that signing a new contract might have on observable individual performance indicators. Future research should possibly examine not only individual performances in isolation but also consider collective performance indicators [ 8 – 9 , 28 ].

The present study is not without limitations. The relationships between previous and actual performances could be affected by the current salary of the players and the opportunities of signing a new contract [ 13 ]. Accordingly, the analysis of football players’ performance should be mediated by the seasonal performances of their career and their market value. These variables should be included in future works. More variables (e.g. technical, tactical and physical indicators) and countries (i.e. specific leagues with the same or different foreign recruitment policies as European competitions) should be considered to provide conclusive evidence on the relationship between performance and contract duration. Further, we did not control for: (i) the duration of the contract signed and years remaining which might result in differential effects depending on the length of the newly signed contract; (ii) if the player moves to another club or stays with the same club; or (iii) the specific playing position during each year for the players analysed. Therefore, a fruitful avenue for further research would be to conduct multifactorial analysis of signing a contract in football [ 12 – 14 ].

In conclusion, this paper investigated the association between performance, contract duration, and nationality of the players in elite soccer. The research does not demonstrate a clear association between performance and contract duration using datasets from the French, German, Italian, and Spanish professional leagues during the seasons 2008 to 2015. Players’ performances did not show a clear decline or improvement during the two years before signing or renegotiating a new contract. Hence, the common assumption of football spectators that players perform better when playing for a new contract and “take a break” once they signed a new contract was not identified in the present data.

Practical applications

From a player’s recruitment or renovation perspective, the performance displayed during the past or the following seasons before or after a new contract can help managers and coaches to decide when a new contract should be signed, the duration of the contract or the salary of the player. While previous work [ 12 – 13 ] might be indicative of having short term contracts for players and waiting until the last year before a contract runs out to resign a player, the present results do not support this reasoning. On the contrary, players sometimes increased performance after signing a new contract.

Supporting information

S1 dataset. ibm spss dataset of players’ performances obtained by optasports company..

https://doi.org/10.1371/journal.pone.0211058.s001

Acknowledgments

The present study was supported by the Ministry of Economy and competitiveness of Spain with the project “Diseño y desarrollo de un software para el análisis del rendimiento en fútbol” (DEP2016-75785-R).

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BRIEF RESEARCH REPORT article

Running performance of high-level soccer player positions induces significant muscle damage and fatigue up to 24 h postgame.

• 1 Department of Fights, Postgraduate Program in Physical Education, School of Physical Education and Sports, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil
• 2 Department of Neuroscience, Rehabilitation, Ophthalmology, Genetics, Maternal and Child Health (DINOGMI), Section of Psychiatry, University of Genoa, Genoa, Italy
• 3 Department of Physical Education and Sport, College of Education, Taif University, Taif, Saudi Arabia
• 4 Laboratory for Industrial and Applied Mathematics, Department of Mathematics and Statistics, York University, Toronto, ON, Canada
• 5 Postgraduate Program in Physical Education, School of Physical Education and Sports, Federal University of Juiz de Fora, Juiz de Fora, Brazil
• 6 Escuela de Kinesiología, Facultad de Salud, Universidad Santo Tomás, Santiago, Chile

This study aimed to determine the impact of a soccer game on the creatine kinase (Ck) response and recovery and the specific Global Positioning System (GPS)-accelerometry-derived performance analysis during matches and comparing playing positions. A sample composed of 118 observations of 24 professional soccer teams of the Brazil League Serie A was recruited and classified according to playing positions, i.e., Left/Right Defenders ( D = 30, age: 25.2 ± 5.8 years, height: 187 ± 5.5 cm, weight: 80 ± 5.8 kg), Offensive Midfielders (OM = 44, age: 25.1 ± 0.2 years, height: 177 ± 0.3 cm, weight: 73 ± 1.2 kg), Forwards ( F = 9, age: 25.1 ± 0.2 years, height: 176.9 ± 4.3 cm, weight: 74.5 ± 2.1 kg), Left/Right Wingers ( M = 23, age: 24.5 ± 0.5 years, height: 175 ± 1.1 cm, weight: 74 ± 4.4 kg), and Strikers ( S = 12, age: 28 ± 0.2 years, height: 184 ± 1.0 cm, weight: 80 ± 1.4 kg). Blood Ck concentration was measured pre-, immediately post-, and 24 h postgame, and the GPS-accelerometry parameters were assessed during games. Findings demonstrated that Ck concentrations were higher at all postgame moments when compared with pregame, with incomplete recovery markers being identified up to 24 h after the game (range: 402–835 U/L). Moreover, Midfielders (108.6 ± 5.6 m/min) and Forwards (109.1 ± 8.3 m/min) had a higher relative distance vs. other positions (100.9 ± 10.1 m/min). Strikers [8.2 (8.1, 9.05) load/min] and Defenders [8.45 (8, 8.8) load/min] demonstrated lower load/min than Wingers [9.5 (9.2, 9.8) load/min], Midfielders [10.6 (9.9, 11.67) load/min], and Forwards [11 (10.65, 11, 15) load/min]. These results could be used to adopt specific training programs and recovery strategies after match according to the playing positions.

Introduction

Studying the determinants of performance outcomes and recoveries of professional soccer players, such as sprints, accelerations, decelerations, changes in direction, jump movement patterns, technical skills, and tactical actions associated with high-intensity efforts translated into metrics, could potentially be useful to inform the construction of specific conditioning drills in an evidence-based fashion ( Ade et al., 2016 ). In quest of best performance, soccer athletes, coaches, and physical trainers have to decide how and when they have to invest their energy ( Akubat et al., 2018 ). The performance analysis of soccer matches has been increasingly utilized during the previous years for this purpose ( Sarmento et al., 2008 ; Enes et al., 2021 ). Due to the difficulties and challenges in conducting physiological measurements during a match, studies interested in the time-motion analysis used running performance [Global Positioning System and Local Positioning System (GPS/LPS)] and factors affecting performance outcomes to infer the metabolic profiles of soccer matches ( Anderson et al., 2019 ; Gantois et al., 2020 ).

The studies of performance analyses showed that soccer match requires many physically demanding performances. The available scholarly literature computed a total and relative distance covered during the game between ∼8,000 and 10,500 m, with a range of ∼100–120 m/min per match ( Reinhardt et al., 2019 ). The low-intensity running performance has not been found determinant in intra-game comparisons, as shown in the published studies ( Di Salvo et al., 2010 ; Modric et al., 2019 ). In contrast, besides scoring the goals, accelerations, decelerations, the number of sprints and distance covered greater than 18 km/h, and other running metrics variables seem to be the key factors to succeed in professional soccer matches ( Mara et al., 2015 ; Abbott et al., 2018 ).

Physical performance during a soccer match is highly variable and depends on many factors, such as match intensity, period of the season, age, and playing positions, among others. Several investigations have studied the physical demands of a soccer match across playing positions. The majority of them categorized positions into defenders, midfielders, and attackers. Felipe et al. (2019) reported that defenders covered greater total distance (10,307.33 ± 1,206.33) when compared with midfielders (7,705.06 ± 3,201.10) and attackers (7,240.61 ± 3,411.31) ( Felipe et al., 2019 ). In contrast, another study showed that defenders had lower total absolute distance and work rates when compared with midfielders in the first half and midfielders and attackers in the second half ( Vescovi and Favero, 2014 ). Regarding moderate- and high-intensity running, contradictory results have been reported in the literature ( Hewitt et al., 2014 ).

More in detail, when specifically categorizing the playing positions, central defenders performed lower total distance covered, high-speed running (HSR), and very-high-speed running (VHSR) compared with full-backs, central midfielders, external midfielders, and forwards. Center-backs reported the lowest values for total distance covered ( Mendez-Villanueva et al., 2012 ) and high-intensity activities ( Andrzejewski et al., 2009 ; Buchheit et al., 2010 ; Brito et al., 2017 ; Varley et al., 2017 ); midfielders and second attackers performed the highest total distance covered; wide midfielders and attackers demonstrated the highest peak game speeds and frequency of high-intensity activities ( Buchheit et al., 2010 ; Al Haddad et al., 2015 ; Izzo and Varde’i, 2017 ).

Furthermore, participation in a soccer match can lead to acute and residual fatigue, characterized by a decline in physical performance over the following hours, which can persist even for days ( Aquino et al., 2016 ). The magnitude of these disturbances increases within the first 24 h after a competition with peaks between 24 and 48 h post-match ( Aquino et al., 2016 ). Together with a decrease in running performance, the potential insurgence of muscle damage and the increased levels of intramuscular enzymes, such as creatine kinase (Ck) and inflammatory/immunological biomarkers, are reported following soccer competition ( Russell et al., 2016 ; Oliveira et al., 2019 ). Some studies have found a significant correlation between running performance outcomes and muscle damage markers (e.g., muscle soreness, Ck) at 24, 48, and 72 h after the soccer matches ( Russell et al., 2016 ; Silva et al., 2018 ). More in detail, Russell et al. (2016) investigated, from a quantitative standpoint, the associations between GPS/LPS-accelerometry findings (i.e., high-intensity distance covered, HSR distance, and the number of sprints carried out) and changes in Ck levels and peak power output during the execution of countermovement jumps 24 and 48 h after the match in a sample of 15 English Premier League Reserve team players. Statistically significant correlations with coefficients ranging from 0.363 to 0.410 were found 24 h but not 48 h post-match. Silva et al. (2018) performed a systematic review and meta-analysis concerning the match-induced fatigue and related recovery profiles of soccer players, taking into account several parameters (i.e., physiological, neuromuscular, biochemical/endocrinological, perceptual, and technical). The authors pooled together 77 studies and computed 1,196 effect sizes (ESs), finding small-to-large variations in the variables under study. These changes could be detected, differently from the previous study ( Russell et al., 2016 ), until 72 h post-match, indicating a persistence of the muscle damage in terms of biochemical, inflammatory, and immunological parameters. These contrasting findings can be reconciled, assuming that some variables (such as those endocrinological/hormonal and technical) can be fully recovered in the short term after the match; for others, the process and dynamics of homeostatic balance are more complex, requiring more than 72 h.

It is well known that the magnitude of muscle damage and the other physiological alterations elicited by matches are associated with oscillations in Ck levels ( Coppalle et al., 2019 ) and related running performance can be assessed with ad hoc performance analytical tools ( Milanović et al., 2020 ; Enes et al., 2021 ; Strauss et al., 2019 ). Furthermore, soccer matches can affect the players differently depending on their playing positions ( Abbott et al., 2018 ). However, the influence of this variable has been relatively overlooked in the available scholarly literature. Furthermore, with the positional difference of physical demand during soccer matches ( Abbott et al., 2018 ), information about the effect of a soccer match on muscle damage postgame was lacking. Consequently, a comprehensive assessment in terms of positional differences of the muscle damage post-match of elite soccer players is necessary to inform applied practitioners working with soccer players to: (1) tailor and personalize interventions based on the specific needs of athletes, rather than relying on a “one-size-fits-it-all” approach; (2) better adopt position-specific recovery strategies; (3) appropriate time between match and session training; and (4) reduce the risk of injuries and achieve optimal performance outcomes. We formulated the working hypotheses that: (1) there is a difference in terms of performance outcomes among players of different playing positions; and (2) game load can impact Ck responses after competition until 24 h after the match. Therefore, this study was devised to fill in this gap in knowledge and aimed to determine the impact of a soccer game on the Ck response, recovery, and specific running performance outcomes, stratifying by playing position.

Materials and Methods

From an initial list of 800 soccer matches performances, this study randomly selected the performances of 118 athletes from professional soccer teams of Rio de Janeiro during the 2018 and 2019 championship seasons. During the games, athletes were classified as Left/Right Defenders ( D = 30, age: 25.2 ± 5.8 years, height: 187 ± 5.5 cm, and weight: 80 ± 5.8 kg), Offensive Midfielders (OM = 44, age: 25.1 ± 0.2 years, height: 177 ± 0.3 cm, and weight: 73 ± 1.2 kg), Forwards ( F = 9, age: 25.1 ± 0.2 years, height: 176.9 ± 4.3 cm, and weight: 74.5 ± 2.1 kg), Left/Right Wingers ( W = 23, age: 24.5 ± 0.5 years, height: 175 ± 1.1 cm, and weight: 74 ± 4.4 kg), and Strikers ( S = 12, age: 28 ± 0.2 years, height: 184 ± 1.0 cm, and weight: 80 ± 1.4 kg). We considered the level and locations of opponents: 39 international games of Copa Libertadores da América , 58 national games of Brasileirão Série A , and 26 state games of Campeonato Carioca de Futebol , with 28 teams in different rounds or championship phases, with competitions occurring once per week. The performance-analysis-related data were collected at single time points while the Ck level was studied before, after, and 24 h after the game. Therefore, a total of 354 samples of Ck concentration of all professional soccer players were analyzed in three moments as follows: first, before the game (Ck n = 118); second, after the game (Ck n = 118); and third, after 24 h of the game (Ck n = 118). The sample size was enough to compute estimates with 95% CI and 5% of margin of error.

Each athlete had a minimum of 6 days of rest from the previous match to prevent stress interference, competed in national and international representative championships once (∼90 min) per week, and had been regularly training 2 h of technical and tactical aspects 4–7 times a week and 1 h of physical preparation 2–3 times a week. Therefore, all participants were from the Brazil league and had previous experience with professional soccer events, rules, and procedures used during the event.

The inclusion criteria were as follows: (1) being from the Brazilian league; (2) having played during ∼85% of the game; (3) having a minimum of 6 days of rest from their previous match to prevent stress interference; (4) having competed in national and international representative championships once (∼90 min) per week; and (5) 2 h of regular training of technical and tactical aspects 4–7 times a week and 1 h of physical preparation 2–3 times a week. We excluded participants who played games for more than 90 min, substituted players, and goalkeepers.

This study was approved by the Local Committee of Ethics in Research (No. 13846919.8.0000.5257), following the rules of resolution of the National Health Council and in accordance with the WMA Declaration of Helsinki. Then, the volunteers (age: >18 years) were contacted by the researchers in such a way to be informed about the aims and procedures of the study and signed an informed consent form to participate in the data collection. Measurements were performed before, after, and 24 h after the game. No modifications were made in the training, nutritional, or hydration status of participants, and they maintained a passive recovery time pattern of 24 h without training efforts between the game, postgame, and 24 h postgame.

Physical Performance Demands

The subjects wore a GPS unit (Catapult Innovations, Scoresby, Australia) during each trial ( Jennings et al., 2010b ). The performance analyses of the professional soccer players were monitored using a portable 5-Hz GPS unit (Catapult, Melbourne, Australia) during games. The GPS unit was positioned via an elasticized shoulder harness to sit between the scapulae of the player at the base of the cervical spine ( Petersen et al., 2009a ). The GPS unit was activated and a GPS satellite lock was established for at least 15 min before the player taking the field, as per the recommendations of the manufacturer ( Petersen et al., 2009b ). The recorded information was downloaded after each session using Catapult Sprint software (Catapult Innovations, Melbourne, Australia) for analysis. Once downloaded, the competition data were edited and split into two 45-min halves ( Abbott et al., 2018 ).

Only subjects completing the entire match were included in the analysis process. The mean number of satellites and the horizontal dilution of position were recorded during the data collection ( Abbott et al., 2018 ). The performance analysis followed a preceding protocol ( Abbott et al., 2018 ). The total distance (m), i.e., distance traveled during all the game; total distance by minute; percentage of distance traveled, low-intensity running and jogging (<14 km/h), running (>14 km/h), and sprinting (>18 km/h), distance and number of sprints (>18 and >24 km/h), maximum speed (km/h), number of accelerations (>9 km/h), and deceleration (<9 km/h), jumps (>30 cm), and efforts (i.e., accelerations, deceleration, and jumps) were the performance analysis factors assessed during professional soccer games with ∼90 min of durations ( Abbott et al., 2018 ).

Blood Ck concentration was measured pre-, post-, and 24 h postgame by reflectance photometry at 37°C using the Reflotron Analyzer Plus (Reflotron Plus, Roche, Germany), previously calibrated. To reduce errors, only one evaluator was responsible for these collected data. A lancet device with an automatic trigger was used for puncturing the finger after finger asepsis using 70% ethyl alcohol, and the blood was drained into a strip for specific examination (using heparinized capillary strips). A blood sample (32 μl) was immediately pipetted into a Ck test strip, which was introduced into the instrument. The absolute values of Ck (U/L) were used for analysis, according to the study of Aquino et al. (2016) .

Statistical Analysis

The descriptive data are presented as mean and SD, using the coefficient of variance (CV, %) as the measure of variability. The Kolmogorov–Smirnov test (K-S) was used to determine the normal distribution of the data, considering p ≤ 0.05. A repeated measure ANOVA was performed to verify the Ck modifications, and a generalized estimating equation (GEE) mixed-linear model accounting for individual (random) effect was conducted, considering the level and location of the opponent (International competitions in South America × National competitions in Brazil × State competitions in Rio de Janeiro) as a control variable. The ES was calculated using eta-squared and interpreted as follows: small (0.01 < ES < 0.06), medium (0.06 < ES < 0.14), or large (ES > 0.14). The significance level of p ≤ 0.05 was used. All analyses were conducted using SPSS for Windows software (version 20.0; SPSS, Inc., Chicago, IL, United States).

Table 1 shows the descriptive analysis of distance and load during the game, with the total/minute ratio separated by the position of the player.

Table 1. Descriptive analysis of behavior and performance factors, considering each player’s position.

Total distance had differences ( F = 14.42, p ≤ 0.001, ES = 0.63, i.e., large ES); the S and D groups had a shorter total length than all groups, and F had a shorter total length than M and W groups ( p ≤ 0.001 for all comparisons).

Statistically significant effects were observed between the positions of players in the distance/min ( F = 15.06, p ≤ 0.001, ES = 0.64, i.e., large ES), where M and F had higher values than all other groups ( p ≤ 0.001 for all comparisons).

The total load also showed differences ( F = 22.39, p ≤ 0.001, ES = 0.73, i.e., large ES); the S and D groups had a lower total load than all the other groups, and F had a lower total load than M ( p ≤ 0.001 for all comparisons).

The comparisons also demonstrated effects in load/min ( F = 19.59, p ≤ 0.001, ES = 0.70, i.e., large ES). The S and D groups had a lower load ratio than all the other groups ( p ≤ 0.001 for all comparisons).

Figure 1 shows the sprint frequencies per soccer match.

Figure 1. Sprint frequencies by player position. $ Significant differences from all other groups; # significant differences from Defenders; ψ significant differences from Forwards; β significant differences from Wingers, p < 0.05 for all comparisons.

Effects were also observed in sprint frequencies above 14 km/h ( F = 10.28, p ≤ 0.001, ES = 0.55, i.e., large ES), where S and D had lower frequencies than all other groups ( p ≤ 0.001 for all comparisons), and S had higher frequencies than D ( p = 0.015).

Moreover, the analysis presented differences in sprint frequencies above 18 km/h between the positions of players ( F = 17.65, p ≤ 0.001, ES = 0.64, i.e., large ES), where S and D had lower frequencies than all other groups ( p ≤ 0.001 for all comparisons), and S had higher frequencies than D ( p = 0.015).

Effects were also observed in sprint frequencies above 24 km/h ( F = 7.72, p ≤ 0.001, ES = 0.48, i.e., large ES), where D had lower frequencies than all groups, while M had lower frequencies than S and W ( p ≤ 0.001 for all comparisons).

Furthermore, the analysis verified differences in maximal velocity comparisons ( F = 2.41, p = 0.007, ES = 0.23, i.e., large ES), D had lower speed than W ( p ≤ 0.001).

The comparison also showed differences in the deceleration of sprints ( F = 7.28, p ≤ 0.001, ES = 0.46, i.e., large ES), where D had a lower frequency than all other groups ( p ≤ 0.001 for all comparisons).

Additionally, the analysis observed effects in the acceleration of sprints between the positions of players ( F = 3.79, p ≤ 0.001, ES = 0.31, i.e., large ES), where D had a lower frequency than S ( p = 0.005), W ( p ≤ 0.001), M ( p = 0.009), and F ( p = 0.003), and M had a lower frequency of acceleration than W ( p = 0.012).

Besides, effects were observed in jump frequencies when comparing the positions of players ( F = 2.46, p = 0.006, ES = 0.23, i.e., large ES), where S and M had lower frequency than D ( p = 0.003 and p = 0.004, respectively).

Finally, the comparisons indicated differences in explosive effort frequencies ( F = 36.43, p ≤ 0.001, ES = 0.56, i.e., large ES); D had a lower frequency than other groups ( p ≤ 0.001 for all), while M had lower values than F ( p = 0.032).

Figure 2 shows Ck values. Significant differences were observed between time points when comparing Ck ( X 2 = 114.67, p ≤ 0.001, ES = 0.59, i.e., large ES). The Ck baseline (255 U/L) time point had lower values than postgame (718.5 U/L, p ≤ 0.001) and 24 h postgame (560.0 U/L, p ≤ 0.001). Postgame presented higher Ck values than the other time points ( p ≤ 0.001 for all comparisons). The positions of players demonstrated differences when comparing Ck baselines ( X 2 = 30.56, p ≤ 0.001, ES = 0.37, i.e., large ES), where D (158.0 U/L) had lower values than W (341.0 U/L, p = 0.003) and M (255.0 U/L, p ≤ 0.001). Significant differences were observed in Ck postgame ( X 2 = 19.89, p ≤ 0.001, ES = 0.218, i.e., large ES) and in Ck 24 h postgame ( X 2 = 20.55, p ≤ 0.001, ES = 0.223, i.e., large ES), where D (522.5 and 325.0 U/L) had lower values than M (718.5 and 560.0 U/L, p ≤ 0.001 for all comparisons) in both Ck postgame and 24 h postgame, respectively.

Figure 2. Ck (U/L –1 ) baseline, post and 24 h post game, by player’s position. *Significant differences from all other moments; # significant differences from Defenders; p < 0.05 for all comparisons.

This study aimed to determine the impact of a soccer game on the Ck response, recovery, and specific running performance outcomes during professional soccer games by comparing playing positions. The main results demonstrated that Ck concentrations were higher at all postgame time points when compared with pregame, with the highest concentrations being observed after the game. Incomplete recovery markers were also identified up to 24 h after the game, especially for midfielders. Significant effects were observed between the positions of players when comparing performance indicators, in which offensive midfielders had higher total and relative distances covered and higher loads during high-level soccer games. The strikers had a lower percentage of submaximum, maximum, and up to maximum limit efforts during the game than other groups. At the same time, middle athletes demonstrated a higher frequency of sprints above 24 km/h, the number of jumps (<30 cm), and the total frequency of explosive efforts. The interactions between the positions and the level and location of opponents were observed for the total distance, relative distance, total load, sprint frequencies above 18 km/h, and decelerations, with higher values in international competitions in South America than at the state level in Rio de Janei ro.

This study showed that midfielders and forwards covered higher distances than other playing positions. This finding is in line with previous studies, which reported a greater distance covered by midfielders, followed by forwards and defenders during a soccer match play ( Mohr et al., 2003 ; Di Salvo et al., 2010 ; Djaoui et al., 2013 ; Vescovi and Favero, 2014 ). The same data were reported in some investigations assessing the French First League, the Spanish La Liga, and the English FA Premier League ( Dellal et al., 2010 , 2011 ). For example, Dellal et al. (2010 , 2011) investigated the physical activities of elite soccer players across six playing positions. The authors showed that the covered total distances were greater in midfielders (i.e., central defensive midfielders, wide midfielders, and central attacking midfielders) than forwards, central defenders, and full-backs. Furthermore, when analyzing running performance during German Bundesliga over three seasons (i.e., 2014/2015, 2015/2016, and 2016/2017) according to five positional roles, Chmura et al. (2018) reported that forwards covered the longer distance in won matches than in drawn and lost matches, while wide midfielders similarly ran a significantly longer distance in drawn and won matches than in lost matches. This finding is also confirmed by Andrzejewski et al. (2019) in their study of 1,178 soccer players taking part in the Polish Premier League matches during the four seasons (from 2010 to 2014). Other data that may support this finding reported that elite midfielders have the biggest intermittent endurance capacity and the maximum rate of oxygen consumption (VO 2 max ) than forwards and defenders ( Slimani and Nikolaidis, 2019 ). This could be explained by the fact that midfielders played in an important position that linked defenders and attackers, which requires them to perform a repetitive moving back and forth between the attack and defense. Practitioners would adopt appropriate specific training plans that adequately elicit heightened cardiovascular demands in midfielders compared with other playing positions.

This study reported that midfielders had a higher sprint frequency and absolute distance sprinting than defenders and attackers. Accordingly, Di Salvo et al. (2010) analyzed 67 European matches (European Champions League and UEFA Cup) over four seasons and compared running performance among five playing positions. The authors found that wide midfielders performed a higher total number of sprints and total sprint distance than other playing positions. In contrast, other studies have shown that wide defenders and attackers covered a significantly greater distance and sprint time than midfielders ( Mohr et al., 2003 ; Rampinini et al., 2008 ). Other studies by Dellal et al. (2010 , 2011) reported that forwards sprinted the greatest distance than other playing positions during the French First League, the Spanish LaLiga, and the English FA Premier League soccer matches. These contradictions may be explained by the fact that each team has a specific playing formation, opposition level, tactics, and physical fitness of players ( Al’Hazzaa et al., 2001 ; Aquino et al., 2017 ; Sarmento et al., 2018 ; Slimani et al., 2019 ; Arjol-Serrano et al., 2021 ). Therefore, it seems that practitioners would adopt position-specific training programs for their players.

Regarding the acceleration and deceleration according to playing positions, our study found that left/right defenders had lower acceleration and deceleration frequencies than the left/right midfielders, wingers, and strikers. These data confirm the data collected by previous authors ( Vigne et al., 2010 ), who analyzed the activity profiles of players of a top-class team in the Italian National Football League over the course of a season and reported that central defenders perform lower accelerations and decelerations than other playing positions. Another study ( Oliva-Lozano et al., 2020 ) conducted a longitudinal study over 13 competitive microcycles recruiting professional footballers from LaLiga and detecting positional differences in terms of sprint, acceleration, and deceleration profiles. More specifically, greater start speeds than high-intensity accelerations were found in wide midfielders while no statistically significant differences could be reported in central defenders, full-backs, and midfielders. The high-intensity decelerations were performed by midfielders, forwards, full-backs, wide midfielders, and central defenders. Therefore, it seems that practitioners would adopt position-specific training programs that elicit higher acceleration/deceleration in outfielders.

Muscle damage markers, notably Ck, were higher in midfielders compared with defenders immediately and 24 h after the soccer match. Similar results have been reported in the existing scholarly literature with higher Ck immediately after the soccer match in midfielders than other playing positions ( Souglis et al., 2018 ). These data could be explained by the fact that midfielders performed higher acceleration, deceleration, and explosive action than defenders. In contrast, another study ( Scott et al., 2016 ) failed to stratify the Ck levels according to playing positions from 15 elite male soccer players competing in the English Premier League, 48 h following a competitive match. However, based on our findings, practitioners would adopt a position-specific recovery program after the soccer match to return to play as fast as possible.

Limitations and Strengths

Few studies have investigated Ck profiles in national team players ( Hecksteden and Meyer, 2020 ; Schuth et al., 2021 ), generally adults ( Hecksteden and Meyer, 2020 ) and more rarely adolescents ( Schuth et al., 2021 ). The present investigation significantly adds to this literature.

However, despite this strength, the sample size is the main limitation of the study since the Strikers and Forwards groups are composed only of three and two individuals. Therefore, individual differences that may modify the outcome distributions of these groups more than the actual differences between groups could influence the ESs reported.

This study presents a further limitation that GPS/LPS substantially underestimated ∼4% of the criterion distance when striding and sprinting over short distances (10 m) at both 1 and 5 Hz ( Jennings et al., 2010a , b ). In contrast, we were able to control the interactions between the positions and the level and location of opponents. The interactions between the level and location of opponents and the positions were observed in total distance, load, and minutes of the game: in this study, international games presented more, i.e., ∼10% of total load and ∼900 m of total distance than the level of state games. This information could improve the periodization of players associated with international championships. However, despite this, these variables were not the determinant for Ck concentrations. Other limitations include the use of Ck as the only biomarker of muscle damage, while a wider array of biological parameters could have been explored. Further high-quality studies are needed to overcome these limitations.

Significant effects were observed in terms of the positions of the player when comparing performance indicators, as offensive midfielders had a higher total and relative distance and load during the high-level soccer games. The strikers had a lower percentage of submaximum, maximum, and up to maximum limit efforts during the game than other groups, while defenders demonstrated a higher frequency of sprints above 24 km/h. The forwards showed a higher number of jumps (<30 cm) and a total frequency of explosive efforts. Muscle damage (as assessed by means of Ck levels) did not differ in terms of playing position, suggesting a relevant muscle involvement for every player regardless of his position, up to 24 h after the match. More specifically, according to these findings, no training game format alone is able to develop the overall soccer fitness, with each format eliciting a unique physical load. These results make it possible to create a specific training game according to playing positions, associated with the predominant activities performed during competition. Consequently, practitioners would adopt a position-specific recovery program after the soccer match, particularly for midfielders who are exposed to higher muscle damage after the soccer match play.

Data Availability Statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

Ethics Statement

The studies involving human participants were reviewed and approved by 13846919.8.000.5257/Hospital Universitário Clementino Fraga Filho-UFRJ. The patients/participants provided their written informed consent to participate in this study.

Author Contributions

LF, NE, and DG conceived the study, planned, carried out, and wrote the manuscript. MT, MS, HZ, NB, and BM performed the statistical analysis and reviewed the manuscript. MB realized the manuscript review and formatting. All authors contributed to the article and approved the submitted version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

We would like to thank all athletes, coaches, and federations for allowing and contributing to the accomplishment of this study and also would like to thank the Taif University Researchers for Supporting Project (No. TURSP-2020/170), Taif University, Taif, Saudi Arabia.

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Keywords : muscle damage, fatigue, muscle strength, sports, task performance and analysis, external loads

Citation: Freire LA, Brito MA, Esteves NS, Tannure M, Slimani M, Znazen H, Bragazzi NL, Brito CJ, Soto DAS, Gonçalves D and Miarka B (2021) Running Performance of High-Level Soccer Player Positions Induces Significant Muscle Damage and Fatigue Up to 24 h Postgame. Front. Psychol. 12:708725. doi: 10.3389/fpsyg.2021.708725

Received: 12 May 2021; Accepted: 09 August 2021; Published: 14 September 2021.

Reviewed by:

Copyright © 2021 Freire, Brito, Esteves, Tannure, Slimani, Znazen, Bragazzi, Brito, Soto, Gonçalves and Miarka. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Michele Andrade de Brito, [email protected]

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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Strength and Kicking Performance in Soccer

Rodríguez-Lorenzo, Lois BA; Fernandez-del-Olmo, Miguel PhD; Martín-Acero, Rafael PhD

Learning and Human Movement Control Group, Department of Sport Science and Physical Education, University of A Coruña, A Coruña, Spain

Address correspondence to Rafael Martín-Acero, [email protected] .

Conflicts of Interest and Source of Funding: The authors report no conflicts of interest and no source of funding.

Lois Rodríguez-Lorenzo is a PhD student of the Faculty of Sports Science and Physical Education (INEF Galicia) at the University of A Coruña.

Miguel Fernandez-del-Olmo is an associate professor of the Faculty of Sports Science and Physical Education (INEF Galicia) at the University of A Coruña.

Rafael Martín-Acero is an associate professor of the Faculty of Sports Science and Physical Education at the University of A Coruña.

THE STUDY OF THE MAXIMAL KICK BALL VELOCITY IN SOCCER IS OF INTEREST DUE TO ITS RELEVANCE IN THE SCORE ACHIEVED IN A SOCCER MATCH. THIS REVIEW FOCUSES ON STUDIES THAT HAVE EXPLORED THE ASSOCIATION BETWEEN STRENGTH AND BALL VELOCITY, AND THE EFFECTS OF STRENGTH TRAINING IN THE MAXIMUM KICKING VELOCITY. THE STUDIES REVIEWED SUGGEST THAT THE RELATIONSHIP BETWEEN STRENGTH AND KICKING VELOCITY IN SOCCER IS INCONSISTENT. IN ADDITION, PLYOMETRIC AND EXPLOSIVE STRENGTH TRAINING CAN BE CARRIED OUT SUCCESSFULLY IN COMBINATION WITH REGULAR SOCCER TRAINING TO IMPROVE THE MAXIMUM KICKING VELOCITY.

INTRODUCTION

Kicking is one of the most frequently used skills in soccer, and the most fundamental for soccer performance ( 5,8,25,32,44,48 ). Soccer performance and other athletic kicking usually depend on kicking ball velocity and kicking accuracy ( 4,8,42 ). This speed could be particularly important while kicking towards the goal, because the chances of scoring increase with an increased ball speed (assuming that the kick is accurate) because the goalkeeper has less time to react ( 19,42 ).

Kicking can be described as a summation of forces ( 58 ). The motion pattern is generally accepted as a proximal-to-distal sequence in which the distal segments are allowed to lag behind the proximal segments as they move forward ( 20 ), in which the foot is the last segment to intervene and the fastest segment in the open kinetic chain ( 65 ). The timing of muscle activation can be described with activation of hip flexors such as the iliopsoas followed by the rectus femoris, which is a hip flexor and knee extensor, and finally by activation of the knee extensors such as the vastus lateralis. Therefore, hip flexor and knee extensor muscles are important for developing a high foot velocity ( 18 ). In addition, the knee flexors (hamstrings) and hip extensors (gluteal muscles), which function as antagonists to decelerate the swinging-leg after ball impact, are also significantly active in a maximum soccer kick ( 18,57 ). In general, these antagonists require fundamentally eccentric strength, whereas agonists require concentric strength ( 18 ).

The ball velocity after a soccer kick is more strongly affected by the foot velocity at the initial instant of the impact phase than any other factors ( 26 ). Furthermore, significant correlation was found between these 2 variables ( 7,17,37 ) and the coefficient of restitution ( 9 ), which is influenced by skill factors, such as the part of the foot that makes contact with the ball and the stiffness of the foot at impact ( 65 ). The mass of the shank–foot segment does not influence the velocity of the ball significantly ( 3 ).

The role of the supporting leg in generation of foot speed is not clear, but it could be speculated that the strength of the support leg is important for providing a stable platform to quickly swing the kicking leg ( 9 ). Significant correlations between single-leg balance and kicking accuracy, but not velocity, were found ( 14 ).

It is clear that the strength of the lower limbs' muscles could be related to the ball velocity. Therefore, several studies have investigated the relationship between the ball velocity after a maximal soccer kick and the strength and power values of the lower limbs ( 10,13,15,28,30,38,44–47,51,60,61 ) as well as the relationship between ball velocity and the velocity values of linear sprint runs ( 15 ) and nonlinear sprint runs ( 13 ). Other studies have explored the influence of different strength training programs on ball velocity values after a soccer kick ( 1,10,12,27,39–41,50,55,62,63 ). Finally, another group of studies have validated new kick test protocols to assess the biomechanics and physiological parameters in soccer kicking ( 2,8,22,36,42,59,63 ).

Therefore, the purpose of this article is to systematically review the existing data about strength and ball velocity, discuss potential limitations of the literature, and suggest directions for future research in kicking velocity performance. Additionally, practical applications of the results will be presented for the strength & conditioning practitioners, because they may have limited knowledge of this topic or are unaware of up to date research.

The following databases were used in this review: Medline, Sport Discus, Dialnet, Google scholar, and Scopus. The keywords used were combinations of “soccer,” “strength,” “training,” “kick,” “kicking,” “maximal,” “ball,” “fatigue,” and “velocity.” Studies in English, Spanish, and Portuguese were included. Studies from 1979 to 2014 were included in this review with a focus on the maximal kicking velocity and one or more of the following factors: soccer, kicking, velocity ball, strength, accuracy, muscle fatigue, and training. We analyzed more than 300 papers, of which 210 were studies about ball velocity and 96 of them were included in this review. The results of this review are classified in 2 categories, studies that describe the relation between strength and kicking velocity and studies that explore the effect of strength training on the kicking velocity.

THE RELATIONSHIP BETWEEN STRENGTH AND BALL VELOCITY

Isokinetic strength in soccer kicking performance.

Differences in strength have been found among players from different divisions ( 11,49 ). However, the relationship between muscle isokinetic strength and kicking performance remains a subject of controversy in the soccer research field ( Table 1 ). Cabri et al. ( 11 ) found a high correlation between kick distances and the isokinetic strength exerted by knee flexor and extensor muscles, but moderate correlation with isokinetic strength of hip flexor and extensor muscles. In addition, in young soccer players, a significant relationship between ball kicking velocity and the maximal isokinetic forces of the thigh and shank was found ( 38 ). In elite soccer players, ball velocity values after a soccer kick correlated significantly with isokinetic muscle torques reached at different angular velocities ( 51 ). These authors also reported that the intensity of the relationship tends to decrease with the increase in angular velocities. Significant correlation between knee extensors isokinetic peak torque and kick velocity score was also found in trained and untrained soccer players. ( 5 ). In addition, Masuda et al. ( 44 ) examined relationships between muscular concentric isokinetic strength and kicking ball velocity achieved with both legs during 3 different approach angles. The mean ball velocities correlated significantly with isokinetic strength of several muscles on the kicking and the supporting legs ( Table 2 ). They concluded that different approach angles could alter the requirement on muscle strength potential of both legs during kicking. Especially, an angled approach toward the kick direction could require greater hip extension and abduction strength from the supporting leg to increase its stabilization capacity.

Maximal strength, explosive strength, and kicking performance

Elite young soccer players can be distinguished from subelite and recreational young soccer players by their higher performance in explosive and strength tests such as maximal isometric force, vertical jump height, pedaling rate, or 10 m sprint time ( 24 ). However, maximal strength in a full squat exercise did not correlate with maximal ball kicking velocity after a soccer kick in university soccer players ( 31 ). There was no correlation between ball velocity and the performance in several jump tests (squat jump; countermovement jump (CMJ)) with young soccer players ( 29 ), elite young soccer players ( 30,61 ), and elite inside soccer players ( 28 ). Furthermore, no relationship was found between the performance in a 10 meter sprint test and ball kicking velocity values. However, 2 studies found a weak relationship between explosive strength and maximal strength with the maximal ball velocity after a soccer kick. ( 13,23 ) ( Tables 2 and 3 ).

Induced fatigue effects on ball kicking velocity

The soccer kick has been mainly studied under nonfatigued conditions. Few researches have examined the effects of fatigue on soccer and maximal ball kicking velocities ( 6,21,33,34,35 ) ( Table 4 ), and there is only one study about fatigue effects on short-passing ability ( 56 ).

In summary, those studies have shown that there is a significant decrease in ball velocity which has been reported after: (a) a 6 minute step exercise protocol ( 35 ); (b) 90 minutes of intermittent exercise protocol ( 33 ); (c) knee extension and flexion motions on a weight-training machine until exhaustion ( 6 ); (d) a soccer specific circuit (with jumps, skipping, multiple changes of direction, dribbling the ball, passing, bursts of sprinting and jogging) ( 21 ), and (e) consecutive soccer instep kicks ( 34 ).

The significantly slower ball velocity observed in the fatigue condition may result from a reduced lower leg swing speed and poorer ball contact ( 6 ). Moreover, fatigue obscured the eccentric action of the knee flexors immediately before ball impact, which might increase the susceptibility to injury ( 6 ). This could be attributed to alterations in the function of the neuromuscular system and force generation capacity ( 33 ), and a poorer intersegmental coordination ( 6,35 ).

Finally, one study ( 34 ) showed that during repetitive kicks there was a significant reduction in the ball velocity between the first and the fifth and subsequent kicks. Therefore, it seems that no more than 4 consecutive kicks would be recommended to evaluate the kicking performance in the absence of fatigue ( 34 ).

Effects of strength training on maximum ball velocity

Several studies have explored the effect of strength training programs on maximal ball kicking velocity. Table 5 summarizes the results and strength protocols used in those studies. Most of these studies have been conducted in nonprofessional soccer players. The studies have reported a significant increase in ball velocity values after the application of different strength training programs in young amateur ( 27,50,53,54 ), young elite ( 16,43,52,55,62 ), adult amateur ( 39–41 ), and elite females soccer players ( 12 ). The strength training program has always been implemented as an extra session to regular soccer training. The strength training programs involved maximal strength training with a loaded kicking simulating exercise ( 62 ), combined strength and kick coordination training ( 39–41 ), an explosive and/or plyometric strength training program without kicking specific exercise ( 12,16,43,50,53–55 ), core strength training performed on stable surfaces ( 52 ), and electrostimulation on both quadriceps ( 10 ).

The studies conducted by Manolopoulos et al. ( 39–41 ) deserve special attention because they record muscle activity by surface electromyography. In those studies, the strength training program led to a significant decrease in joint angular velocities with an increase in biceps femoris electromyography of the kicking leg during the backswing phase ( 40,62 ). In addition, an increase in segmental and joint velocities and muscle activation of the same leg during the forward swing phase were found ( 40,63 ). They also reported a significantly higher vertical ground reaction force on the supported leg together with an increase in the rectus femoris and gastrocnemius activations ( 40,63 ). Therefore, the authors reported that the increase in ball velocity values after a soccer kick was accompanied by changes in kinetic and kinematic indices of the kicking performance ( 10,40,41 ) mainly due to an altered soccer kick movement pattern, characterized by a more explosive backward–forward swinging movement and higher muscle activation during the final kicking phase ( 39 ).

In other studies, the enhancement in maximum kicking performance was accompanied by increases in the performance of muscular strength tests like sprint ( 41,50,52–55,64 ), CMJ ( 12,39–41,53–55 ), and other strength (Isometric) measurements ( 10,39–41,62 ).

It should be noted that a few studies have been conducted in professional soccer players. Trolle et al. ( 63 ) and Aagaard ( 1 ) did not find a significant improvement in maximal kicking velocity in professional soccer players after strength training with several intensities with ( 1 ) and without ( 63 ) regular soccer training. Although, both studies reported improvements in knee extension strength, those results were not accompanied by an increase in the maximal kicking velocity.

In this article, we review the existing data about strength and ball kicking velocity in soccer. We focus on the relationship between both parameters and also the effects of strength training on maximum ball velocity. The main outcomes of this review indicate that the relationship between strength and kicking velocity in soccer is inconsistent. In addition, strength training programs seem to enhance maximal ball kicking velocity although its effectiveness is questionable in elite soccer players.

STRENGTH IN SOCCER KICKING PERFORMANCE

In general, soccer players are stronger than nonsoccer players and differences in strength can be found among players from different divisions ( 11,49 ). However, it is not possible to establish a cause-and-effect relationship between the maximal strength and kicking performance ( 65 ) according to studies using maximal isokinetic or full squat tests ( 5,11,15,19,38,44,46–47,51,60 ). Therefore, neither isokinetic torque nor maximal full squat seems to be a good predictor of ball velocity. It is likely that those tests do not reproduce the muscular demands involved in the soccer kick.

It is reasonable to assume that tests involving explosive muscular actions could be more suitable when exploring the relationship between strength training and ball kicking velocity, than the above-mentioned tests. Vertical jumps or sprints have been the most common tests used to explore the relationship between explosive strength and ball kicking velocity. However, the small number of studies and their contradictory findings ( 11,13,15,23,28–31,61 ) make it difficult to reach a definitive conclusion about this relationship. It is important to note that the performance of vertical jumps required familiarization sessions to obtain reliable data. Most of those studies did not report whether or not such familiarization sessions were conducted, thus, we cannot assess an association between vertical jumps and ball kicking velocities.

We suggest that soccer-specific strength tests, like a maximal kicking test, should be included in the selection of young soccer players and in the elite adult soccer player's training process to guide and control their progress, because this test closely reflects the role of muscles used in this action.

INDUCED FATIGUE EFFECTS ON BALL KICKING VELOCITY

The effects of fatigue on soccer kick performance have only been studied by a few researchers. There is an agreement that after implementation of different fatigue protocols, there is a significant decrease in ball kicking velocity ( 6,33,34 ). This could be attributed to alterations of the force generation capacity and a poorer intersegmental coordination. However, no researchers have examined the effects of actual match fatigue in maximum kicking velocity. Therefore, we do not have available information about how muscle fatigue could be related to impairment in ball kicking velocity during a real match. This is because, in part, to explore that relationship during an actual match would be a challenge. The simulation of a real game with small pauses to perform explosive strength tests (i.e., vertical jumps and maximal kicking test) could be an interesting approach.

Previously, we have suggested that less than 5 trials seems to be adequate to avoid the fatigue effect and maintain high kinematics and kinetic responses, when measuring the maximum kicking velocity ( 34 ). Although this is interesting for evaluation purposes, it does not represent a real situation, where it is unlikely that a player can perform more than 4 maximal kicks in a 2 minute period as in Khorasani's study ( 34 ).

EFFECTS OF STRENGTH TRAINING ON MAXIMUM BALL VELOCITY

Based on the review of literature, it is difficult to reach a final conclusion about the efficacy of strength training programs in improving the ball kicking velocity. Several factors can account for the different results across studies such as: (a) the heterogeneous profile of the samples (from amateurs to elite soccer players); (b) a disparity of the features of the strength training programs (from isokinetic to plyometric exercises), and (c) the inclusion of regular soccer training sessions during the strength programs. Nevertheless, we can speculate in the following paragraphs the role of those factors in the efficacy of the strength training programs.

There is evidence in the literature that the application of different strength training programs can increase ball velocity values in soccer players, depending on gender and playing level ( 12 ). In amateur adult, young elite, and young soccer players, strength training programs in combination with technical training lead to significant increases in maximum ball velocity values ( 39–41,62 ). However, the scarce information on adult elite soccer players ( Table 5 ), may be due to the difficulty in accessing this type of sample. Only a small pool of studies has been conducted in that population ( 1,12,63 ). One of them found a significant improvement with female soccer players ( 12 ), while no studies found any effect of a strength training program in male soccer players ( 1,63 ). Therefore, it seems that the years of experience and expertise can affect the potential enhanced effect of strength training programs. This is reasonable because the margin for improvement becomes narrower with higher levels of performance. However, this does not necessarily mean that there is still no opportunity for improvement, but it is clear that the development of strength training programs for elite soccer players is more complex.

From the diversity of training programs reviewed in the current study, it seems that the most effective approaches to improve ball kicking velocity of the programs reviewed are those that combine explosive strength training with regular soccer training. The lack of improvement in training programs that include isokinetic strength exercises ( 1,63 ) could be expected to be according to the poor specificity between isokinetic and kicking performances (see previous discussion about this issue). However, the use of plyometric exercises in the form of vertical or horizontal jumps could lead to improvements in ball kicking velocity ( 23,53,54 ) even in combination with isometric strength exercises ( 23 ). In those studies, the increases in maximum kicking velocity were accompanied by an increase in the performance of countermovement jumps. Another important factor is whether or not the strength training program is combined with regular soccer training or technical kicking training. From all the studies that lead to improvements in ball kicking velocity, 94.1% of them ( 1,4,10,12,16,23,27,39–41,50,52–55,62 ) combined strength and regular soccer training. Therefore, this supports Aagaard's suggestion ( 1 ) that strength training should be integrated with other types of training, involving the actual movement pattern to increase the performance within more complex movement patterns.

Studies about the effect of strength training programs on ball kicking velocity are focused on the dominant leg, whereas studies evaluating the nondominant leg are clearly limited. Only 3 studies have explored or compared the improvements between legs ( 12,16,23 ). Interestingly, their results showed higher gains in the kicking ball velocities for the nondominant in comparison with the dominant legs. It is likely that diminished baseline performance kicking with the nondominant leg could explain this result. Nevertheless, more studies must be conducted to explore the effects of strength training programs in kicking ball velocity with the nondominant leg because the ability to kick with both legs leads to an advantage for the soccer player.

PRACTICAL APPLICATIONS

The current review explores the existing data about the relationship between strength tests and ball kicking velocity and the efficacy of the strength training programs. The results of this review suggest that tests such as isokinetic, 1RM, or vertical jumps are not strongly related to maximal kicking velocity. Thus, their role in understanding the force requirements during soccer ball kicking is questionable. In addition, the development of a strength training program with the goal of improving the ball kicking velocity must include explosive strength exercises in combination with regular soccer training. We also recommend evaluation in the efficacy of those programs by a direct measurement of ball kicking movement (i.e., by radar) rather than other strength measurements.

strength training; jumping performance; maximal ball velocity; nondominant leg; fatigue effects

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• Review Article
• Open access
• Published: 02 April 2015

Strength training in soccer with a specific focus on highly trained players

• João R Silva 1 , 2 ,
• George P Nassis 1 &
• Antonio Rebelo 2  

Sports Medicine - Open volume  1 , Article number:  17 ( 2015 ) Cite this article

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Data concerning the physical demands of soccer (e.g., activity pattern) suggest that a high level of performance requires well-developed neuromuscular function (NF). Proficient NF may be relevant to maintain and/or increase players’ short- (intense periods of soccer-specific activity; accelerations, decelerations, and sprinting) and long-term performance during a match and throughout the season.

This review examines the extent to which distinct modes of strength training improve soccer players’ performance, as well as the effects of concurrent strength and endurance training on the physical capacity of players.

Data sources

A selection of studies was performed in two screening phases. The first phase consisted of identifying articles through a systematic search using relevant databases, including the US National Library of Medicine (PubMed), MEDLINE, and SportDiscus. Several permutations of keywords were utilized (e.g., soccer; strength; power; muscle function), along with the additional scanning of the reference lists of relevant manuscripts. Given the wide range of this review, additional researchers were included. The second phase involved applying six selection criteria to the articles.

Results and conclusions

After the two selection phases, 24 manuscripts involving a total sample of 523 soccer players were considered. Our analysis suggests that professional players need to significantly increase their strength to obtain slight improvements in certain running-based actions (sprint and change of direction speed). Strength training induces greater performance improvements in jump actions than in running-based activities, and these achievements varied according to the motor task [e.g., greater improvements in acceleration (10 m) than in maximal speed (40 m) running movements and in non-squat jump (SJ) than in SSC-based actions (countermovement jump)]. With regard to the strength/power training methods used by soccer players, high-intensity resistance training seems to be more efficient than moderate-intensity resistance training (hypertrophic). From a training frequency perspective, two weekly sessions of strength training are sufficient to increase a player’s force production and muscle power-based actions during pre-season, with one weekly session being adequate to avoid in-season detraining. Nevertheless, to further improve performance during the competitive period, training should incorporate a higher volume of soccer-specific power-based actions that target the neuromuscular system. Combined strength/power training programs involving different movement patterns and an increased focus on soccer-specific power-based actions are preferred over traditional resistance exercises, not only due to their superior efficiency but also due to their ecological value. Strength/power training programs should incorporate a significant number of exercises targeting the efficiency of stretch-shortening-cycle activities and soccer-specific strength-based actions. Manipulation of training surfaces could constitute an important training strategy (e.g., when players are returning from an injury). In addition, given the conditional concurrent nature of the sport, concurrent high-intensity strength and high-intensity endurance training modes (HIT) may enhance a player’s overall performance capacity.

Our analysis suggests that neuromuscular training improves both physiological and physical measures associated with the high-level performance of soccer players.

Neuromuscular training improves both physiological and physical measures associated with high-level performance.

It seems that strength and power training programs should target all the force-velocity potential/spectrum of the neuromuscular system.

Due to the conditioned concurrent nature of the sport, combined strength and combined high-intensity training approaches may constitute a good training approach within a football periodized process.

Introduction

The central goal of strength/power training in a highly competitive sport is to improve the players’ specific and relevant athletic activities inherent in their sport. To achieve this outcome, different strength/power training modes with i) distinct movement patterns (traditional resistance exercises, ballistic exercises, plyometrics, weight lifting, and/or sport-specific strength-based actions), ii) different combinations of the temporal organization of strength/power training loads (e.g., microcycle and training session variations), iii) distinct loads, iv) a wide range of movement velocities, v) specific biomechanical characteristics, and vi) different training surfaces have been adopted with the final end point of achieving an improvement in players’ performance in relevant motor tasks (e.g., jumping, sprinting, and changing direction) [ 1 - 24 ].

Certain training methods combine different exercise modes (e.g., weight training, plyometric training, and sport-specific force-based actions) and allow for optimal power development and transfer to athletic activities due to both the neural and morphological adaptations typically associated with advanced training [ 25 ]. In fact, the intrinsic characteristics of soccer activity patterns (a varied range of motor actions that involve both breaking and propulsive forces as well as distinct contraction modes and velocities that require the all force-velocity potential of the neuromuscular system) that highlight the importance of the principle of specificity in strength and muscle power training cannot be understated [ 26 , 27 ].

A combination of different methods, including high-intensity strength training involving traditional resistance exercises (TRE; squats) and plyometrics [ 6 ], TRE and sprint training [ 10 ], and complex strength training (CT) [ 11 , 15 , 19 ], have all recently received considerable attention. Although some similarities exist between the previous modes of strength and power training, there are important differences. In this review, we found that complex training refers to training protocols that are comprised of the alternation of biomechanically comparable strength exercises and sport-specific drills in the same workout (e.g., six repetitions of calf extension exercise at 90% of one repetition maximum (1RM) + 5 s of rest + eight vertical jumps + 5 s of rest + three high ball headers) [ 25 ].

By focusing on more effective periodization techniques, researchers have investigated the effectiveness of different loading schemes throughout the power training phase (from high-force/low-velocity end to low-force/high-velocity end or vice versa) [ 22 ]. The training-induced effects of exercises with distinct biomechanical and technical characteristics during the plyometric-based component (e.g., purely vertically or a combination of vertically and horizontally oriented exercises [ 12 , 16 , 21 , 23 ], as well as the effects of plyometric training on different ground surfaces (grass vs. sand) [ 12 ], have both garnered significant attention. Furthermore, the adaptiveness of the functional and muscle structure of professional players (e.g., myosin heavy chain composition) to high-intensity strength training in the isokinetic contraction mode has also been investigated. However, the implementation of this analysis during the off-season resulted in lower ecological validity of these findings [ 7 ]. With regard to the search for complementary procedures and/or less stressful interventions, the effects of other methodologies (e.g., effects of electrostimulation training on semi-professional players) on physical fitness have also been investigated [ 28 ].

In general, most studies have examined the training-induced performance effects of two [ 1 , 6 , 8 , 10 , 14 , 16 , 19 ] to three [ 2 , 3 , 11 , 12 , 21 , 28 ] sessions per week. Given the multi-component requisites of soccer players’ training (e.g., endurance, speed endurance, strength, power, and agility) that coincide with the increased amount of training time, some researchers examined the short-term effect of a lower weekly volume program (one session) [ 1 , 15 , 19 ] and the effect of training-induced adaptations of different weekly training frequencies (e.g., one vs. two sessions and one session per week vs. one session every second week) on both physiological and performance parameters during pre-season [ 19 ] and throughout the in-season in well-trained soccer players [ 1 ].

Nevertheless, despite an increase in the body of evidence regarding the applicability of strength/power training programs to routine soccer training, the short-term duration of interventions (e.g., 4 to 12 weeks) [ 2 , 3 , 6 , 8 , 10 - 12 , 14 - 16 , 19 , 21 - 23 , 28 ], the wide variety of training methods, the distinct season time lines used throughout the pre-season [ 2 , 3 , 6 , 12 , 19 ] and in-season [ 8 , 14 - 16 , 21 , 24 , 28 ] periods, the different weekly training loads, and the absence of control groups make the drawing of precise conclusions very difficult. With regard to the latter aspect, it is accepted that due to the importance of winning matches, technical staff of semi-professional and professional teams are unable to implement different training scenarios based on research interests. Nevertheless, in this review, our aim is to contribute to the understanding of the present state of the art of strength/power training and concurrent training in soccer to motivate future studies.

Search strategy: databases and inclusion criteria

The selection of studies was performed in two consecutive screening phases. The first phase consisted of identifying articles through a systematic search using the US National Library of Medicine (PubMed), MEDLINE, and SportDiscus databases. Literature searches were performed from January 2013 until June 2014, and this review comprises papers from 1985 to 2014 (N 1985-2009  = 76 papers, N 2010  = 7 papers, N 2011  = 17 papers, N 2012  = 4 papers, N 2013  = 21 papers, N 2014  = 11 papers). The following keywords were used in combination: ‘elite soccer’, ‘professional soccer’, ‘first division soccer,’ ‘highly trained players,’ ‘seasonal alterations’, ‘performance analysis’, ‘soccer physiology’, ‘football’, ‘strength training’, ‘concurrent training’, ‘training transfer’, ‘neuromuscular performance,’ ‘muscular power’, ‘jump ability’, ‘sprint ability’, ‘agility’, ‘repeated sprint’, ‘intermittent endurance’. Further searching of the relevant literature was performed by using the ‘related citations’ function of PubMed and by scanning the reference lists. The second phase involved applying the selection criteria to the articles. Studies were chosen if they fulfilled the following six selection criteria: (i) the studied athletic population consisted of highly trained soccer players, ii) the players in the sample were not under 17 years of age, (iii) detailed physiological and performance tests were included, iv) the training programs applied were specified, (v) appropriate statistical analyses were used, and (vi) the article was written in the English language and published as an article in a peer-reviewed journal or a peer-review soccer-specific book edition.

Data extraction and presentation

Data related to the players’ physiological parameters (e.g., lean leg volume, body fat percentage, running economy, anaerobic threshold, maximum absolute and relative oxygen consumption and strength values, peak and mean power values, and rate force development measures) and performance parameters (e.g., soccer-specific endurance tests, maximal aerobic speed, repeated and single sprint tests, jump ability exercises, agility, and ball speed) were extracted. All data are presented as the percentage of change in the means (∆) unless otherwise specified.

Search data and study characteristics

The aim of providing players with updated data and training approaches in modern scenarios was fulfilled by 23 of the 24 papers published in the last 10 years. There were a total of 24 manuscripts fulfilling the five selection criteria, and the total sample population consisted of 523 soccer players. The distribution of players by competition level was as follows: 322 adults, 145 U-20 players, 12 U-19 players, and 44 U-18 players.

General physiological considerations of strength/power training

Strength training has become an integral component of the physical preparation for the enhancement of sports performance [ 29 ]. While strength is defined as the integrated result of several force-producing muscles performing maximally, either isometrically or dynamically during a single voluntary effort of a defined task, power is the product of force and the inverse of time, i.e., the ability to produce as much force as possible in the shortest possible time [ 9 ]. Nevertheless, strength and power are not distinct entities, as power performance is influenced by training methods that maximize both strength and stretch-shortening cycle activity (SSC) [ 30 ]. The ability of a muscle to produce force and power is determined by the interaction of biomechanical and physiological factors, such as muscle mechanics (e.g., type of muscle action) and morphological (e.g., muscle fiber type) and neural (e.g., motor unit recruitment) factors, and by the muscle environment itself (e.g., biochemical composition) [ 31 ].

The mechanisms underlying strength/power adaptations are largely associated with increases in the cross-sectional area of the muscle (hypertrophy methods) [ 32 ]. However, muscular strength increments can be observed without noticeable hypertrophy and serve as the first line of evidence for the neural involvement in the acquisition of muscular strength [ 32 ]. Thus, despite the notion that hypertrophy and neural adaptations are the basis of muscle strength development [ 33 ], their respective mechanisms of adaptation in the neuromuscular system are distinct [ 34 ]. In fact, ‘more strength’, i.e., the adaptational effect, does not necessarily imply an increase in muscle mass, as several distinct adaptations can lead to the same effect [ 33 ]. In this regard, the trainable effects of explosive/ballistic and/or heavy-resistance strength training causing enhanced force/power production have been primarily attributed to neural adaptations, such as motor unit recruitment, rate coding (frequency or rate of action potentials), synchronization, and inter-muscular coordination [ 31 , 35 , 36 ].

Physiological adaptations in soccer players

Our analysis suggests that the physiological adaptations underlining strength/power training may result in improvements in different motor tasks and performance qualities in high- and low-level players (Table  1 and Figure  1 ). In fact, independent of the players’ standard, an enhanced dynamic [ 1 - 7 , 10 , 14 , 22 , 23 ] and static maximum force production [ 4 , 5 , 28 ] and increased muscle power outputs during different physical movements can be obtained through the implementation of strength/power training routines [ 2 - 8 , 14 , 22 , 37 ]. Specifically, increases in 1RM were observed during isoinertial assessments of half-squat exercises [ 1 - 3 , 6 , 10 , 14 , 22 ], hamstring leg curls, and one-leg step-up bench exercises [ 10 ]. Additionally, in our analysis, we observed a large range of improvements in the 1RM of well-trained players after short-term intervention periods (e.g., pre-season, Figure.  1 , from 11% to 52% during the squat exercise) with average increments of approximately 21% [ 1 - 3 , 6 , 22 , 37 , 38 ]. Only Helgerud et al. [ 37 ] reported considerably larger gains in 1RM compared with other studies (11% to 26%; Table  1 ). Moreover, increments in maximal isometric voluntary contraction (MIVC) in the leg press task after CT training [ 11 ] and in knee extension strength after electrostimulation [ 28 ] and isokinetic training [ 4 , 5 ] have also been reported. Interestingly, not only were improvements in absolute force production (1RM) achieved, but an increased efficiency was also evident after allometric scaling of the results; 1RM per lean leg volume (LLV; 1RM/LLV) improved after high- and moderate-intensity modes of strength training [ 2 ], and relative force (maximum force divided by body mass) improved after complex strength training [ 11 ].

The gains in strength and d ifferent motor abilities of high-level players after 5 to 10 weeks. Squares represent the average squat jump performance [ 1 , 6 , 14 , 22 ]; rhombi represent the average countermovement jump performance [ 2 , 22 , 37 ]; triangles represent the average four bounce test performance [ 6 ]; circles represent the average 10-m sprint performance [ 2 , 22 , 37 , 38 ]; x symbols represent the average 40-m sprint performance [ 1 , 2 , 6 ]; + symbols represent the average change in direction ability [ 2 , 38 ]; and lines represent the average of all the previous motor tasks.

According to Harris et al. [ 27 ], intervention studies should use a specific isoinertial loading scheme, and test protocols should assess performance over the force-velocity continuum to gain a better understanding of the effect of load on muscular function. Moreover, neuromuscular-related qualities, such as impulse, rate of force development (RFD), and explosive strength, can better predict athletic performance; thus, the development of these approaches should be targeted [ 27 ]. The functional performance of soccer players seems to be more significantly associated with variables that are measured within the power-training load range (75% to 125% of body weight [BW] in half-squats) at which peak power (PP) is obtained (60% 1RM = 112% of BW) [ 39 ]. The PPs of highly trained soccer players were shown to occur with loads of 45% and 60% 1RM during jump- and half-squat exercises, respectively [ 22 , 39 ]. It is likely that superior improvements in power performance may be achieved by working on these optimal power training load ranges [ 22 , 39 ].

One particular muscle strength/power training adaptation involves an increase in the force-velocity relationships and the mechanical parabolic curves of power vs. velocity after high-intensity training programs, both in isoinertial [ 14 ] and isokinetic [ 4 ] exercises. Ronnestad et al. [ 6 ] and Gorostiaga et al. [ 8 ] observed increases in the force-velocity curve after high-intensity TRE and explosive-type strength training among professional and amateurs players, respectively. In the former study, the analysis of the pooled groups revealed increases in all measures of PP [ 6 ]. It seems that high-intensity strength training significantly increases performance in professional players at both the high-force end (increases in 1RM and sprint acceleration) and the high-velocity end (improvements in peak sprint velocity and four bounce test; 4BT) but only as long as the subjects perform concurrent plyometric and explosive exercises during their soccer sessions [ 6 ]. Furthermore, Los Arcos et al. (2013) recently found that professional players performing 5 weeks of pre-season and 3 weeks of in-season strength/power training increased the load at which PP was achieved during the half-squat exercise [ 11 ]. Additionally, 10 weeks of complex strength training, consisting of soccer-specific strength and skill exercises (soccer kick), improved measures of explosive strength and RFD during the isometric leg press in low-level players, with an increase in the electromyography (EMG) activity of certain muscles involved in the task also reported [ 11 ].

Adaptations in sport-specific efforts

The effectiveness of a strength/power program is evaluated by the magnitude of sport-specific improvements. Although the predominant activities during training and matches are performed at low and medium intensities, sprints, jumps, duels, and kicking, which are mainly dependent on the maximum strength and anaerobic power of the neuromuscular system, are essential skills [ 40 ]. Power and speed usually support the decisive decision-making situations in professional football, e.g., straight sprinting is the most frequent physical action in goal situations [ 41 ]. Furthermore, a high degree of stress is imposed on the neuromuscular system of players to enable them to cope with these essential force-based actions required during training and competition (e.g., accelerations and decelerations) [ 42 , 43 ].

Although not universally confirmed, there is evidence of associations between the measures of maximal (1RM) [ 44 ] and relative strength (1RM/BM) [ 45 ], as well as between certain muscle mechanical properties, such as peak torque [ 46 , 47 ] and PP [ 39 ], and the ability of soccer players to perform complex multi-joint dynamic movements, e.g., jumping and sprinting actions. Independently of a player’s level, strength-related interventions represent a powerful training stimulus by promoting adaptations in a wide range of athletic skills (e.g., jumping, Table  1 , Figures 1 , 2 and 3 and Additional file 1 : Figure S1-5) [ 2 , 3 , 6 , 8 , 10 , 12 , 14 , 15 , 19 , 21 - 23 , 48 ] and soccer-specific skills (soccer kick) [ 21 , 28 ] (Tables  1 and 2 ). Interestingly, the addition of a long-term strength/power training program to normal soccer training routines seems to result in a higher long-term increase in the physical performance of elite youth players [ 45 , 49 ]. Furthermore, to have a clear picture of the effect of strength training on physical performance, different motor tasks should be assessed; jumping, sprinting, and change of direction abilities may represent separate and independent motor abilities, and concentric and slow SSC jumping actions are shown to be relatively independent of fast SSC abilities [ 50 ].

Gains in strength and motor abilities of high level players after different training modes (5 to 10 weeks). x and dashed x symbols represent the change of direction ability performance after traditional resistance exercises programs (TRE) [ 2 ] and combined programs (COM) [ 38 ], respectively; filled and unfilled squares represent the 40-m sprint performance after TRE [ 1 , 2 ] and COM [ 6 ], respectively; + and dashed + symbols represent the 10-m sprint performance after TRE [ 2 , 37 ] and COM [ 22 , 38 ], respectively; filled and unfilled triangles represent the four bounce test performance after TRE [ 6 ] and COM [ 6 ], respectively; filled and unfilled rhombi represent the squat jump performance after TRE [ 1 , 14 ] and COM [ 6 , 22 ], respectively; and filled and unfilled circles represent the countermovement jump performance after TRE [ 2 , 37 ] and COM [ 22 ], respectively.

Percentage of improvement by training program and training session. Percentage of improvement by training program and training session after traditional resistance exercises programs (TRE), combined programs (COM), and strength/power training programs in the different motor tasks and overall functional performance (FP) of high-level players. Countermovement jump (CMJ) after TRE (CMJ-TRE) [ 2 , 20 , 37 ]; CMJ after COM (CMJ-COM) [ 22 , 23 , 38 ]; CMJ [ 2 , 20 - 23 , 37 , 38 ]; squat jump (SJ) after TRE (SJ-TRE) [ 1 , 14 ]; SJ after COM (SJ-COM) [ 6 , 19 , 22 ]; SJ [ 1 , 6 , 14 , 19 , 22 ]; 40-m sprint performance after TRE (40m-TRE) [ 1 , 2 ]; 40-m sprint performance after COM (40m-COM) [ 6 ]; 40-m sprint performance (40-m) [ 1 , 2 , 6 ]; 10-m sprint performance after TRE (10m-TRE) [ 2 , 20 , 37 ]; 10-m sprint performance after COM (10m-COM) [ 22 , 38 ]; 10-m sprint performance (10m) [ 2 , 20 - 22 , 37 , 38 ]; change of direction ability (COD) after TRE (COD-TRE) [ 2 ]; COD after COM (COD-COM) [ 38 ]; COD [ 2 , 38 ]; FP after TRE (FP-TRE) [ 1 , 2 , 6 , 14 , 20 , 37 ]; FP after COM (FP-COM) [ 6 , 19 , 22 , 23 , 38 ]; and FP [ 1 , 2 , 6 , 14 , 19 - 23 , 37 , 38 ].

Sprint ability

With regard to adaptations in sprint qualities (e.g., acceleration and maximal speed, Table  1 and Additional file 1 : Figure S1), improvements in different sprint distances (5- to 40-m distances) [ 1 , 2 , 6 , 10 - 12 , 14 , 15 , 19 , 21 , 22 , 48 , 51 ] have been reported in different levels of players. On average, highly trained players [ 1 , 2 , 6 , 22 , 37 , 38 ] need to increase their 1RM half-squat by 23.5% to achieve an approximately 2% improvement in sprint performance at 10- and 40-m distances (Figure  2 ). Excluding the study of Helgerud et al. [ 37 ], which reported significantly larger increments in strength, studies have demonstrated that lower increments in 1RM (19%) are required to achieve a similar improvement in sprint performance (1.9%) after short-term training interventions (in average, an 18% increments in 1RM resulted in a 2% average improvements in 10-m sprint performance [ 2 , 22 , 38 ] and 17% average increments in 1RM resulted in 1.6% improvements in 40-m distance time [ 1 , 2 , 6 ]). Nevertheless, improvements in sprint performance have not been entirely confirmed [ 1 , 6 , 8 , 10 , 16 , 22 , 28 ]. Notwithstanding, factors associated with the training status of various players, players’ background, and/or the characteristics of the training modes adopted should be considered as the most likely factors. For example, the sole performance of one type of plyometric exercise [ 16 ] and of electrostimulation training [ 28 ], which has an apparent lower level of specificity, may explain, at least in part, the lack of transfer of training adaptations to dynamic and complex activities, where the coordination and force production of different body muscles, as is the case of sprint performance, are essential.

Jump ability

Our analysis suggests that strength/power training induces adaptations in the jump abilities of high-level players (Table  1 and Figure  1 and Additional file 1 : Figure S2). On average, 24.4% 1RM improvements during squats result in a CMJ increase of approximately 6.8% [ 2 , 22 , 37 ]. Lower performance improvements in four bounce test (4-BT; 3.8%) were found with similar increments in 1RM (24.5%) [ 6 ], and similar improvements in SJ (6.8%) occurred with an average 1RM increase of 21.8% [ 1 , 6 , 14 , 22 ]. Curiously, the plotted data of all studies assessing the improvement in jump abilities in high-level players revealed that, on average (Figure  2 , Additional file 1 : Figure S5), a 23.5% 1RM increase may result in a 6.2% improvement in jump ability tasks after 6 to 10 weeks of strength/power training [ 1 , 2 , 6 , 14 , 22 , 37 ]. The previous results suggest that, on average, higher increments in force are needed to improve CMJ to the same extent as SJ (figure  1 ). This result may reflect the fact that the current programs were not able to increase (at the same relative rate) performance ability in the positive and negative phases of the SSC component and may explain, at least in part, the smaller improvements in sprint performance.

Improvements in the squat jump (SJ) [ 1 , 10 , 12 , 14 , 19 , 22 ], four bounce test (4BT) [ 6 ], five jump test (5-JT) [ 14 ], countermovement jump test (CMJ) [ 2 , 8 , 10 , 12 , 16 , 21 , 22 ], CMJ with free arms [ 21 ], and eccentric utilization ratio (CMJ/SJ) [ 12 ] have been observed in different players. Nevertheless, contradictions regarding improvements in SJ after plyometric [ 21 ] and in CMJ after high-intensity strength protocols performed by well-trained players can be found in the literature [ 1 , 14 ]. Additionally, no significant increases in CMJ were observed after CT involving workouts with high [ 19 ] or low loads [ 15 ] or in drop jumps from a 40-cm height (DJ 40 ) [ 10 ] following TRE and TRE plus sprint training.

Change of direction speed (COD)

According to the literature, it is difficult to discern which force/power qualities (e.g., horizontal and lateral) and technical factors influence event- or sport-specific COD ability [ 52 ]. To date, limited research has been conducted on agility/COD adaptations, with even less known about high-level athletes. Despite the limitations initially described see Introduction our results suggest that, on average, an increase of 15% in 1RM results in a 1.3% improvement in COD abilities after 5 to 6 weeks of training (Table  1 ; Figure  1 and Additional file 1 : Figure S3) [ 2 , 38 ]. Bogdanis et al. [ 2 ] observed that applying TRE-targeting hypertrophic or neural adaptations was effective in increasing COD (Table  1 , Additional file 1 : Figure S3). Nevertheless, improvements in COD performance evaluated by the 505 agility test after different plyometric techniques [ 16 ] were not found after CT [ 19 ]. Additionally, in a study by Mujika et al. [ 15 ] where players performed CT, no improvements in COD, evaluated by the agility 15-m test, were observed. The spectrum of possible factors associated with this discrepancy in results is ample and includes the players’ background and initial training status, the different training periods during which the intervention was carried out, the structure of the training intervention, game exposure, and distinct force/power qualities and technical factors that influence event- or sport-specific COD. For example, the study of Maio Alves et al. [ 19 ] was implemented during pre-season, and the research of Thomas et al. [ 16 ] was carried out during in-season. Consequently, the accumulated effect of COD actions performed during training sessions and games may influence these results [ 46 , 53 ]. Although the players are from the same age groups, the differences in the competitive levels of the players from previous studies should not be ignored. Moreover, the lack of improvements in COD after in-season CT that are reported by Mujika et al. [ 15 ] may be related to the fact that only six sessions were performed in a 7-week period. As will be further analyzed (‘Training efficiency’), this fact, among others, may suggest that higher training volumes may be necessary to induce adaptations in COD.

Sport-specific skills

One of the most important indicators of a successful soccer kick is the speed of the ball. Studies involving amateur players observed that CT [ 11 ] and electrostimulation training [ 28 ] increase ball speed with [ 11 , 28 ] and without (Table  1 ) run up [ 28 ]. Nevertheless, these improvements were examined in lower standard players. Moreover, elite U-19 players performing plyometric training increased ball speed with the dominant and non-dominant leg [ 21 ]. Other studies involving elite players performing different modes of strength training (isokinetic strength training or functional training) did not report improvements in ball speed [ 4 , 5 ]. Nevertheless, in studies performed during the off-season period, training stimulus consists of the exercise mode of the experimental designs and no other types of soccer routines are undertaken. Thus, the results should be analyzed with caution as the scenarios for training transfer to occur during this period are constricted (off-season); the increases in certain strength parameters were not reflected in positive transference to consecutive gains in ball speed.

Comparing different training variables in strength/power interventions in soccer

The multi-factorial constructs of soccer performance (technical, tactical, and physical performance) and their associated components bring a higher complexity to the designing of the training process. In fact, professionals involved in the preparation of soccer teams have to reflect on several questions associated with the manipulation of the individual variables that affect each of these relevant constructs and how they can affect each other. With regard to physical performance, several potential questions arise: What are the most beneficial movement patterns and type of training? How many sessions do athletes need to improve and maintain the performance outcome? Does ground surface have an effect on adaptations? We will analyze these and other relevant questions in the following sections.

Force production and movement pattern specificity: traditional resistance exercises vs. combined programs

Our analysis suggests that the activity patterns of applied exercises may influence performance outcomes (Figures  2 and 3 and Additional file 1 : Figure S4 to S5). Therefore, we compared programs involving mainly traditional resistance exercises (TREs) with programs that combine different activity patterns during the training intervention (COM; programs including TRE and ballistic exercises, plyometrics, weight lifting, body weight exercises, and/or sprint training during training cycles). Despite the fact that some limitations can be ruled out from this type of analysis (e.g., differences in session and weekly training volumes and load, the density of different intrinsic activity patterns, and the 1RM percentage used during the loaded exercises), we believe that it will aid in challenging research designs in this field.

Effects on sprint performance

On average, despite TRE resulting in superior strength gains compared with COM, greater performance improvements in the 10-m sprint are observed after COM (TRE = in average, 26.8% increments in 1RM resulted in 1.93% average improvements in 10-m sprint [ 2 , 37 ]; COM = in average, 19.9% increments in 1RM resulted in 2.4% average improvements in 10-m sprint [ 2 , 22 , 38 ]; Figure  2 and Additional file 1 : Figure S5). However, our analysis suggests the opposite with regard to 40-m sprint performance (TRE = in average, 15.8% increments in 1RM resulted in 1.9% average improvements in 40-m time [ 1 , 2 ] COM = in average, 23% increments in 1RM resulted in 1.1% average improvements in 40-m sprint time [ 6 ]). Nevertheless, all pooled data suggest that despite the TRE result of greater increases in 1RM (26%) than COM (21%), this may not translate into superior improvements in the sprint performance of high-level players (1.9% TRE vs. 2.1% COM; Additional file 1 : Figure S4).

Effects on jump ability

By performing the same analysis for jump ability exercises (Figure  2 and Additional file 1 : Figure S5), we found that there is a tendency toward greater strength increases after TRE (in average, 26.8% increments in 1RM resulted in 6.8% average improvements in CMJ; in average, 22% increments in 1RM resulted in 6.7% average enhancement in SJ; in average, 25% increments in 1RM resulted in 6% average improvements in 4BT) that are not translated into superior performance gains compared with the results observed following COM (in average, 21% increments of 1RM resulted in 6.8% average improvements in CMJ; in average, 22% increments in 1RM resulted in 6.9% average enhancements in SJ; in average, 22% increments of 1RM resulted in 6.4% average improvements in 4BT). In fact, all pooled data show that greater improvements in jump ability may be obtained with lower strength increases after COM than TRE only (Additional file 1 : Figure S5; in average, 21.6% increments in 1RM resulted in 6.4% average improvements in jump ability and a 25% average increments in 1RM resulted in 6% average improvements in jump ability, respectively). This higher efficacy of transfer of strength gains to performance improvements after COM seems to be more evident in SSC jump ability (CMJ). Taking into consideration, among other factors, the described associations between physiological and mechanical characteristics (e.g., post-activation potentiation and peak torque) and CMJ and running-based actions in professional players [ 44 , 46 , 54 ], this fact may suggest that COM may represent a superior method for improving sport-specific actions compared with TRE alone. Additional studies on this topic are necessary.

Effects on COD ability

Given the scarcity of literature assessing the effect of COD training modes and the reported small to moderate associations between strength and power variables with COD performance and different characteristics (e.g., test duration, COD number, and primary application of force throughout the test) of the agility tests commonly used to evaluate COD [ 52 ], conclusions should be drawn with caution. In fact, within programs involving only TRE, as will be discussed later in this review (‘ Manipulation of loading schemes ’), it seems that manipulating different mechano-biological descriptors of strength/power stimuli may influence performance adaptations in COD actions [ 2 ]. Nevertheless, our analysis shows that, on average, lower strength increases after TRE [ 2 ] produce greater performance improvements in the agility t -test than after COM [ 38 ] (in average, 14.2% increments in 1RM resulted in 1.7% average improvements in t -test and a 19.9% average increment in 1RM resulted in 1% average improvement in t -test, respectively; Figure  2 ).

Two studies are particularly relevant with regard to this topic: TRE vs. TRE plus plyometrics [ 6 ] and TRE vs. TRE plus sprint training [ 10 ]. In the study of Ronnestad et al. [ 6 ], although no significant differences between groups were observed, the group of players who utilized combined approaches broadly improved their performance. Additionally, Kotzamanidis et al. [ 10 ] observed that the jump and sprint performance of low-level players only improved in the combined program approach. Thus, it seems that combining heavy and light load training schemes may be an effective method for improving muscular function and may be particularly useful when force application is required in a wide range of functional tasks [ 27 ].

Training efficiency

To estimate the improvement in the different motor tasks and in overall functional performance, as well as the efficiency (efficiency = percentage of improvement/number of training sessions) of strength/power interventions and the effects of the different types of programs (TRE vs. COM) on specific motor tasks and functional performance, we performed an analysis involving all studies in highly trained players where performance outcomes were reported despite no references to changes in force production (Figure  3 ). Despite the limitations already highlighted, our analysis suggests that even though TRE slightly increases overall functional performance, the efficiency (gains by session) is lower than in COM modes. These uncertainties make this research topic particularly crucial. In summary, considering the high demands of high-level competition, the increase in different motor tasks (1.3% to 7.2%) and overall functional performance (4%) observed in highly trained players following strength/power training programs makes strength/power programs an essential training component. In general, it seems that strength/power training induces greater improvements in jump abilities than in running-based activities. Moreover, combining resistance- and speed-training or plyometric- and soccer-specific strength programs in the same session seems to be more effective than the resistance-training program alone [ 6 , 10 , 48 ].

Manipulation of loading schemes

Bogdanis et al. [ 2 , 3 ] analyzed the effects of high-repetition/moderate-load (hypertrophy) and low-repetition/high-load (neural adaptations) programs on anthropometric, neuromuscular, and endurance performance. These last studies [ 2 , 3 ] and others [ 4 , 5 , 23 ] suggest that the manipulation of different mechano-biological descriptors of strength/power stimuli (e.g., load magnitude, number of repetitions) is associated with different physiological and performance adaptations in highly trained soccer players. The hypertrophic mode was associated with increases in lower limb muscle mass, while the neural mode was more effective in improving 1RM/LLV, sprint, and COD performance [ 2 ]. In another study, Bogdanis et al. [ 3 ] found that even though both groups (hypertrophic group vs. neural group) improved the total work performed during a repeated cycle ergometer sprint test (RST; 10 × 6-s sprint with 24-s passive recovery), the neural mode group had a significantly greater improvement in work capacity during the second half (sprint 6 to 10; 8.9% ± 2.6%) compared with the first half of RST (sprint 1 to 5; 3.2% ± 1.7%). These results suggest that the neural mode confers a higher fatigue resistance during RST [ 3 ]. In addition, the mean power output expressed per lean leg volume (MPO/LLV) was better maintained during the last six sprint post-training only in the neural group, and there was no change in MPO/LLV in the hypertrophic group in the RST [ 3 ]. These results suggest, at least in part, a better efficacy of neural-based programs in high-level players [ 2 , 3 ] that could be linked to several adaptive mechanisms that are not associated with increases in muscle volume. However, the most likely adaptations are at the neuro-physiological level, i.e., changes in the pattern of motor unit recruitment and increases in rate coding [ 2 , 32 ].

Other researchers observed that physiological and performance outcomes can be independent of the kinetics of the power loading scheme used (from the high-force/low-velocity end to the low-force/high-velocity end and vice versa) because the loading scheme components spanned the optimal power training spectrum [ 22 ].

Contraction modes

The analysis of the impact of high- vs. low-intensity isokinetic strength vs. functional strength showed that professional players who performed a high-load, low angular velocity program had a higher improvement in maximal isometric and isokinetic strength and in PP at different knee angles and velocities [ 4 , 5 ]. Although the increases in dynamic muscle strength were generally associated with the specific velocities used in the training programs, the high-load/low-velocity group also exhibited improvements in muscle force and power at high knee extension velocities [ 4 , 5 ]. Although several explanations can be offered to clarify the greater adaptations associated with a wide range of velocities observed after the high-load/low-velocity strength training program, the most likely explanation is the occurrence of changes in neural and morphological factors associated with this type of training (e.g., increases in RFD, muscle mass, and/or fiber pennation angle).

Training frequency

As previously mentioned, high-level soccer players are usually involved in weekly matches of national leagues and are often involved in international commitments, thus limiting the time available for fitness training. Maio Alves et al. [ 19 ] found that different weekly volumes (two vs. one session per week) of complex training performed by high-level junior players resulted in similar improvements in sprint, jump, and COD ability. Ronnestad et al. [ 1 ] observed that one high-intensity strength training session per week during the first 12 weeks of the in-season period represented a sufficient training stimulus for maintaining the pre-season (two sessions per week for 10 weeks) gains in strength, jump, and sprint performance of professional players. However, a lower weekly in-season volume (one session every two weeks) only prevented detraining in jump performance [ 1 ]. Accordingly, a recent study [ 48 ] involving a larger sample of players showed that professional teams subject to distinct weekly strength training stress (all performed one resistance strength session a week) exhibit higher neuromuscular performance in the middle of the season than at the start of the season. Nevertheless, only the team that performed a higher number of sessions targeting the neuromuscular system showed improved neuromuscular performance during the second phase of the season. Despite the distinct individual variables that constituted the weekly resistance training session performed by the teams (e.g., percentage of 1RM, number of repetitions and exercises), differences in strength/power training stress were mainly due to the higher employed volume of both soccer-specific strength and sprint sessions [ 48 ]. This result again established the important role of the specificity of the training stimulus. Given the important role of circulating levels of androgens in strength and power performance, it is relevant to mention that only the high neuromuscular training scheme positively affected the circulation and activation (increase in 3a Diol G) of the androgen pool (total testosterone) [ 48 ].

However, Mujika et al. [ 15 ] observed that a low volume of combined forms of strength/power training is more effective in improving sprint performance (15-m sprint time) than the sole performance of lower volumes of sprint training in elite U-19 players.

Manipulation of biomechanical components of plyometric-based exercises

Performance outcomes may also be influenced by the biomechanical nature of the exercises employed in a single or combined program. Los Arcos et al. [ 23 ] observed that weight training plus plyometric and functional exercises involving vertically and horizontally oriented movements were more effective in enhancing the CMJ performance of highly trained players than exercises involving purely vertically oriented movements. Nevertheless, both groups improved their PP and showed small, although non-significant, improvements in 5- and 15-m sprint performance [ 23 ]. In contrast, Thomas et al. [ 16 ] examined that both plyometric training involving drop jumps or CMJs were effective in improving the jump (CMJ) and COD ability (505 agility test) of semi-professional players, regardless of the lack of change in short sprint distances. It is important to highlight that although no between-group differences were reported, the improvements in COD ability were twofold greater in the CMJ group. Nevertheless, given the age group of the players (U-18), it is important to be cautious in extrapolating these findings to professional adult players.

Training surface

There is also evidence that the ground surface used during plyometrics (sand vs. grass) may influence adaptations [ 12 ]. Impellizzeri et al. [ 12 ] observed that performing plyometrics on grass produced greater effects in CMJ and in the eccentric utilization ratio CMJ/SJ than when performed on sand. However, a trend toward higher adaptations was observed in SJ when the training program was performed on sand (Table  1 ). Additionally, sand was found to induce lower levels of muscle soreness compared with grass [ 12 ]. The fatigue development and recovery kinetics during and after a game have been well characterized in recent years. A reduction in the players’ ability to produce force toward the end of the match and in the match recovery period, an increase in some indirect markers of muscle damage, and longer periods of post-match muscle soreness have all been described [ 55 - 68 ]. In light of these findings, it may be expected that sandy surfaces may be a good alternative for the execution of plyometric programs during periods of high-volume, high-intensity, or high-frequency training (e.g., pre-season) and when athletes are recovering from injury and trying to regain physical capacity. In fact, in addition to improving neuromuscular capabilities, sand has been shown to produce lower levels of muscle soreness compared with grass [ 12 ]. Accordingly, compared with natural grass or artificial turf, the performance of dynamic powerful actions on sand, despite the known higher energy expenditures and metabolic power values, results in smaller impact shocks and limited stretching of the involved muscles [ 69 ].

Interference between concurrent strength and endurance training

Concurrent training involves the incorporation of both resistance and endurance exercises in a designed, periodized training regime [ 70 ]. The current dogma is that muscle adaptations to RE are blunted when combined with endurance [ 71 ], resulting in lower strength and power gains than those achieved by resistance exercise alone. When the modes of strength and endurance training focus on the same location of adaptation (e.g., peripheral adaptations), the muscle is required to adapt in distinctly different physiological ways [ 72 ]. However, when the modes of strength/power and endurance training are at opposite ends of the biomechanical and neuro-coordinative spectrum, the anatomical and performance adaptations may be reduced, and the accuracy of the intended movement, fluidity, and elegance that characterize excellence may be compromised. In fact, it is the entire spectrum of characteristics (e.g., metabolic and neuro-coordinative) of the upstream stimulus (resistance vs. endurance exercise; RE vs. E) that determines the downstream events necessary for training adaptations to occur. The range of factors that may be associated with the interference phenomenon or the incapability of achieving/maintaining higher levels of strength/power during concurrent strength and endurance training is ample and spans from excessive fatigue or increments in catabolic environments to differences in motor unit recruitment patterns, possible shifts in fiber type, and conflicts with the direction of adaptation pathways required by the muscle [ 34 , 70 , 72 , 73 ].

Molecular events

RE stimulates a cascade of events leading to the induction or inhibition of muscle atrophy [ 74 ]. From a molecular standpoint, these adaptations result from the downstream events promoted by the phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3-k/Akt/mTOR) pathway [ 74 , 75 ]. However, three kinases [p38 mitogen-activated protein kinase (MAPK), AMP-activated protein kinase (AMPK), and calmodulin-dependent protein kinase] are particularly relevant in the signaling pathways that mediate skeletal muscle adaptations to endurance-based training [ 75 , 76 ].

A few studies highlight the notion that both translation efficiency and protein synthesis may be compromised due to the incompatibility of the two different intracellular signaling networks, i.e., activation of AMPK during endurance exercise impairs muscle growth by inhibiting mTOR [ 74 , 75 ]. Nevertheless, other studies revealed that endurance performed after RE did not compromise the signaling pathways of RE (mTORC1-S6K1) [ 71 ] and may amplify the adaptive response of mitochondrial biogenesis [ 76 ]. Moreover, the translational capacity for protein synthesis can be reinforced rather than compromised when aerobic exercise precedes RE and molecular events are not compromised; mTOR and P70S6K shown greater phosphorylation in response to concurrent aerobic exercise compared with RE alone [ 77 ]. Furthermore, chronic concurrent aerobic exercise and RE may increase aerobic capacity and promote a greater increase in muscle size than RE alone [ 78 ]. Nevertheless, taking into account the complexity and the several molecular interactions that constitute the cascade of events associated with resistance and endurance exercise, conclusions should be drawn with caution. Additionally, studies have been performed primarily in healthy adults (physically active college students, moderately trained and recreationally active subjects) and not high-level athletes; although not universally confirmed, athletes with more extensive training backgrounds may have distinct phenotypes [ 79 - 81 ] and genotypes than normally active subjects [ 82 ]. Moreover, to the authors’ best knowledge, there is no research concerning how the distinct genotypes that can be found within a high-level group of athletes [ 82 - 84 ] may influence the individual responses to concurrent training.

Methodological considerations

Given the divergent physiological nature of strength and endurance training [ 34 ], the methodology applied, the volume and frequency of training, and the target goal all play key roles in increasing the degree of compatibility between these two key physical fitness determinants [ 34 , 72 ]. Slow long-duration sustained aerobic conditioning (SLDC) has been shown to be potentially detrimental to the overall performance of athletes involved in power sports and, for example, may have a negative impact on strength and power development [ 85 ]. Excessive training volumes may contribute to high metabolic stress, leading to high levels of substrate depletion and catabolic states (e.g., increased cortisol responses) [ 85 ]. Furthermore, SLDC may compromise recovery and regeneration, leading to a progression in the overtraining continuum [ 85 ]. Moreover, the high levels of oxidative stress (e.g., damaging proteins, lipids, and DNA) that are associated with high-volume training may increase reactive oxygen species (ROS) production to a level that overcomes the positive adaptations that may be triggered by ROS, i.e., there is a range in which ROS may represent an optimal redox state for greater performance, as with force production capacity [ 86 ]. Additionally, these previous factors associated with SLDC that limit force production may compromise skill acquisition by reducing the quality of execution (e.g., the technical ability of force application) and, thus, motor learning [ 85 ]. It is reasonable to consider that there may be certain mechanisms associated with the combination of training modalities that produce positive improvements and are additive in nature [ 87 ].

A low-volume, high-intensity approach, such as sprint interval training, may favor an anabolic environment (e.g., growth hormone, insulin-like growth factor-I, IGF binding protein-3, and testosterone) [ 88 - 92 ], maintain a muscle fiber phenotype associated with strength and power capabilities [ 93 ], and increase endurance and neuromuscular-related outcomes [ 94 - 96 ]. In fact, HIT and/or combined forms of HIT seem to promote adaptations in skeletal muscle and improvements in laboratory and field endurance-related parameters that are comparable to the effects of high-volume endurance training [ 94 , 97 - 101 ] and may improve muscle power-based actions [ 94 , 102 ]. Interestingly, the type of previously observed hormonal responses to HIT (e.g., sprint interval training) [ 88 - 92 ] constitutes one of the paradigms of resistance exercise biology, namely, an increase in cellular signaling pathways as well as satellite cell activation that contributes to an increase in translation and transcription processes associated with protein synthesis [ 74 ]. In this regard, supramaximal interval training is shown to be superior to high-intensity interval training for concurrent improvements in endurance, sprint, and repeated sprint performance in physically active individuals [ 103 ].

Does the magnitude of neuromuscular involvement during training sessions reduce possible incompatibilities associated with concurrent training? Are the biomechanical and neuro-coordinative demands (e.g., accelerations/decelerations impacting mechanical load and neuromuscular demands) of different training modes with similar physiological responses the same (e.g., 4 × 4-min interval running with 2-min rest vs. 4 × 4-min SSG with 2-min rest vs. 4 × 4-min intermittent situational drill with 2-min rest)? It is possible that, from a biomechanical and neuromuscular standpoint, more specific training methods to develop strength/power and endurance performance with higher biomechanical and neuromuscular demands may improve both adaptations and performance outcomes, as well as reduce the negative effect of this interference from a molecular point of view; human-based studies to date are far from agreement regarding the molecular interference after acute concurrent exercise [ 70 ]. In fact, strength/power and HIT are characterized by brief intermittent bouts of intense muscle contractions. Questions related to training transfer should be observed with greater attention when extrapolating the applicability of concurrent training to sport-specific settings. In fact, several factors can influence the transfer of strength training in endurance performance and the impact of endurance workloads on strength and power performances [ 104 ].

Soccer: a concurrent modality

A soccer player’s performance is intimately associated with the efficiency of different energy-related systems [ 105 - 107 ]. During the season, players perform intense programs with multiple goals of increasing strength, power, speed, speed endurance, agility, aerobic fitness, and game skills [ 108 ]. In fact, despite the predominant activity patterns of the game being aerobic in nature, the most deterministic factors of match outcome depend on anaerobic mechanisms [ 41 ]. It is common sense that the most intense match periods and worst-case match scenarios are associated with periods of high mechanical and metabolic stress. In fact, recently developed techniques of match analysis provide a body of evidence that supports the belief that neuromuscular demands of training and competition are higher than initially suspected (e.g., accelerations/decelerations) [ 42 , 43 , 109 ] and give further support to the viewpoint that strength/power-related qualities are crucial for high-level performance.

There is a belief that by stressing the neuromuscular system, adaptive mechanisms that are neurological, morphological, and biomechanical in nature will be triggered, thus increasing the player’s neuromuscular performance and providing him/her with a superior short- and long-term endurance capacity [ 17 , 110 - 113 ]. In this regard, associations between neuromuscular qualities (e.g., CMJ peak power) and intermittent endurance exercise [ 114 ] and repeated sprint ability performance [ 115 ] have also been observed. Moreover, there has been evidence supporting the association between team success and jump abilities (e.g., CMJ and SJ) [ 116 ]. Additionally, starter players demonstrate higher strength [ 108 ] and power performance capabilities than non-starters [ 117 ], and greater neuromuscular capabilities have been associated with game-related physical parameters and lower fatigue development during matches [ 118 ]. Moreover, Meister et al. [ 119 ] observed that after a match congestion period, players with a higher exposure time show better scores in certain neuromuscular parameters (CMJ, drop jump height, and drop jump contact) than players with a lower exposure time, although this result is not significant. Interestingly, recent reports revealed that neuromuscular-based actions, such as sprinting, have improved more in recent years than physiological endurance parameters. Professional players tested during the 2006 to 2012 seasons actually had a 3.2% lower VO 2 max than those tested during 2000 to 2006 [ 120 , 121 ]. Although with the obvious limitations and the universal consensus of the importance of aerobic fitness in soccer, these observations suggest that anaerobic power is ‘ stealing space ’ from aerobic power with regard to the constructs relevant in soccer performance. All of these previous facts highlight the role of neuromuscular exercise during soccer training and suggest that soccer routines should be performed concurrently as they are concurrent by nature. In fact, the physiological systems associated with endurance fitness development and maintenance are generally largely targeted in any match competition, friendly game, tactical exercise, circuit technical drills that often involve frequent displacements, and/or small side game exercises performed during a 90-min soccer competition/training session [ 106 , 122 , 123 ].

Physiological and performance adaptations

The summary of changes in physiological and functional parameters resulting from concurrent strength and endurance training are presented in Table  2 . Wong et al. [ 20 ] observed that 8 weeks of pre-season high-intensity strength training and SE resulted in a significant improvement in endurance markers, soccer-specific endurance (SSE), and soccer-specific neuromuscular (SSN) parameters. Helgerud et al. found that 8 weeks of other modes of HIT (aerobic high-intensity training) and high-intensity strength training during the preseason of non-elite [ 51 ] and elite [ 37 ] football players improved VO 2 max (8.6% and 8.9%), running economy (3.5% and 4.7%), and 1RM during half-squat strength exercise (52%), respectively. Moreover, the 10- and 20-m sprint performance (3.2% and 1.6%, respectively) and CMJ (5.2%) of elite players also improved [ 37 ]. These strength improvements occurred with minor increases in body mass (average 1%) and a substantial increase in relative strength [ 37 ]. More recently, McGawley et al. [ 38 ] found that a high-frequency program (three times a week) of concurrent high-intensity running-based training with strength/power-based training in the same session resulted in a positive training effect on all evaluated measures, ranging from flexibility, anthropometric, endurance, and neuromuscular-related parameters (Table  2 ). Moreover, these results suggested that the order of completion of the program, E + RE or RE + E, did not influence the performance adaptations. These results [ 38 ] and others [ 2 , 37 ] may support, at least in part, the better compatibility between high-intensity modes of strength and endurance training.

It is reasonable to assume that the players in the studies examining the effects of strength training programs (Table  1 ) had performed training with significantly high weekly endurance-based loads (e.g., pre-season). In this regard, Bogdanis et al. [ 3 ], when examining the strength training effects of the hypertrophic and neural modes in professional soccer players during pre-season, reported that the weekly cycle also involved a considerable amount of interval training and small-sided games, which have been described as effective methodologies targeting endurance fitness and SSE development (for a review, see [ 95 , 122 ]). The authors [ 3 ] observed that both aerobic fitness parameters (e.g., VO 2 max and MAS) and SSE, evaluated by the Yo-Yo intermittent endurance test and Hoff’s dribbling track test, respectively, were significantly improved in both groups (Table  1 ). Furthermore, other researchers [ 23 ] found that strength/power training performed in parallel with endurance training resulted in improvements in the individual anaerobic threshold and muscle/power parameters. Additionally, the performance of explosive-type strength training with routine soccer training did not interfere with the aerobic capacity of amateur young players [ 8 ], e.g., sub-maximal blood lactate values. These findings suggest that performing concurrent strength/power training and routine soccer training is advisable because, in addition to an increase in neuromuscular performance and the anabolic environment, this training did not interfere with the development of aerobic capacity [ 8 ]. Nevertheless, the question of whether this compatibility is related to the type of endurance and strength performed is highlighted in the distinct between-group results presented in the study of Bogdanis et al. [ 3 ], e.g., point ‘ Manipulation of loading schemes ’, where only the neural group significantly improved with respect to running economy and a trend toward a better performance in the YYIE2 in the neural group than in the hypertrophic group was reported.

In another study [ 13 ], semi-professional male soccer players performed both endurance and strength sessions as part of the annual periodization (four cycles of 12 weeks). This type of periodization was effective in improving both the endurance performance (Probst test) and SSN parameters, e.g., CMJ. These results suggested that no adaptation conflicts occur when one or two sessions of strength/power and endurance are simultaneously combined during a soccer training cycle (endurance block composed of two endurance training sessions and one strength training session and vice versa).

Additionally, Lopez-Segovia et al. [ 18 ] examined training adaptations in elite U-19 players during a 4-month period. The training program consisted of four sessions per week, targeting the improvement of player’s aerobic performance. Training was complemented with one or two specific strength training sessions per week performed at the start of the training session. This type of periodization improved loaded CMJ performance and the speed of movement in full squats, with loads ranging from 20 to 40 kg. Nevertheless, significant decrements in different sprint abilities were found. According to the researchers, the lack of improvement in the former sprint variables was attributed to the high volume of aerobic work performed. Nevertheless, an increase in MAS (3.2%) was observed after the intervention period [ 18 ].

Conclusions

Our analysis suggests that, independent of the methodology applied (Table  1 ) and the form of concurrent endurance and strength/power training (Table  2 ), pre-season training resulted in an improvement in physiological and soccer-specific and non-specific performance parameters. The large responsiveness to training may be associated with the fact that most of the studies were conducted during an early stage of pre-season, with off-season detraining negatively affecting several physical attributes, such as anthropometric characteristics (e.g., decreases in LBM and increases in BF) [ 124 - 126 ], endurance-related markers [ 53 , 101 , 126 , 127 ], soccer-specific endurance [ 101 , 128 ], and neuromuscular parameters [ 126 , 129 ]. With this in mind, the overall conclusion of the analyzed literature is that the addition of strength/power training programs to routine soccer training favors a more integral physical fitness development of the player. The associated improvements in physiological (e.g., 1RM/LLV, PP) and performance (e.g., jump, sprint, COD) parameters may, at least in part, increase a player’s ability to cope with training and competition demands. Our analysis suggests that high-intensity strength training (HIST) may be a more efficient method than moderate-intensity methods (hypertrophic). In addition, the compatibility between strength and endurance training may be greater when high-intensity or explosive strength training is combined with high-intensity endurance training to favor a more soccer-specific phenotype.

One of the most sensitive periods of training implementation is the in-season period. As the match is the most important part of the soccer-training schedule, technical staff often view the in-season periodization with particular prudence. They want to maintain or even increase the pre-season gains obtained throughout the short pre-season period (5 to 7 weeks). However, they face the constant dilemma of determining the proper dose/response that allows for the cycle of training-recovering/competing-recovering to be effective; a high volume of training and/or competition interspersed by insufficient recovery favors fatigue development [ 130 ], resulting in a transition from a functional to a non-functional overreaching state or, in more severe cases, an overtraining state [ 131 , 132 ]. Unfortunately, studies implemented during in-season are scarce [ 1 , 8 , 13 - 16 , 18 , 21 , 24 , 28 , 48 ]; seven were conducted with U-19 players, and only four were conducted with adult soccer players [ 1 , 13 , 28 , 48 ]. Our analysis suggests that two weekly sessions allow for highly trained players to obtain significant performance enhancements and that one session a week is sufficient to avoid in-season detraining. It may be possible that, in parallel with a higher volume of neuromuscular training (soccer-specific strength/power-based efforts), further in-season improvements could be observed. Moreover, manipulations of the training surface could constitute an important strategy (e.g., players returning from injury and the management of biochemical and perceptual disturbances).

We found that the results of high-force increments vs. low-performance enhancements and the respective efficiency of the programs (jump vs. running-based actions and non-SSC abilities (SJ) vs. SSC-based actions (e.g., CMJ)) suggest that current approaches may overlook some essential aspects required to achieve an increase in a player’s performance capacities. According to Komi [ 133 ], an effective SSC is obtained with ‘a well-timed pre-activation of the muscle(s) before the eccentric phase, a short and fast eccentric phase, and an immediate transition (short delay) between stretch (eccentric) and shortening (concentric phase).’ The observed increments in force production will most likely occur to a greater extent in the positive phase of the SSC. We suggest that to achieve greater improvements, weight training should be combined with more soccer-specific strength exercises (e.g., the player’s ability to use strength and power effectively and consistently [ 134 ], allowing for the application of force/power in a larger range of planes (horizontal) and specific angles). Therefore, a conditioning method such as Speed, Agility and Quickness (SAQ) may be useful, as it incorporates plyometric and soccer-specific strength exercises and can, therefore, constitute a good conditioning tool for this type of outcome (acting on the entire spectrum of the SSC and on the transition from eccentric to concentric movements; it should be kept in mind that plyometric training is a technique demonstrated to increase musculo-tendinous stiffness, which can optimize power output in explosive movements) [ 135 ]. The greater ecological validity of COM approaches make combined methods a preferred training strategy for strength training in soccer; targeting the intra- and inter-muscular aspects of athletic performance should occur in parallel and begin at the start of the preparation period. In fact, hypertrophy and general power exercises can enhance sports performance, but optimal transfer from football-specific activities also requires football-specific exercise programs [ 29 ] in which the biomechanical and neuro-coordinative patterns of sport-specific motor tasks are taxed.

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Additional file 1: figure s1..

The gains in strength and sprint performance of high-level players after 5 to 10 weeks. Squares represent the 10-m distance [ 2 , 22 , 37 , 38 ]; circles represent the 20-m distance [ 37 ]; rhombi represent the 40-m distance [ 1 , 2 , 6 ]; + symbols represent the average of all distances; triangles represent the average of the 10-m distance; and lines represent the average of the 40-m distance. Figure S2. The gains in strength and jump performance of high-level players after 6 to 10 weeks. Squares represent the squat jump performance (SJ) [ 1 , 6 , 14 , 22 ]; triangles represent the countermovement jump (CMJ) performance [ 2 , 22 , 37 ]; rhombi represent the four bounce test (4BT) performance [ 6 ]; lines represent the five jump test [ 14 ]; circles represent the average CMJ; x symbols represent the average SJ performance; and + symbols represent the average 4BT performance. Figure S3. The gains in strength and change of direction ability of high-level players after 5 to 6 weeks. Squares represent the t -test performance [ 2 , 38 ]; circles represent the Zig-Zag test performance [ 2 ]; and rhombi represent the Illinois agility test performance [ 2 ]. Red-filled triangles represent average of all tests. Figure S4. The gains in strength and overall sprint performance of high-level players following traditional resistance exercise programs (TRE; 6 to 10 weeks) and combined programs (COM; 5 to 7 weeks). Filled circles represent the TRE results; empty circles represent the COM results; red-filled circles represent the average TRE [ 1 , 2 , 37 ]; empty red circles represent the average COM [ 6 , 22 , 38 ]. Figure S5. The gains in strength and overall jump ability of high-level players following traditional resistance exercise programs (TRE; 6 to 10 weeks) and combined programs (COM; 6 to 7 weeks). Blue-filled and unfilled triangles represent the countermovement jump (CMJ) results after TRE and COM, respectively; red-filled and unfilled triangles represent the squat jump (SJ) results after TRE and COM, respectively; green-filled and unfilled triangles represent the four bounce test (4BT) results after TRE and COM, respectively; yellow-filled triangles represent the five jump test (5JT) results after TRE; blue-filled and unfilled circles represent the average CMJ results after TRE [ 2 , 37 ] and COM [ 22 ], respectively; red-filled and unfilled circles represent the average SJ results after TRE [ 1 , 14 ] and COM [ 6 , 22 ], respectively; black-filled and unfilled circles represent the average overall jump ability increases after TRE [ 1 , 2 , 6 , 14 , 37 ] and COM [ 6 , 22 ], respectively.

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Silva, J.R., Nassis, G.P. & Rebelo, A. Strength training in soccer with a specific focus on highly trained players. Sports Med - Open 1 , 17 (2015). https://doi.org/10.1186/s40798-015-0006-z

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Received : 23 July 2014

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DOI : https://doi.org/10.1186/s40798-015-0006-z

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Soccer - Free Essay Samples And Topic Ideas

Soccer, known as football outside of North America, is a globally cherished sport with a rich history and significant cultural impact. Essays on soccer might explore its origins, the evolution of soccer rules and organizations, and the sport’s influence on international relations and national identities. Additionally, discussions might extend to notable soccer events like the FIFA World Cup, iconic soccer players, and the societal and economic aspects of soccer fandom and commercialization. A vast selection of complimentary essay illustrations pertaining to Soccer you can find at PapersOwl Website. You can use our samples for inspiration to write your own essay, research paper, or just to explore a new topic for yourself.

Informative Essay about Soccer

Soccer is the most played sport in the world today. As Alex Morgan one said, "It doesn't matter if its soccer, football, or futbol, This game brings people together". Soccer has a lot of background for its history, including the rules and regulations, has proven health benefits for playing the sport, and shows a lot of characteristics on why it's a team sport. Soccer is a sport that welcomes anyone to play and brings positivity and unity out in one's […]

Topic Soccer Balls Bounce

A ball will bounce differently depending on the surface because some surfaces are harder than others like grass, turf, concrete they are all different. Some surfaces absorb more energy than others do. A hard surface, such as concrete, absorbs less energy compared with a soft surface, such as a grass floor. The more energy absorbed by the surface, the less that remains in the ball for it to bounce. When a ball is dropped gravity pulls the ball toward the […]

The CAC 40 – the Paris Stock Exchange

The CAC 40: stands for "Cotation Assistée en Continu", which may be translated to continuous aided mercantilism, and is that the hottest benchmark index for funds finance within the French securities market. This index provides a general plan regarding the direction of the "Euronext Paris", that is that the largest stock market in France once referred to as the Paris stock exchange. CAC 40 represents the capitalization-weighted measure of the foremost forty vital values among the very best one hundred […]

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The Rules and Process of Soccer

When you hear the word "soccer" do you right away freak out like most of Americans do, well I will explain why you shouldn't. Soccer is a beautiful sport that is played across five continents. Americans have a wrong understanding of playing soccer because they believe their kids will get hurt playing soccer, but they encourage their kids to play American Football when it is even more dangerous than soccer. There is a rules and process of playing soccer. Soccer […]

The Effects of Soccer on Health

The world is filled up and down with soccer. With no doubt it is one of the most popular sports in the entire world. That is because it can be played practically anywhere you can think of, backyards, parks, streets, and stadiums. During a soccer match players work their body to the maximum. They do a variety of physical activity like jumping, sprinting, shooting, and dribbling for a whole ninety minutes. Soccer has a great impact on not only physical […]

A Football Player Homare Sawa

Homare Sawa was a Japanese football player who led Japan to victory as captain in the 2011 Women's World Cup, she hopes the disparity between men's and women's teams in Japan will close. About 25,000 girls play football in Japan, but there are no professional leagues for women. Sawa herself played on the boy's team. The pay gap between the genders is stark and most of the members of Sawa's World Cup team have full-time jobs and could only train […]

Racism in Soccer

Racism has been a part of civilization for a very long time now and will be something that will always be around. It has even found its way into sports all around the world and the problem continues to grow. Sports of all kinds have this problem, from organizations such as the NBA and NFL, to the leagues in other countries. One of the biggest sports in the world, soccer, has one of the worst issues with racism. Since soccer […]

About Motion of Soccer Ball

When a soccer ball is kicked or in motion it is determined by newton's laws of motion. Newton's first law of motion states that an object will move in a straight line unless acted on by other external forces. Newton's second law of motion explains that the velocity, the speed of an object in a given direction, changes when it interacts with an external force. The forces that can stop and interfere with the motion of the soccer ball is […]

Why Soccer is the Sport most Famous

For many it is not a mystery that the sport of the Soccer (or fut-bol in other countries) is the physical activity that has more followers around the world, which raises passions, emotions, joys, rivalries, etc; all that is what the fut-bol brings. But why is this? In this essay from a very particular point of view I, because soccer is the most watched sport in the world, all aspects that for me make football the king sport in the […]

History of Soccer

Soccer is among the most loved and celebrated sports worldwide. People will spent a lot of money to buy soccer jerseys with the same number worn by their favorite soccer stars Soccer has evolved throughout time, from being a game without rules to a game that has a visual assistant referee.The game get its name from that people uses their foot to kick the ball. It was started more than two thousand years ago as some other games. The only […]

The Evolution of Soccer

The Evolution of Soccer around the world and how it gained its popularity throughout countries Soccer is a favorite sport played all over the world. Although it has just been prevalent in North America for as far back as 30 years, soccer has been quite a while most loved most wherever else. It is the national game of many European and Latin-American nations, and numerous different countries. The game goes back to the Egyptians, who played amusements including the kicking […]

The Psychology Side of Soccer

The psychology side of soccer has helped me through learning how to deal with situations, ways on how to solve problems together with my teammates, allowing myself to adapt and meet new people from different countries. Many athletes are confused about the role of soccer psychology and how mental training can improve your performance. The goal of the psychology side of soccer is to help other athletes and teams perform their best by improving mental skills that help us excel […]

Soccer is a Highly Contested Cut-throat Game

Many people hold a notion that soccer is a highly contested, cut-throat game. However, soccer has a great role in hosting competitions and being a mediator betweens nations at an international level (Kunczik, 2016). Football touches lives both on a regional and global scale. At times it inspires revolutions, but it also has the capability to create an everlasting peace and lift the participating nations. However, a blend of politics and soccer has significant and far-reaching implications on the international […]

Soccer and Goalies

Description In the game of soccer, the objective is to score as many goals as possible (more than the opposing team) by the end of the ninety-minutes in order to win the game. If there is a tie at the end of the ninety minutes, depending on the situation, the referee will either declare it a tie, add additional time (also known as stoppage time), or the game will go into penalty kicks. Components of the Field In terms of […]

Soccer as the most Famous Sport

Soccer is known to be the most famous sport in the world. In soccer there is 2 teams with 11 players. Soccer is played on a large grass field but you can also play indoor. The point of the game it to get the ball into the opposite team's goal. You can player soccer by only using your feet to dribble it and the goalie can only touch the ball. Soccer is played by all ages around the world for […]

Soccer in the US

Football is the king of sports, is the most popular and playable sport in the world. In the USA they play the same exact sport but with different name soccer, which is an acronym of the word Football Association. Although soccer was not very popular in 50's or 60's, in the 70's and 80's it gained some publicity from the creation of New York Cosmos and the arrival of some of the greatest soccer players of that time. In the […]

Major League Soccer

The United States has realized several impacts following the creation of the Major League Soccer (MLS) which was initially formed from an ownership model which was specifically a single-entity one. The ownership model acted as an antidote to the failing investments which were made in the history of professional soccer in North America. The creation of the MLS and its sustainability in the American sports culture is outstanding considering the competition it has been subjected to by other domestic sports […]

Playing Soccer

Panting as she dribbles the ball down the field, making sure the ball goes off the right part of her foot, and trying to figure out who is the best player to pass to make up the amazing sport of soccer. Soccer is a rewarding sport to play because it teaches coordination, increases cardiovascular health, and has cognitive benefits. Soccer uses the whole body, therefore, requires coordination from your eyes to the rest of your body. Cardiovascular health is increased […]

Twelve Thai Soccer Players

On June 23, twelve Thailand soccer players and their coach, Ekapol Chanthawong, went missing. The name of the soccer team was Wild Boars. The soccer team was from the northern part of Thailand. The soccer team got trapped in a cave that was half a mile below the surface and was prone to flooding. The twelve Thai soccer players were roughly around the ages of 14 and 16, with their coach being 25 years old. The situation in June 2018 […]

Soccer and Stock Markets

According to earlier studies, emotions are omnipresent. They have an essential effect on the individuals' behaviour and thus the decision making. Take for example the impact of anger on judgment and it's influence on cognition, and how the incidental anger in one situation can result in misattributed blame in another (Tedeschi and Quigley 1996). Again,According to Aristotle, "Anyone can become angry,that is easy. But to be angry with the right person, to the right degree, at the right time, for […]

Sizes of a Soccer Ball

Soccer Balls come in many different sizes. They also have different weight to all of them, but all soccer balls have one thing in common they all have air in them which allows the ball to be tough or soft. In the beginning it will fly exactly in the direction of the kick, as it slows down due to the friction of the air. The spinning motion will cause the air on one side to move faster than the other […]

Stock Market and Soccer

The interest in the role of sentiment, mood, feelings and emotions in finance and business stems from the work of (Kahneman and Tversky, 1979). Results in this area was built on evidence from experimental psychology and economics and studies to explore how investors are affected in light of information's evaluation, risk, gains. The applying the direct and the indirect measurement on the sentiment and feelings of the investors as an attempt to discover its role on the performance of stock […]

Soccer and Stock Market

When reviewing previous papers, we can touch the remarkable role of emotions that would suggest alternative courses of action affecting behavior of individuals and thus the decision-making, As a simple example the incidental anger happening in one situation will elicits automatically a motive to blame individuals in other situations even though the targets of such anger have nothing to do with the source of the anger (Tedeschi & Quigley 1996), and that typically occurs without awareness, as if Emotions play […]

The Global Reach of Soccer Viewership

Soccer, known as football outside of North America, is undeniably the world's most popular sport. Its appeal transcends geographical, cultural, and socio-economic boundaries, captivating millions across the globe. The question of how many people watch soccer is both intriguing and complex, reflecting the sport's vast reach and its profound impact on societies worldwide. The sheer scale of soccer's viewership is staggering. According to FIFA, the sport's global governing body, over half of the world's population watched some part of the […]

Kicking it at Angelo’s Soccer Corner: more than Just a Store

Nestled within the core of the local soccer community lies Angelo's Soccer Corner, an oasis for devotees of the beautiful game. Far beyond a mere emporium, Angelo's has evolved into a vibrant nucleus for players, enthusiasts, and families, embodying the fervor and ardor that soccer evokes universally. This discourse delves into the distinctive role Angelo's Soccer Corner assumes in nurturing the local soccer milieu, fostering a sense of camaraderie, and contributing to the sport's proliferation at its grassroots. From its […]

Neymar’s First Name: the Story Behind the Soccer Sensation

In the world of soccer, few names resonate as strongly as Neymar. He's a figure known globally, not just for his skill on the pitch but for his larger-than-life persona. However, many fans, even the most ardent ones, might pause when asked, "What is Neymar's first name?" It's a question that invites us to explore not just a name, but the story and the culture behind one of soccer's most iconic figures. Neymar's full name is Neymar da Silva Santos […]

Unraveling the Origins of Soccer: a Journey through History

Soccer, or football as it's known outside of North America, is more than just a game; it's a global phenomenon that captivates billions. But who can we thank for this ball-kicking frenzy? The answer isn't as straightforward as you might think. Tracing soccer's roots is like walking through a maze of history, culture, and evolution. Long before soccer became the sport we know today, ancient civilizations were already playing ball games. The Chinese game of 'cuju,' recorded during the Han […]

Kwame Appiah: a Philosopher’s Perspective on Soccer

When Kwame Appiah, a renowned philosopher, turns his attention to soccer, you know you're in for an insightful ride. It's not every day that a thinker of his caliber delves into the world of sports. Appiah, known for his work on cosmopolitanism and identity, brings a unique perspective to the game of soccer, a sport celebrated and loved by millions worldwide. His approach to soccer is not just about the game itself but also about what it represents in our […]

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Essay About Soccer U.S. Soccer, the governing body for the sport in America, pays the members of the men's and women's teams for international matches. The men's team earns higher wages in comparison to the women’s team. A perfect example of this case is when the women only earned $2 million the summer of 2015 for winning the World Cup while the men's team earned $9 million without advancing past the Round of 16 in the 2014 World Cup. Five of the women's players filed a federal complaint accusing the U.S. of discrimination of wages even with their revenue increase. The soccer world and others should care and be mindful of this because of the equal employment opportunities and the equal protection clause for both men and women. In the end, we are in search of equal opportunities for our future and for the little boys and girls who have dreams to succeed as well. U.S. Soccer may think they can get away with this because the majority of sports that pay their women players are always lower than the men's sports. There was a time where the women’s national soccer team was making more than the men's team. There will always be a controversy within the wage difference between men and women. At some point, there should be a solution and an agreement on how much everyone makes. “I won’t settle for less than equal pay”, Former National Women’s Soccer Goalie Hope Solo, said with strong words fighting for equal pay as a professional women’s soccer player. This has been a constant debate throughout time about equal pay between men and women in the professional sports industry. Many women argue that they do just as much as men do in their field. From training camps to similar training periods and even equivalent working conditions. Throughout this paper, we will touch on some major factors as to why women should get paid the same amount as men do. To begin, throughout the duration of a soccer player’s career they go through many different levels before becoming a professional. They typically come up in the youth academy programs and then they move into high school level, from here they normally select a college to attend and they can opt-out after their first year to enter the draft. From here, this is where we run into major issues with differentiating between both genders in this sport. Professional soccer players on the men's side of things tend to make on average about $60,000, and they can see upwards of up to $300,000 in wages per year. As for women soccer players they make on average about $30,000 and can get paid upwards of $80,000 a year. Almost a $220,000 drop off and the question is why? Women prepare just as hard, and the demand of the head coaches for Women soccer player doesn’t differ because of their gender. This is still an ongoing debate as to why women aren’t being paid the same as men. To continue on the requirements of both men and women, they both have a similar playing field as men do. They train on the same size field as men, play with the same size ball, and even run training sessions that are just as long. So why is there such a gap in the pay for women soccer compared to men? Well, many argue that the fanbase of men’s soccer game brings compared to that of a women’s soccer game explains why there is a pay gap. For example, the last world cup for both men and women respectively both had a large number of viewers. During the Men’s world cup final in 2014 over 3.2 billion viewers across the globe. As for the women they had over 40 million across the world. With the men’s soccer final seeing such a difference in views across the world, this is the argument as to why there is a pay gap between both genders in the sport. On the other hand, U.S. women's soccer is much more popular than that on the men's side. In fact, the last Women’s World Cup finals had over 23 million views across the U.S. This was the most-watched soccer game for both men’s and women’s in history. So, holding the argument that women are just as equal to men in soccer especially in the U.S. is a valid point. So why are we so off on paying women? If they are playing under equivalent working conditions, what’s stopping women from having more money. This argument has now been ongoing for decades, but when will we start to see changes in the current professional field of soccer? To continue, just to compare some numbers. In research constructed by a business insider, they compared the annual pay based on 20 games between both men and women. If a men’s soccer team lost all 20 games, they are expected to bring in at least $100,000 in pay, whereas if the women’s team lost all 20 games, they bring in approximately $72,000. In addition, if each team were to win 10 games both on the Men's and the Women's side, the men’s team would get a payout of up to $181, 660, whereas the Women would get up to $85,500. This is almost a $100,000 in pay difference for achieving the exact same number of wins. Lastly, if each team were to hypothetically win all 20 games, the men’s team payout would be up to $263,320, whereas, the women would get $99,000 if they accomplished the same. This is almost a $180,000 difference in pay according to a business insider. This means that even if the women’s team went 20-0 and the men’s team went 10-10, the men would still get paid almost twice as much as that of the women’s team. This is why many professional women soccer athletes are infuriated with the current pay and inequality in between the two and demand equal compensation. Another huge compensation gap is the world cup bonuses. This is one of the most surprising numbers between the two and the difference is unimaginable. For instance, if a men's team finished third in the world cup, they would receive $52.083 in bonuses, whereas if a women's team finished in the same place, they would receive less than half of that at about $20,000. An even more remarkable number is if the men’s team finished in second, they would be compensated for $260,417 according to business insider, whereas the women would receive just about $32,500 in bonuses. This is almost $230,000 more than that of women. The only difference between the two would be their gender. Lastly, a first-place finisher in the world cup would get over $390, 625 on the men’s side, whereas the women’s team would only get $75,000. Compared to the men, this is almost an offensive offer by FIFA when it comes to compensating their players for their work. This is not only an embarrassment for FIFA as an organization but for the sport and what they represent as a whole for the women’s teams. 

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• v.1; 2015 Dec

Strength training in soccer with a specific focus on highly trained players

João r silva.

1 National Sports Medicine Programme Excellence in Football Project, Aspetar-Qatar Orthopaedic and Sports Medicine Hospital, P.O BOX 29222, Doha, Qatar

2 Center of Research, Education, Innovation and Intervention in Sport (CIFI2D), Porto, Portugal

George P Nassis

Antonio rebelo.

Data concerning the physical demands of soccer (e.g., activity pattern) suggest that a high level of performance requires well-developed neuromuscular function (NF). Proficient NF may be relevant to maintain and/or increase players’ short- (intense periods of soccer-specific activity; accelerations, decelerations, and sprinting) and long-term performance during a match and throughout the season.

This review examines the extent to which distinct modes of strength training improve soccer players’ performance, as well as the effects of concurrent strength and endurance training on the physical capacity of players.

Data sources

A selection of studies was performed in two screening phases. The first phase consisted of identifying articles through a systematic search using relevant databases, including the US National Library of Medicine (PubMed), MEDLINE, and SportDiscus. Several permutations of keywords were utilized (e.g., soccer; strength; power; muscle function), along with the additional scanning of the reference lists of relevant manuscripts. Given the wide range of this review, additional researchers were included. The second phase involved applying six selection criteria to the articles.

Results and conclusions

After the two selection phases, 24 manuscripts involving a total sample of 523 soccer players were considered. Our analysis suggests that professional players need to significantly increase their strength to obtain slight improvements in certain running-based actions (sprint and change of direction speed). Strength training induces greater performance improvements in jump actions than in running-based activities, and these achievements varied according to the motor task [e.g., greater improvements in acceleration (10 m) than in maximal speed (40 m) running movements and in non-squat jump (SJ) than in SSC-based actions (countermovement jump)]. With regard to the strength/power training methods used by soccer players, high-intensity resistance training seems to be more efficient than moderate-intensity resistance training (hypertrophic). From a training frequency perspective, two weekly sessions of strength training are sufficient to increase a player’s force production and muscle power-based actions during pre-season, with one weekly session being adequate to avoid in-season detraining. Nevertheless, to further improve performance during the competitive period, training should incorporate a higher volume of soccer-specific power-based actions that target the neuromuscular system. Combined strength/power training programs involving different movement patterns and an increased focus on soccer-specific power-based actions are preferred over traditional resistance exercises, not only due to their superior efficiency but also due to their ecological value. Strength/power training programs should incorporate a significant number of exercises targeting the efficiency of stretch-shortening-cycle activities and soccer-specific strength-based actions. Manipulation of training surfaces could constitute an important training strategy (e.g., when players are returning from an injury). In addition, given the conditional concurrent nature of the sport, concurrent high-intensity strength and high-intensity endurance training modes (HIT) may enhance a player’s overall performance capacity.

Our analysis suggests that neuromuscular training improves both physiological and physical measures associated with the high-level performance of soccer players.

Electronic supplementary material

The online version of this article (doi:10.1186/s40798-015-0006-z) contains supplementary material, which is available to authorized users.

• Neuromuscular training improves both physiological and physical measures associated with high-level performance.
• It seems that strength and power training programs should target all the force-velocity potential/spectrum of the neuromuscular system.
• Due to the conditioned concurrent nature of the sport, combined strength and combined high-intensity training approaches may constitute a good training approach within a football periodized process.

Introduction

The central goal of strength/power training in a highly competitive sport is to improve the players’ specific and relevant athletic activities inherent in their sport. To achieve this outcome, different strength/power training modes with i) distinct movement patterns (traditional resistance exercises, ballistic exercises, plyometrics, weight lifting, and/or sport-specific strength-based actions), ii) different combinations of the temporal organization of strength/power training loads (e.g., microcycle and training session variations), iii) distinct loads, iv) a wide range of movement velocities, v) specific biomechanical characteristics, and vi) different training surfaces have been adopted with the final end point of achieving an improvement in players’ performance in relevant motor tasks (e.g., jumping, sprinting, and changing direction) [ 1 - 24 ].

Certain training methods combine different exercise modes (e.g., weight training, plyometric training, and sport-specific force-based actions) and allow for optimal power development and transfer to athletic activities due to both the neural and morphological adaptations typically associated with advanced training [ 25 ]. In fact, the intrinsic characteristics of soccer activity patterns (a varied range of motor actions that involve both breaking and propulsive forces as well as distinct contraction modes and velocities that require the all force-velocity potential of the neuromuscular system) that highlight the importance of the principle of specificity in strength and muscle power training cannot be understated [ 26 , 27 ].

A combination of different methods, including high-intensity strength training involving traditional resistance exercises (TRE; squats) and plyometrics [ 6 ], TRE and sprint training [ 10 ], and complex strength training (CT) [ 11 , 15 , 19 ], have all recently received considerable attention. Although some similarities exist between the previous modes of strength and power training, there are important differences. In this review, we found that complex training refers to training protocols that are comprised of the alternation of biomechanically comparable strength exercises and sport-specific drills in the same workout (e.g., six repetitions of calf extension exercise at 90% of one repetition maximum (1RM) + 5 s of rest + eight vertical jumps + 5 s of rest + three high ball headers) [ 25 ].

By focusing on more effective periodization techniques, researchers have investigated the effectiveness of different loading schemes throughout the power training phase (from high-force/low-velocity end to low-force/high-velocity end or vice versa) [ 22 ]. The training-induced effects of exercises with distinct biomechanical and technical characteristics during the plyometric-based component (e.g., purely vertically or a combination of vertically and horizontally oriented exercises [ 12 , 16 , 21 , 23 ], as well as the effects of plyometric training on different ground surfaces (grass vs. sand) [ 12 ], have both garnered significant attention. Furthermore, the adaptiveness of the functional and muscle structure of professional players (e.g., myosin heavy chain composition) to high-intensity strength training in the isokinetic contraction mode has also been investigated. However, the implementation of this analysis during the off-season resulted in lower ecological validity of these findings [ 7 ]. With regard to the search for complementary procedures and/or less stressful interventions, the effects of other methodologies (e.g., effects of electrostimulation training on semi-professional players) on physical fitness have also been investigated [ 28 ].

In general, most studies have examined the training-induced performance effects of two [ 1 , 6 , 8 , 10 , 14 , 16 , 19 ] to three [ 2 , 3 , 11 , 12 , 21 , 28 ] sessions per week. Given the multi-component requisites of soccer players’ training (e.g., endurance, speed endurance, strength, power, and agility) that coincide with the increased amount of training time, some researchers examined the short-term effect of a lower weekly volume program (one session) [ 1 , 15 , 19 ] and the effect of training-induced adaptations of different weekly training frequencies (e.g., one vs. two sessions and one session per week vs. one session every second week) on both physiological and performance parameters during pre-season [ 19 ] and throughout the in-season in well-trained soccer players [ 1 ].

Nevertheless, despite an increase in the body of evidence regarding the applicability of strength/power training programs to routine soccer training, the short-term duration of interventions (e.g., 4 to 12 weeks) [ 2 , 3 , 6 , 8 , 10 - 12 , 14 - 16 , 19 , 21 - 23 , 28 ], the wide variety of training methods, the distinct season time lines used throughout the pre-season [ 2 , 3 , 6 , 12 , 19 ] and in-season [ 8 , 14 - 16 , 21 , 24 , 28 ] periods, the different weekly training loads, and the absence of control groups make the drawing of precise conclusions very difficult. With regard to the latter aspect, it is accepted that due to the importance of winning matches, technical staff of semi-professional and professional teams are unable to implement different training scenarios based on research interests. Nevertheless, in this review, our aim is to contribute to the understanding of the present state of the art of strength/power training and concurrent training in soccer to motivate future studies.

Search strategy: databases and inclusion criteria

The selection of studies was performed in two consecutive screening phases. The first phase consisted of identifying articles through a systematic search using the US National Library of Medicine (PubMed), MEDLINE, and SportDiscus databases. Literature searches were performed from January 2013 until June 2014, and this review comprises papers from 1985 to 2014 (N 1985-2009  = 76 papers, N 2010  = 7 papers, N 2011  = 17 papers, N 2012  = 4 papers, N 2013  = 21 papers, N 2014  = 11 papers). The following keywords were used in combination: ‘elite soccer’, ‘professional soccer’, ‘first division soccer,’ ‘highly trained players,’ ‘seasonal alterations’, ‘performance analysis’, ‘soccer physiology’, ‘football’, ‘strength training’, ‘concurrent training’, ‘training transfer’, ‘neuromuscular performance,’ ‘muscular power’, ‘jump ability’, ‘sprint ability’, ‘agility’, ‘repeated sprint’, ‘intermittent endurance’. Further searching of the relevant literature was performed by using the ‘related citations’ function of PubMed and by scanning the reference lists. The second phase involved applying the selection criteria to the articles. Studies were chosen if they fulfilled the following six selection criteria: (i) the studied athletic population consisted of highly trained soccer players, ii) the players in the sample were not under 17 years of age, (iii) detailed physiological and performance tests were included, iv) the training programs applied were specified, (v) appropriate statistical analyses were used, and (vi) the article was written in the English language and published as an article in a peer-reviewed journal or a peer-review soccer-specific book edition.

Data extraction and presentation

Data related to the players’ physiological parameters (e.g., lean leg volume, body fat percentage, running economy, anaerobic threshold, maximum absolute and relative oxygen consumption and strength values, peak and mean power values, and rate force development measures) and performance parameters (e.g., soccer-specific endurance tests, maximal aerobic speed, repeated and single sprint tests, jump ability exercises, agility, and ball speed) were extracted. All data are presented as the percentage of change in the means (∆) unless otherwise specified.

Search data and study characteristics

The aim of providing players with updated data and training approaches in modern scenarios was fulfilled by 23 of the 24 papers published in the last 10 years. There were a total of 24 manuscripts fulfilling the five selection criteria, and the total sample population consisted of 523 soccer players. The distribution of players by competition level was as follows: 322 adults, 145 U-20 players, 12 U-19 players, and 44 U-18 players.

General physiological considerations of strength/power training

Strength training has become an integral component of the physical preparation for the enhancement of sports performance [ 29 ]. While strength is defined as the integrated result of several force-producing muscles performing maximally, either isometrically or dynamically during a single voluntary effort of a defined task, power is the product of force and the inverse of time, i.e., the ability to produce as much force as possible in the shortest possible time [ 9 ]. Nevertheless, strength and power are not distinct entities, as power performance is influenced by training methods that maximize both strength and stretch-shortening cycle activity (SSC) [ 30 ]. The ability of a muscle to produce force and power is determined by the interaction of biomechanical and physiological factors, such as muscle mechanics (e.g., type of muscle action) and morphological (e.g., muscle fiber type) and neural (e.g., motor unit recruitment) factors, and by the muscle environment itself (e.g., biochemical composition) [ 31 ].

The mechanisms underlying strength/power adaptations are largely associated with increases in the cross-sectional area of the muscle (hypertrophy methods) [ 32 ]. However, muscular strength increments can be observed without noticeable hypertrophy and serve as the first line of evidence for the neural involvement in the acquisition of muscular strength [ 32 ]. Thus, despite the notion that hypertrophy and neural adaptations are the basis of muscle strength development [ 33 ], their respective mechanisms of adaptation in the neuromuscular system are distinct [ 34 ]. In fact, ‘more strength’, i.e., the adaptational effect, does not necessarily imply an increase in muscle mass, as several distinct adaptations can lead to the same effect [ 33 ]. In this regard, the trainable effects of explosive/ballistic and/or heavy-resistance strength training causing enhanced force/power production have been primarily attributed to neural adaptations, such as motor unit recruitment, rate coding (frequency or rate of action potentials), synchronization, and inter-muscular coordination [ 31 , 35 , 36 ].

Physiological adaptations in soccer players

Our analysis suggests that the physiological adaptations underlining strength/power training may result in improvements in different motor tasks and performance qualities in high- and low-level players (Table  1 and Figure  1 ). In fact, independent of the players’ standard, an enhanced dynamic [ 1 - 7 , 10 , 14 , 22 , 23 ] and static maximum force production [ 4 , 5 , 28 ] and increased muscle power outputs during different physical movements can be obtained through the implementation of strength/power training routines [ 2 - 8 , 14 , 22 , 37 ]. Specifically, increases in 1RM were observed during isoinertial assessments of half-squat exercises [ 1 - 3 , 6 , 10 , 14 , 22 ], hamstring leg curls, and one-leg step-up bench exercises [ 10 ]. Additionally, in our analysis, we observed a large range of improvements in the 1RM of well-trained players after short-term intervention periods (e.g., pre-season, Figure.  1 , from 11% to 52% during the squat exercise) with average increments of approximately 21% [ 1 - 3 , 6 , 22 , 37 , 38 ]. Only Helgerud et al. [ 37 ] reported considerably larger gains in 1RM compared with other studies (11% to 26%; Table  1 ). Moreover, increments in maximal isometric voluntary contraction (MIVC) in the leg press task after CT training [ 11 ] and in knee extension strength after electrostimulation [ 28 ] and isokinetic training [ 4 , 5 ] have also been reported. Interestingly, not only were improvements in absolute force production (1RM) achieved, but an increased efficiency was also evident after allometric scaling of the results; 1RM per lean leg volume (LLV; 1RM/LLV) improved after high- and moderate-intensity modes of strength training [ 2 ], and relative force (maximum force divided by body mass) improved after complex strength training [ 11 ].

Physiological and functional adaptations to strength training

↑, significant improvement; ↓, significant decrement; ↔, no significant alterations; †, significant differences between groups; ~, approximately and data extracted from graphs; NS, not specified; F/D, frequency and duration of training protocols; P, period of the soccer season; rec, recovery; RST, resistance strength training; PT, plyometric training; SP, sprint training; wk, week; PS, performed during preseason; IS, performed during in-season; ND, not defined; rep., repetitions; 1RM, one repetition maximum; 1RM/LLV, maximal strength in half-squat strength per lean leg volume; PPO, peak power output; F 0 , individual theoretical maximal force generated at zero pedal speed; V opt , speed were the highest value of power is achieve; V 0 , maximal cycling speed corresponding to zero load; LLV, lean leg volume; m, meters; COD, change of direction; CMJ( 10-20-30-40-50-60-70kg) , countermovement jump with or without external (load); RSA, repeated sprint ability test; RE, running economy; VO 2 max, maximal oxygen consumption; MAS, maximal aerobic speed; YYIE2, Yo-Yo intermittent endurance test level 2; DTT, Holff’s dribbling track test; MP 60%-1RM-squat , mean power; MPP 45%-1RM , jump squat; mean propulsive power; SJ, squat jump; ALB, alternate leg bound; DLHJ, double leg hurdle jump; SLFH, single leg forward hop; 4BT, four bounce test; SST, soccer-specific strength; SAQ, speed, agility and quickness; BF, body fat; Wpeak, leg cycling peak power; LMV, leg muscle volume; TMV, thigh muscle volume; MTCSA, mean thigh cross-sectional area; V first step , velocity during the first step after the start of sprint test; V first5-m , average running velocity during the first 5 m of the sprint test; Vmax, maximal running velocity; 5J, five jump test; MPV, maximal pedaling velocity; BHS, back half-squat at 90°; SU, step up on a bench with one leg; LCH, leg curls for hamstrings; DJ40cm, drop jump from 40-cm height; VH, vertical oriented exercises; VHS, vertical and horizontal oriented exercises; BM, body mass; IAT, individual anaerobic threshold; CMJ D, countermovement jump dominant leg; CMJ ND, countermovement jump non-dominant leg; OS, off-season; CON IKE (0. 4.18 and 5.24 rad/s) , concentric isokinetic knee extensor peak torque (angular velocity) ; CON IKE 50° (0 and 0.52 rad/s) , concentric isokinetic knee extensor peak torque at 50° knee extension (angular velocity); PPO ↑3.14 rad/s , peak power at angular velocity higher than 3.14 rad/s; PPO 50°(↑3.14 rad/s) , peak power at 50° knee extension (angular velocity); CON IKE Vpeak(↑5.24 rad/s) , concentric isokinetic knee extensor peak torque exerted at the instance of peak velocity (angular velocities higher than 5.24 rad/s); PPO Vpeak (↑5.24 rad/s) , peak power output exerted at the instance of peak velocity (angular velocities higher than 5.24 rad/s); MIVC 50° knee extension , maximal isometric voluntary contraction of knee extensors (angle); BS with or without run up , ball speed after kicking with or without previous run up; CT, complex strength training; CMJWAS, counter movement jump with arm swing; CMJ 15-s , counter movement jump during 15-s period; RRB, resistance provided by a rubber band; SSG, small sided game; PB, player on the back; MIVC leg press , maximal isometric voluntary contraction in the leg press machine (knee and hip angles of 110° and 90°, respectively; 180° = full extension); MIVC/BW, maximal force divided by body weight; F 60−100 , maximal force value during the first 60 or 100 ms of the contraction; EMG VL, electromyography activity of vastus medialis of the swinging leg (phase 3) normalized relatively to the maximal EMG value during kick; MCS, maximal cycling speed; CMJ/SJ, eccentric utilization ratio; BSdl, ball speed after kicking with dominant leg; BSndl, ball speed after kicking with non-dominant leg; UK, United Kingdom; Hr 13- 14 km.h-1(bpm) , - heart rate at 13 and 14 km.h −1 ; La 13- 14 km.h-1(mM) , blood lactate concentration at 13 and 14 km.h −1 ; V 10m , velocity at 10-m sprint.

The gains in strength and d ifferent motor abilities of high-level players after 5 to 10 weeks. Squares represent the average squat jump performance [ 1 , 6 , 14 , 22 ]; rhombi represent the average countermovement jump performance [ 2 , 22 , 37 ]; triangles represent the average four bounce test performance [ 6 ]; circles represent the average 10-m sprint performance [ 2 , 22 , 37 , 38 ]; x symbols represent the average 40-m sprint performance [ 1 , 2 , 6 ]; + symbols represent the average change in direction ability [ 2 , 38 ]; and lines represent the average of all the previous motor tasks.

According to Harris et al. [ 27 ], intervention studies should use a specific isoinertial loading scheme, and test protocols should assess performance over the force-velocity continuum to gain a better understanding of the effect of load on muscular function. Moreover, neuromuscular-related qualities, such as impulse, rate of force development (RFD), and explosive strength, can better predict athletic performance; thus, the development of these approaches should be targeted [ 27 ]. The functional performance of soccer players seems to be more significantly associated with variables that are measured within the power-training load range (75% to 125% of body weight [BW] in half-squats) at which peak power (PP) is obtained (60% 1RM = 112% of BW) [ 39 ]. The PPs of highly trained soccer players were shown to occur with loads of 45% and 60% 1RM during jump- and half-squat exercises, respectively [ 22 , 39 ]. It is likely that superior improvements in power performance may be achieved by working on these optimal power training load ranges [ 22 , 39 ].

One particular muscle strength/power training adaptation involves an increase in the force-velocity relationships and the mechanical parabolic curves of power vs. velocity after high-intensity training programs, both in isoinertial [ 14 ] and isokinetic [ 4 ] exercises. Ronnestad et al. [ 6 ] and Gorostiaga et al. [ 8 ] observed increases in the force-velocity curve after high-intensity TRE and explosive-type strength training among professional and amateurs players, respectively. In the former study, the analysis of the pooled groups revealed increases in all measures of PP [ 6 ]. It seems that high-intensity strength training significantly increases performance in professional players at both the high-force end (increases in 1RM and sprint acceleration) and the high-velocity end (improvements in peak sprint velocity and four bounce test; 4BT) but only as long as the subjects perform concurrent plyometric and explosive exercises during their soccer sessions [ 6 ]. Furthermore, Los Arcos et al. (2013) recently found that professional players performing 5 weeks of pre-season and 3 weeks of in-season strength/power training increased the load at which PP was achieved during the half-squat exercise [ 11 ]. Additionally, 10 weeks of complex strength training, consisting of soccer-specific strength and skill exercises (soccer kick), improved measures of explosive strength and RFD during the isometric leg press in low-level players, with an increase in the electromyography (EMG) activity of certain muscles involved in the task also reported [ 11 ].

Adaptations in sport-specific efforts

The effectiveness of a strength/power program is evaluated by the magnitude of sport-specific improvements. Although the predominant activities during training and matches are performed at low and medium intensities, sprints, jumps, duels, and kicking, which are mainly dependent on the maximum strength and anaerobic power of the neuromuscular system, are essential skills [ 40 ]. Power and speed usually support the decisive decision-making situations in professional football, e.g., straight sprinting is the most frequent physical action in goal situations [ 41 ]. Furthermore, a high degree of stress is imposed on the neuromuscular system of players to enable them to cope with these essential force-based actions required during training and competition (e.g., accelerations and decelerations) [ 42 , 43 ].

Although not universally confirmed, there is evidence of associations between the measures of maximal (1RM) [ 44 ] and relative strength (1RM/BM) [ 45 ], as well as between certain muscle mechanical properties, such as peak torque [ 46 , 47 ] and PP [ 39 ], and the ability of soccer players to perform complex multi-joint dynamic movements, e.g., jumping and sprinting actions. Independently of a player’s level, strength-related interventions represent a powerful training stimulus by promoting adaptations in a wide range of athletic skills (e.g., jumping, Table  1 , Figures ​ Figures1, 1 , ​ ,2 2 and ​ and3 3 and Additional file 1 : Figure S1-5) [ 2 , 3 , 6 , 8 , 10 , 12 , 14 , 15 , 19 , 21 - 23 , 48 ] and soccer-specific skills (soccer kick) [ 21 , 28 ] (Tables  1 and ​ and2). 2 ). Interestingly, the addition of a long-term strength/power training program to normal soccer training routines seems to result in a higher long-term increase in the physical performance of elite youth players [ 45 , 49 ]. Furthermore, to have a clear picture of the effect of strength training on physical performance, different motor tasks should be assessed; jumping, sprinting, and change of direction abilities may represent separate and independent motor abilities, and concentric and slow SSC jumping actions are shown to be relatively independent of fast SSC abilities [ 50 ].

Gains in strength and motor abilities of high level players after different training modes (5 to 10 weeks). x and dashed x symbols represent the change of direction ability performance after traditional resistance exercises programs (TRE) [ 2 ] and combined programs (COM) [ 38 ], respectively; filled and unfilled squares represent the 40-m sprint performance after TRE [ 1 , 2 ] and COM [ 6 ], respectively; + and dashed + symbols represent the 10-m sprint performance after TRE [ 2 , 37 ] and COM [ 22 , 38 ], respectively; filled and unfilled triangles represent the four bounce test performance after TRE [ 6 ] and COM [ 6 ], respectively; filled and unfilled rhombi represent the squat jump performance after TRE [ 1 , 14 ] and COM [ 6 , 22 ], respectively; and filled and unfilled circles represent the countermovement jump performance after TRE [ 2 , 37 ] and COM [ 22 ], respectively.

Percentage of improvement by training program and training session. Percentage of improvement by training program and training session after traditional resistance exercises programs (TRE), combined programs (COM), and strength/power training programs in the different motor tasks and overall functional performance (FP) of high-level players. Countermovement jump (CMJ) after TRE (CMJ-TRE) [ 2 , 20 , 37 ]; CMJ after COM (CMJ-COM) [ 22 , 23 , 38 ]; CMJ [ 2 , 20 - 23 , 37 , 38 ]; squat jump (SJ) after TRE (SJ-TRE) [ 1 , 14 ]; SJ after COM (SJ-COM) [ 6 , 19 , 22 ]; SJ [ 1 , 6 , 14 , 19 , 22 ]; 40-m sprint performance after TRE (40m-TRE) [ 1 , 2 ]; 40-m sprint performance after COM (40m-COM) [ 6 ]; 40-m sprint performance (40-m) [ 1 , 2 , 6 ]; 10-m sprint performance after TRE (10m-TRE) [ 2 , 20 , 37 ]; 10-m sprint performance after COM (10m-COM) [ 22 , 38 ]; 10-m sprint performance (10m) [ 2 , 20 - 22 , 37 , 38 ]; change of direction ability (COD) after TRE (COD-TRE) [ 2 ]; COD after COM (COD-COM) [ 38 ]; COD [ 2 , 38 ]; FP after TRE (FP-TRE) [ 1 , 2 , 6 , 14 , 20 , 37 ]; FP after COM (FP-COM) [ 6 , 19 , 22 , 23 , 38 ]; and FP [ 1 , 2 , 6 , 14 , 19 - 23 , 37 , 38 ].

Physiological and functional adaptations to concurrent strength and endurance training

↑, significant improvement; ↓, significant decrement; ↔, no significant alterations; ~ , approximately; NS , not specified; F/D , frequency and duration of training protocols; P , period of the soccer season; ST, strength training; ET, endurance training; SJ, squat jump; CMJ, countermovement jump; CMJWAS, countermovement jump with arm swing; MAS, maximal aerobic speed; VJ, vertical jump; Fsquats (20-40kg) , speed of movement during full squats exercise (range of the external load); T 5-30m , sprint performance; T st/10-30 , sprint performance in predetermined split distances; VO 2 max, maximal oxygen consumption; RE (11km.h) , running economy (velocity); 1RM HS . one repetition maxim in half-squat strength exercise; 1RM/BW, strength per kilogram of body weight; rec, recovery; CJS, continuous jumps with legs extended; YYIR1, Yo-Yo intermittent recovery level one; MAS distance , maximal aerobic distance; SSG, small-sided game; CMJ (20kg) , countermovement jump (external load); SAQ , speed, agility and quickness; HRmax, maximal heart rate; IS , performed during in-season; RSA , repeated sprint ability; PT , plyometric training; perf dec RSA, performance decrement in the repeated sprint ability test; YYIR2 , Yo-Yo intermittent recovery level 2.

Sprint ability

With regard to adaptations in sprint qualities (e.g., acceleration and maximal speed, Table  1 and Additional file 1 : Figure S1), improvements in different sprint distances (5- to 40-m distances) [ 1 , 2 , 6 , 10 - 12 , 14 , 15 , 19 , 21 , 22 , 48 , 51 ] have been reported in different levels of players. On average, highly trained players [ 1 , 2 , 6 , 22 , 37 , 38 ] need to increase their 1RM half-squat by 23.5% to achieve an approximately 2% improvement in sprint performance at 10- and 40-m distances (Figure  2 ). Excluding the study of Helgerud et al. [ 37 ], which reported significantly larger increments in strength, studies have demonstrated that lower increments in 1RM (19%) are required to achieve a similar improvement in sprint performance (1.9%) after short-term training interventions (in average, an 18% increments in 1RM resulted in a 2% average improvements in 10-m sprint performance [ 2 , 22 , 38 ] and 17% average increments in 1RM resulted in 1.6% improvements in 40-m distance time [ 1 , 2 , 6 ]). Nevertheless, improvements in sprint performance have not been entirely confirmed [ 1 , 6 , 8 , 10 , 16 , 22 , 28 ]. Notwithstanding, factors associated with the training status of various players, players’ background, and/or the characteristics of the training modes adopted should be considered as the most likely factors. For example, the sole performance of one type of plyometric exercise [ 16 ] and of electrostimulation training [ 28 ], which has an apparent lower level of specificity, may explain, at least in part, the lack of transfer of training adaptations to dynamic and complex activities, where the coordination and force production of different body muscles, as is the case of sprint performance, are essential.

Jump ability

Our analysis suggests that strength/power training induces adaptations in the jump abilities of high-level players (Table  1 and Figure  1 and Additional file 1 : Figure S2). On average, 24.4% 1RM improvements during squats result in a CMJ increase of approximately 6.8% [ 2 , 22 , 37 ]. Lower performance improvements in four bounce test (4-BT; 3.8%) were found with similar increments in 1RM (24.5%) [ 6 ], and similar improvements in SJ (6.8%) occurred with an average 1RM increase of 21.8% [ 1 , 6 , 14 , 22 ]. Curiously, the plotted data of all studies assessing the improvement in jump abilities in high-level players revealed that, on average (Figure  2 , Additional file 1 : Figure S5), a 23.5% 1RM increase may result in a 6.2% improvement in jump ability tasks after 6 to 10 weeks of strength/power training [ 1 , 2 , 6 , 14 , 22 , 37 ]. The previous results suggest that, on average, higher increments in force are needed to improve CMJ to the same extent as SJ (figure  1 ). This result may reflect the fact that the current programs were not able to increase (at the same relative rate) performance ability in the positive and negative phases of the SSC component and may explain, at least in part, the smaller improvements in sprint performance.

Improvements in the squat jump (SJ) [ 1 , 10 , 12 , 14 , 19 , 22 ], four bounce test (4BT) [ 6 ], five jump test (5-JT) [ 14 ], countermovement jump test (CMJ) [ 2 , 8 , 10 , 12 , 16 , 21 , 22 ], CMJ with free arms [ 21 ], and eccentric utilization ratio (CMJ/SJ) [ 12 ] have been observed in different players. Nevertheless, contradictions regarding improvements in SJ after plyometric [ 21 ] and in CMJ after high-intensity strength protocols performed by well-trained players can be found in the literature [ 1 , 14 ]. Additionally, no significant increases in CMJ were observed after CT involving workouts with high [ 19 ] or low loads [ 15 ] or in drop jumps from a 40-cm height (DJ 40 ) [ 10 ] following TRE and TRE plus sprint training.

Change of direction speed (COD)

According to the literature, it is difficult to discern which force/power qualities (e.g., horizontal and lateral) and technical factors influence event- or sport-specific COD ability [ 52 ]. To date, limited research has been conducted on agility/COD adaptations, with even less known about high-level athletes. Despite the limitations initially described see Introduction our results suggest that, on average, an increase of 15% in 1RM results in a 1.3% improvement in COD abilities after 5 to 6 weeks of training (Table  1 ; Figure  1 and Additional file 1 : Figure S3) [ 2 , 38 ]. Bogdanis et al. [ 2 ] observed that applying TRE-targeting hypertrophic or neural adaptations was effective in increasing COD (Table  1 , Additional file 1 : Figure S3). Nevertheless, improvements in COD performance evaluated by the 505 agility test after different plyometric techniques [ 16 ] were not found after CT [ 19 ]. Additionally, in a study by Mujika et al. [ 15 ] where players performed CT, no improvements in COD, evaluated by the agility 15-m test, were observed. The spectrum of possible factors associated with this discrepancy in results is ample and includes the players’ background and initial training status, the different training periods during which the intervention was carried out, the structure of the training intervention, game exposure, and distinct force/power qualities and technical factors that influence event- or sport-specific COD. For example, the study of Maio Alves et al. [ 19 ] was implemented during pre-season, and the research of Thomas et al. [ 16 ] was carried out during in-season. Consequently, the accumulated effect of COD actions performed during training sessions and games may influence these results [ 46 , 53 ]. Although the players are from the same age groups, the differences in the competitive levels of the players from previous studies should not be ignored. Moreover, the lack of improvements in COD after in-season CT that are reported by Mujika et al. [ 15 ] may be related to the fact that only six sessions were performed in a 7-week period. As will be further analyzed (‘Training efficiency’), this fact, among others, may suggest that higher training volumes may be necessary to induce adaptations in COD.

Sport-specific skills

One of the most important indicators of a successful soccer kick is the speed of the ball. Studies involving amateur players observed that CT [ 11 ] and electrostimulation training [ 28 ] increase ball speed with [ 11 , 28 ] and without (Table  1 ) run up [ 28 ]. Nevertheless, these improvements were examined in lower standard players. Moreover, elite U-19 players performing plyometric training increased ball speed with the dominant and non-dominant leg [ 21 ]. Other studies involving elite players performing different modes of strength training (isokinetic strength training or functional training) did not report improvements in ball speed [ 4 , 5 ]. Nevertheless, in studies performed during the off-season period, training stimulus consists of the exercise mode of the experimental designs and no other types of soccer routines are undertaken. Thus, the results should be analyzed with caution as the scenarios for training transfer to occur during this period are constricted (off-season); the increases in certain strength parameters were not reflected in positive transference to consecutive gains in ball speed.

Comparing different training variables in strength/power interventions in soccer

The multi-factorial constructs of soccer performance (technical, tactical, and physical performance) and their associated components bring a higher complexity to the designing of the training process. In fact, professionals involved in the preparation of soccer teams have to reflect on several questions associated with the manipulation of the individual variables that affect each of these relevant constructs and how they can affect each other. With regard to physical performance, several potential questions arise: What are the most beneficial movement patterns and type of training? How many sessions do athletes need to improve and maintain the performance outcome? Does ground surface have an effect on adaptations? We will analyze these and other relevant questions in the following sections.

Force production and movement pattern specificity: traditional resistance exercises vs. combined programs

Our analysis suggests that the activity patterns of applied exercises may influence performance outcomes (Figures  2 and ​ and3 3 and Additional file 1 : Figure S4 to S5). Therefore, we compared programs involving mainly traditional resistance exercises (TREs) with programs that combine different activity patterns during the training intervention (COM; programs including TRE and ballistic exercises, plyometrics, weight lifting, body weight exercises, and/or sprint training during training cycles). Despite the fact that some limitations can be ruled out from this type of analysis (e.g., differences in session and weekly training volumes and load, the density of different intrinsic activity patterns, and the 1RM percentage used during the loaded exercises), we believe that it will aid in challenging research designs in this field.

Effects on sprint performance

On average, despite TRE resulting in superior strength gains compared with COM, greater performance improvements in the 10-m sprint are observed after COM (TRE = in average, 26.8% increments in 1RM resulted in 1.93% average improvements in 10-m sprint [ 2 , 37 ]; COM = in average, 19.9% increments in 1RM resulted in 2.4% average improvements in 10-m sprint [ 2 , 22 , 38 ]; Figure  2 and Additional file 1 : Figure S5). However, our analysis suggests the opposite with regard to 40-m sprint performance (TRE = in average, 15.8% increments in 1RM resulted in 1.9% average improvements in 40-m time [ 1 , 2 ] COM = in average, 23% increments in 1RM resulted in 1.1% average improvements in 40-m sprint time [ 6 ]). Nevertheless, all pooled data suggest that despite the TRE result of greater increases in 1RM (26%) than COM (21%), this may not translate into superior improvements in the sprint performance of high-level players (1.9% TRE vs. 2.1% COM; Additional file 1 : Figure S4).

Effects on jump ability

By performing the same analysis for jump ability exercises (Figure  2 and Additional file 1 : Figure S5), we found that there is a tendency toward greater strength increases after TRE (in average, 26.8% increments in 1RM resulted in 6.8% average improvements in CMJ; in average, 22% increments in 1RM resulted in 6.7% average enhancement in SJ; in average, 25% increments in 1RM resulted in 6% average improvements in 4BT) that are not translated into superior performance gains compared with the results observed following COM (in average, 21% increments of 1RM resulted in 6.8% average improvements in CMJ; in average, 22% increments in 1RM resulted in 6.9% average enhancements in SJ; in average, 22% increments of 1RM resulted in 6.4% average improvements in 4BT). In fact, all pooled data show that greater improvements in jump ability may be obtained with lower strength increases after COM than TRE only (Additional file 1 : Figure S5; in average, 21.6% increments in 1RM resulted in 6.4% average improvements in jump ability and a 25% average increments in 1RM resulted in 6% average improvements in jump ability, respectively). This higher efficacy of transfer of strength gains to performance improvements after COM seems to be more evident in SSC jump ability (CMJ). Taking into consideration, among other factors, the described associations between physiological and mechanical characteristics (e.g., post-activation potentiation and peak torque) and CMJ and running-based actions in professional players [ 44 , 46 , 54 ], this fact may suggest that COM may represent a superior method for improving sport-specific actions compared with TRE alone. Additional studies on this topic are necessary.

Effects on COD ability

Given the scarcity of literature assessing the effect of COD training modes and the reported small to moderate associations between strength and power variables with COD performance and different characteristics (e.g., test duration, COD number, and primary application of force throughout the test) of the agility tests commonly used to evaluate COD [ 52 ], conclusions should be drawn with caution. In fact, within programs involving only TRE, as will be discussed later in this review (‘ Manipulation of loading schemes ’), it seems that manipulating different mechano-biological descriptors of strength/power stimuli may influence performance adaptations in COD actions [ 2 ]. Nevertheless, our analysis shows that, on average, lower strength increases after TRE [ 2 ] produce greater performance improvements in the agility t -test than after COM [ 38 ] (in average, 14.2% increments in 1RM resulted in 1.7% average improvements in t -test and a 19.9% average increment in 1RM resulted in 1% average improvement in t -test, respectively; Figure  2 ).

Two studies are particularly relevant with regard to this topic: TRE vs. TRE plus plyometrics [ 6 ] and TRE vs. TRE plus sprint training [ 10 ]. In the study of Ronnestad et al. [ 6 ], although no significant differences between groups were observed, the group of players who utilized combined approaches broadly improved their performance. Additionally, Kotzamanidis et al. [ 10 ] observed that the jump and sprint performance of low-level players only improved in the combined program approach. Thus, it seems that combining heavy and light load training schemes may be an effective method for improving muscular function and may be particularly useful when force application is required in a wide range of functional tasks [ 27 ].

Training efficiency

To estimate the improvement in the different motor tasks and in overall functional performance, as well as the efficiency (efficiency = percentage of improvement/number of training sessions) of strength/power interventions and the effects of the different types of programs (TRE vs. COM) on specific motor tasks and functional performance, we performed an analysis involving all studies in highly trained players where performance outcomes were reported despite no references to changes in force production (Figure  3 ). Despite the limitations already highlighted, our analysis suggests that even though TRE slightly increases overall functional performance, the efficiency (gains by session) is lower than in COM modes. These uncertainties make this research topic particularly crucial. In summary, considering the high demands of high-level competition, the increase in different motor tasks (1.3% to 7.2%) and overall functional performance (4%) observed in highly trained players following strength/power training programs makes strength/power programs an essential training component. In general, it seems that strength/power training induces greater improvements in jump abilities than in running-based activities. Moreover, combining resistance- and speed-training or plyometric- and soccer-specific strength programs in the same session seems to be more effective than the resistance-training program alone [ 6 , 10 , 48 ].

Manipulation of loading schemes

Bogdanis et al. [ 2 , 3 ] analyzed the effects of high-repetition/moderate-load (hypertrophy) and low-repetition/high-load (neural adaptations) programs on anthropometric, neuromuscular, and endurance performance. These last studies [ 2 , 3 ] and others [ 4 , 5 , 23 ] suggest that the manipulation of different mechano-biological descriptors of strength/power stimuli (e.g., load magnitude, number of repetitions) is associated with different physiological and performance adaptations in highly trained soccer players. The hypertrophic mode was associated with increases in lower limb muscle mass, while the neural mode was more effective in improving 1RM/LLV, sprint, and COD performance [ 2 ]. In another study, Bogdanis et al. [ 3 ] found that even though both groups (hypertrophic group vs. neural group) improved the total work performed during a repeated cycle ergometer sprint test (RST; 10 × 6-s sprint with 24-s passive recovery), the neural mode group had a significantly greater improvement in work capacity during the second half (sprint 6 to 10; 8.9% ± 2.6%) compared with the first half of RST (sprint 1 to 5; 3.2% ± 1.7%). These results suggest that the neural mode confers a higher fatigue resistance during RST [ 3 ]. In addition, the mean power output expressed per lean leg volume (MPO/LLV) was better maintained during the last six sprint post-training only in the neural group, and there was no change in MPO/LLV in the hypertrophic group in the RST [ 3 ]. These results suggest, at least in part, a better efficacy of neural-based programs in high-level players [ 2 , 3 ] that could be linked to several adaptive mechanisms that are not associated with increases in muscle volume. However, the most likely adaptations are at the neuro-physiological level, i.e., changes in the pattern of motor unit recruitment and increases in rate coding [ 2 , 32 ].

Other researchers observed that physiological and performance outcomes can be independent of the kinetics of the power loading scheme used (from the high-force/low-velocity end to the low-force/high-velocity end and vice versa) because the loading scheme components spanned the optimal power training spectrum [ 22 ].

Contraction modes

The analysis of the impact of high- vs. low-intensity isokinetic strength vs. functional strength showed that professional players who performed a high-load, low angular velocity program had a higher improvement in maximal isometric and isokinetic strength and in PP at different knee angles and velocities [ 4 , 5 ]. Although the increases in dynamic muscle strength were generally associated with the specific velocities used in the training programs, the high-load/low-velocity group also exhibited improvements in muscle force and power at high knee extension velocities [ 4 , 5 ]. Although several explanations can be offered to clarify the greater adaptations associated with a wide range of velocities observed after the high-load/low-velocity strength training program, the most likely explanation is the occurrence of changes in neural and morphological factors associated with this type of training (e.g., increases in RFD, muscle mass, and/or fiber pennation angle).

Training frequency

As previously mentioned, high-level soccer players are usually involved in weekly matches of national leagues and are often involved in international commitments, thus limiting the time available for fitness training. Maio Alves et al. [ 19 ] found that different weekly volumes (two vs. one session per week) of complex training performed by high-level junior players resulted in similar improvements in sprint, jump, and COD ability. Ronnestad et al. [ 1 ] observed that one high-intensity strength training session per week during the first 12 weeks of the in-season period represented a sufficient training stimulus for maintaining the pre-season (two sessions per week for 10 weeks) gains in strength, jump, and sprint performance of professional players. However, a lower weekly in-season volume (one session every two weeks) only prevented detraining in jump performance [ 1 ]. Accordingly, a recent study [ 48 ] involving a larger sample of players showed that professional teams subject to distinct weekly strength training stress (all performed one resistance strength session a week) exhibit higher neuromuscular performance in the middle of the season than at the start of the season. Nevertheless, only the team that performed a higher number of sessions targeting the neuromuscular system showed improved neuromuscular performance during the second phase of the season. Despite the distinct individual variables that constituted the weekly resistance training session performed by the teams (e.g., percentage of 1RM, number of repetitions and exercises), differences in strength/power training stress were mainly due to the higher employed volume of both soccer-specific strength and sprint sessions [ 48 ]. This result again established the important role of the specificity of the training stimulus. Given the important role of circulating levels of androgens in strength and power performance, it is relevant to mention that only the high neuromuscular training scheme positively affected the circulation and activation (increase in 3a Diol G) of the androgen pool (total testosterone) [ 48 ].

However, Mujika et al. [ 15 ] observed that a low volume of combined forms of strength/power training is more effective in improving sprint performance (15-m sprint time) than the sole performance of lower volumes of sprint training in elite U-19 players.

Manipulation of biomechanical components of plyometric-based exercises

Performance outcomes may also be influenced by the biomechanical nature of the exercises employed in a single or combined program. Los Arcos et al. [ 23 ] observed that weight training plus plyometric and functional exercises involving vertically and horizontally oriented movements were more effective in enhancing the CMJ performance of highly trained players than exercises involving purely vertically oriented movements. Nevertheless, both groups improved their PP and showed small, although non-significant, improvements in 5- and 15-m sprint performance [ 23 ]. In contrast, Thomas et al. [ 16 ] examined that both plyometric training involving drop jumps or CMJs were effective in improving the jump (CMJ) and COD ability (505 agility test) of semi-professional players, regardless of the lack of change in short sprint distances. It is important to highlight that although no between-group differences were reported, the improvements in COD ability were twofold greater in the CMJ group. Nevertheless, given the age group of the players (U-18), it is important to be cautious in extrapolating these findings to professional adult players.

Training surface

There is also evidence that the ground surface used during plyometrics (sand vs. grass) may influence adaptations [ 12 ]. Impellizzeri et al. [ 12 ] observed that performing plyometrics on grass produced greater effects in CMJ and in the eccentric utilization ratio CMJ/SJ than when performed on sand. However, a trend toward higher adaptations was observed in SJ when the training program was performed on sand (Table  1 ). Additionally, sand was found to induce lower levels of muscle soreness compared with grass [ 12 ]. The fatigue development and recovery kinetics during and after a game have been well characterized in recent years. A reduction in the players’ ability to produce force toward the end of the match and in the match recovery period, an increase in some indirect markers of muscle damage, and longer periods of post-match muscle soreness have all been described [ 55 - 68 ]. In light of these findings, it may be expected that sandy surfaces may be a good alternative for the execution of plyometric programs during periods of high-volume, high-intensity, or high-frequency training (e.g., pre-season) and when athletes are recovering from injury and trying to regain physical capacity. In fact, in addition to improving neuromuscular capabilities, sand has been shown to produce lower levels of muscle soreness compared with grass [ 12 ]. Accordingly, compared with natural grass or artificial turf, the performance of dynamic powerful actions on sand, despite the known higher energy expenditures and metabolic power values, results in smaller impact shocks and limited stretching of the involved muscles [ 69 ].

Interference between concurrent strength and endurance training

Concurrent training involves the incorporation of both resistance and endurance exercises in a designed, periodized training regime [ 70 ]. The current dogma is that muscle adaptations to RE are blunted when combined with endurance [ 71 ], resulting in lower strength and power gains than those achieved by resistance exercise alone. When the modes of strength and endurance training focus on the same location of adaptation (e.g., peripheral adaptations), the muscle is required to adapt in distinctly different physiological ways [ 72 ]. However, when the modes of strength/power and endurance training are at opposite ends of the biomechanical and neuro-coordinative spectrum, the anatomical and performance adaptations may be reduced, and the accuracy of the intended movement, fluidity, and elegance that characterize excellence may be compromised. In fact, it is the entire spectrum of characteristics (e.g., metabolic and neuro-coordinative) of the upstream stimulus (resistance vs. endurance exercise; RE vs. E) that determines the downstream events necessary for training adaptations to occur. The range of factors that may be associated with the interference phenomenon or the incapability of achieving/maintaining higher levels of strength/power during concurrent strength and endurance training is ample and spans from excessive fatigue or increments in catabolic environments to differences in motor unit recruitment patterns, possible shifts in fiber type, and conflicts with the direction of adaptation pathways required by the muscle [ 34 , 70 , 72 , 73 ].

Molecular events

RE stimulates a cascade of events leading to the induction or inhibition of muscle atrophy [ 74 ]. From a molecular standpoint, these adaptations result from the downstream events promoted by the phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3-k/Akt/mTOR) pathway [ 74 , 75 ]. However, three kinases [p38 mitogen-activated protein kinase (MAPK), AMP-activated protein kinase (AMPK), and calmodulin-dependent protein kinase] are particularly relevant in the signaling pathways that mediate skeletal muscle adaptations to endurance-based training [ 75 , 76 ].

A few studies highlight the notion that both translation efficiency and protein synthesis may be compromised due to the incompatibility of the two different intracellular signaling networks, i.e., activation of AMPK during endurance exercise impairs muscle growth by inhibiting mTOR [ 74 , 75 ]. Nevertheless, other studies revealed that endurance performed after RE did not compromise the signaling pathways of RE (mTORC1-S6K1) [ 71 ] and may amplify the adaptive response of mitochondrial biogenesis [ 76 ]. Moreover, the translational capacity for protein synthesis can be reinforced rather than compromised when aerobic exercise precedes RE and molecular events are not compromised; mTOR and P70S6K shown greater phosphorylation in response to concurrent aerobic exercise compared with RE alone [ 77 ]. Furthermore, chronic concurrent aerobic exercise and RE may increase aerobic capacity and promote a greater increase in muscle size than RE alone [ 78 ]. Nevertheless, taking into account the complexity and the several molecular interactions that constitute the cascade of events associated with resistance and endurance exercise, conclusions should be drawn with caution. Additionally, studies have been performed primarily in healthy adults (physically active college students, moderately trained and recreationally active subjects) and not high-level athletes; although not universally confirmed, athletes with more extensive training backgrounds may have distinct phenotypes [ 79 - 81 ] and genotypes than normally active subjects [ 82 ]. Moreover, to the authors’ best knowledge, there is no research concerning how the distinct genotypes that can be found within a high-level group of athletes [ 82 - 84 ] may influence the individual responses to concurrent training.

Methodological considerations

Given the divergent physiological nature of strength and endurance training [ 34 ], the methodology applied, the volume and frequency of training, and the target goal all play key roles in increasing the degree of compatibility between these two key physical fitness determinants [ 34 , 72 ]. Slow long-duration sustained aerobic conditioning (SLDC) has been shown to be potentially detrimental to the overall performance of athletes involved in power sports and, for example, may have a negative impact on strength and power development [ 85 ]. Excessive training volumes may contribute to high metabolic stress, leading to high levels of substrate depletion and catabolic states (e.g., increased cortisol responses) [ 85 ]. Furthermore, SLDC may compromise recovery and regeneration, leading to a progression in the overtraining continuum [ 85 ]. Moreover, the high levels of oxidative stress (e.g., damaging proteins, lipids, and DNA) that are associated with high-volume training may increase reactive oxygen species (ROS) production to a level that overcomes the positive adaptations that may be triggered by ROS, i.e., there is a range in which ROS may represent an optimal redox state for greater performance, as with force production capacity [ 86 ]. Additionally, these previous factors associated with SLDC that limit force production may compromise skill acquisition by reducing the quality of execution (e.g., the technical ability of force application) and, thus, motor learning [ 85 ]. It is reasonable to consider that there may be certain mechanisms associated with the combination of training modalities that produce positive improvements and are additive in nature [ 87 ].

A low-volume, high-intensity approach, such as sprint interval training, may favor an anabolic environment (e.g., growth hormone, insulin-like growth factor-I, IGF binding protein-3, and testosterone) [ 88 - 92 ], maintain a muscle fiber phenotype associated with strength and power capabilities [ 93 ], and increase endurance and neuromuscular-related outcomes [ 94 - 96 ]. In fact, HIT and/or combined forms of HIT seem to promote adaptations in skeletal muscle and improvements in laboratory and field endurance-related parameters that are comparable to the effects of high-volume endurance training [ 94 , 97 - 101 ] and may improve muscle power-based actions [ 94 , 102 ]. Interestingly, the type of previously observed hormonal responses to HIT (e.g., sprint interval training) [ 88 - 92 ] constitutes one of the paradigms of resistance exercise biology, namely, an increase in cellular signaling pathways as well as satellite cell activation that contributes to an increase in translation and transcription processes associated with protein synthesis [ 74 ]. In this regard, supramaximal interval training is shown to be superior to high-intensity interval training for concurrent improvements in endurance, sprint, and repeated sprint performance in physically active individuals [ 103 ].

Does the magnitude of neuromuscular involvement during training sessions reduce possible incompatibilities associated with concurrent training? Are the biomechanical and neuro-coordinative demands (e.g., accelerations/decelerations impacting mechanical load and neuromuscular demands) of different training modes with similar physiological responses the same (e.g., 4 × 4-min interval running with 2-min rest vs. 4 × 4-min SSG with 2-min rest vs. 4 × 4-min intermittent situational drill with 2-min rest)? It is possible that, from a biomechanical and neuromuscular standpoint, more specific training methods to develop strength/power and endurance performance with higher biomechanical and neuromuscular demands may improve both adaptations and performance outcomes, as well as reduce the negative effect of this interference from a molecular point of view; human-based studies to date are far from agreement regarding the molecular interference after acute concurrent exercise [ 70 ]. In fact, strength/power and HIT are characterized by brief intermittent bouts of intense muscle contractions. Questions related to training transfer should be observed with greater attention when extrapolating the applicability of concurrent training to sport-specific settings. In fact, several factors can influence the transfer of strength training in endurance performance and the impact of endurance workloads on strength and power performances [ 104 ].

Soccer: a concurrent modality

A soccer player’s performance is intimately associated with the efficiency of different energy-related systems [ 105 - 107 ]. During the season, players perform intense programs with multiple goals of increasing strength, power, speed, speed endurance, agility, aerobic fitness, and game skills [ 108 ]. In fact, despite the predominant activity patterns of the game being aerobic in nature, the most deterministic factors of match outcome depend on anaerobic mechanisms [ 41 ]. It is common sense that the most intense match periods and worst-case match scenarios are associated with periods of high mechanical and metabolic stress. In fact, recently developed techniques of match analysis provide a body of evidence that supports the belief that neuromuscular demands of training and competition are higher than initially suspected (e.g., accelerations/decelerations) [ 42 , 43 , 109 ] and give further support to the viewpoint that strength/power-related qualities are crucial for high-level performance.

There is a belief that by stressing the neuromuscular system, adaptive mechanisms that are neurological, morphological, and biomechanical in nature will be triggered, thus increasing the player’s neuromuscular performance and providing him/her with a superior short- and long-term endurance capacity [ 17 , 110 - 113 ]. In this regard, associations between neuromuscular qualities (e.g., CMJ peak power) and intermittent endurance exercise [ 114 ] and repeated sprint ability performance [ 115 ] have also been observed. Moreover, there has been evidence supporting the association between team success and jump abilities (e.g., CMJ and SJ) [ 116 ]. Additionally, starter players demonstrate higher strength [ 108 ] and power performance capabilities than non-starters [ 117 ], and greater neuromuscular capabilities have been associated with game-related physical parameters and lower fatigue development during matches [ 118 ]. Moreover, Meister et al. [ 119 ] observed that after a match congestion period, players with a higher exposure time show better scores in certain neuromuscular parameters (CMJ, drop jump height, and drop jump contact) than players with a lower exposure time, although this result is not significant. Interestingly, recent reports revealed that neuromuscular-based actions, such as sprinting, have improved more in recent years than physiological endurance parameters. Professional players tested during the 2006 to 2012 seasons actually had a 3.2% lower VO 2 max than those tested during 2000 to 2006 [ 120 , 121 ]. Although with the obvious limitations and the universal consensus of the importance of aerobic fitness in soccer, these observations suggest that anaerobic power is ‘ stealing space ’ from aerobic power with regard to the constructs relevant in soccer performance. All of these previous facts highlight the role of neuromuscular exercise during soccer training and suggest that soccer routines should be performed concurrently as they are concurrent by nature. In fact, the physiological systems associated with endurance fitness development and maintenance are generally largely targeted in any match competition, friendly game, tactical exercise, circuit technical drills that often involve frequent displacements, and/or small side game exercises performed during a 90-min soccer competition/training session [ 106 , 122 , 123 ].

Physiological and performance adaptations

The summary of changes in physiological and functional parameters resulting from concurrent strength and endurance training are presented in Table  2 . Wong et al. [ 20 ] observed that 8 weeks of pre-season high-intensity strength training and SE resulted in a significant improvement in endurance markers, soccer-specific endurance (SSE), and soccer-specific neuromuscular (SSN) parameters. Helgerud et al. found that 8 weeks of other modes of HIT (aerobic high-intensity training) and high-intensity strength training during the preseason of non-elite [ 51 ] and elite [ 37 ] football players improved VO 2 max (8.6% and 8.9%), running economy (3.5% and 4.7%), and 1RM during half-squat strength exercise (52%), respectively. Moreover, the 10- and 20-m sprint performance (3.2% and 1.6%, respectively) and CMJ (5.2%) of elite players also improved [ 37 ]. These strength improvements occurred with minor increases in body mass (average 1%) and a substantial increase in relative strength [ 37 ]. More recently, McGawley et al. [ 38 ] found that a high-frequency program (three times a week) of concurrent high-intensity running-based training with strength/power-based training in the same session resulted in a positive training effect on all evaluated measures, ranging from flexibility, anthropometric, endurance, and neuromuscular-related parameters (Table  2 ). Moreover, these results suggested that the order of completion of the program, E + RE or RE + E, did not influence the performance adaptations. These results [ 38 ] and others [ 2 , 37 ] may support, at least in part, the better compatibility between high-intensity modes of strength and endurance training.

It is reasonable to assume that the players in the studies examining the effects of strength training programs (Table  1 ) had performed training with significantly high weekly endurance-based loads (e.g., pre-season). In this regard, Bogdanis et al. [ 3 ], when examining the strength training effects of the hypertrophic and neural modes in professional soccer players during pre-season, reported that the weekly cycle also involved a considerable amount of interval training and small-sided games, which have been described as effective methodologies targeting endurance fitness and SSE development (for a review, see [ 95 , 122 ]). The authors [ 3 ] observed that both aerobic fitness parameters (e.g., VO 2 max and MAS) and SSE, evaluated by the Yo-Yo intermittent endurance test and Hoff’s dribbling track test, respectively, were significantly improved in both groups (Table  1 ). Furthermore, other researchers [ 23 ] found that strength/power training performed in parallel with endurance training resulted in improvements in the individual anaerobic threshold and muscle/power parameters. Additionally, the performance of explosive-type strength training with routine soccer training did not interfere with the aerobic capacity of amateur young players [ 8 ], e.g., sub-maximal blood lactate values. These findings suggest that performing concurrent strength/power training and routine soccer training is advisable because, in addition to an increase in neuromuscular performance and the anabolic environment, this training did not interfere with the development of aerobic capacity [ 8 ]. Nevertheless, the question of whether this compatibility is related to the type of endurance and strength performed is highlighted in the distinct between-group results presented in the study of Bogdanis et al. [ 3 ], e.g., point ‘ Manipulation of loading schemes ’, where only the neural group significantly improved with respect to running economy and a trend toward a better performance in the YYIE2 in the neural group than in the hypertrophic group was reported.

In another study [ 13 ], semi-professional male soccer players performed both endurance and strength sessions as part of the annual periodization (four cycles of 12 weeks). This type of periodization was effective in improving both the endurance performance (Probst test) and SSN parameters, e.g., CMJ. These results suggested that no adaptation conflicts occur when one or two sessions of strength/power and endurance are simultaneously combined during a soccer training cycle (endurance block composed of two endurance training sessions and one strength training session and vice versa).

Additionally, Lopez-Segovia et al. [ 18 ] examined training adaptations in elite U-19 players during a 4-month period. The training program consisted of four sessions per week, targeting the improvement of player’s aerobic performance. Training was complemented with one or two specific strength training sessions per week performed at the start of the training session. This type of periodization improved loaded CMJ performance and the speed of movement in full squats, with loads ranging from 20 to 40 kg. Nevertheless, significant decrements in different sprint abilities were found. According to the researchers, the lack of improvement in the former sprint variables was attributed to the high volume of aerobic work performed. Nevertheless, an increase in MAS (3.2%) was observed after the intervention period [ 18 ].

Conclusions

Our analysis suggests that, independent of the methodology applied (Table  1 ) and the form of concurrent endurance and strength/power training (Table  2 ), pre-season training resulted in an improvement in physiological and soccer-specific and non-specific performance parameters. The large responsiveness to training may be associated with the fact that most of the studies were conducted during an early stage of pre-season, with off-season detraining negatively affecting several physical attributes, such as anthropometric characteristics (e.g., decreases in LBM and increases in BF) [ 124 - 126 ], endurance-related markers [ 53 , 101 , 126 , 127 ], soccer-specific endurance [ 101 , 128 ], and neuromuscular parameters [ 126 , 129 ]. With this in mind, the overall conclusion of the analyzed literature is that the addition of strength/power training programs to routine soccer training favors a more integral physical fitness development of the player. The associated improvements in physiological (e.g., 1RM/LLV, PP) and performance (e.g., jump, sprint, COD) parameters may, at least in part, increase a player’s ability to cope with training and competition demands. Our analysis suggests that high-intensity strength training (HIST) may be a more efficient method than moderate-intensity methods (hypertrophic). In addition, the compatibility between strength and endurance training may be greater when high-intensity or explosive strength training is combined with high-intensity endurance training to favor a more soccer-specific phenotype.

One of the most sensitive periods of training implementation is the in-season period. As the match is the most important part of the soccer-training schedule, technical staff often view the in-season periodization with particular prudence. They want to maintain or even increase the pre-season gains obtained throughout the short pre-season period (5 to 7 weeks). However, they face the constant dilemma of determining the proper dose/response that allows for the cycle of training-recovering/competing-recovering to be effective; a high volume of training and/or competition interspersed by insufficient recovery favors fatigue development [ 130 ], resulting in a transition from a functional to a non-functional overreaching state or, in more severe cases, an overtraining state [ 131 , 132 ]. Unfortunately, studies implemented during in-season are scarce [ 1 , 8 , 13 - 16 , 18 , 21 , 24 , 28 , 48 ]; seven were conducted with U-19 players, and only four were conducted with adult soccer players [ 1 , 13 , 28 , 48 ]. Our analysis suggests that two weekly sessions allow for highly trained players to obtain significant performance enhancements and that one session a week is sufficient to avoid in-season detraining. It may be possible that, in parallel with a higher volume of neuromuscular training (soccer-specific strength/power-based efforts), further in-season improvements could be observed. Moreover, manipulations of the training surface could constitute an important strategy (e.g., players returning from injury and the management of biochemical and perceptual disturbances).

We found that the results of high-force increments vs. low-performance enhancements and the respective efficiency of the programs (jump vs. running-based actions and non-SSC abilities (SJ) vs. SSC-based actions (e.g., CMJ)) suggest that current approaches may overlook some essential aspects required to achieve an increase in a player’s performance capacities. According to Komi [ 133 ], an effective SSC is obtained with ‘a well-timed pre-activation of the muscle(s) before the eccentric phase, a short and fast eccentric phase, and an immediate transition (short delay) between stretch (eccentric) and shortening (concentric phase).’ The observed increments in force production will most likely occur to a greater extent in the positive phase of the SSC. We suggest that to achieve greater improvements, weight training should be combined with more soccer-specific strength exercises (e.g., the player’s ability to use strength and power effectively and consistently [ 134 ], allowing for the application of force/power in a larger range of planes (horizontal) and specific angles). Therefore, a conditioning method such as Speed, Agility and Quickness (SAQ) may be useful, as it incorporates plyometric and soccer-specific strength exercises and can, therefore, constitute a good conditioning tool for this type of outcome (acting on the entire spectrum of the SSC and on the transition from eccentric to concentric movements; it should be kept in mind that plyometric training is a technique demonstrated to increase musculo-tendinous stiffness, which can optimize power output in explosive movements) [ 135 ]. The greater ecological validity of COM approaches make combined methods a preferred training strategy for strength training in soccer; targeting the intra- and inter-muscular aspects of athletic performance should occur in parallel and begin at the start of the preparation period. In fact, hypertrophy and general power exercises can enhance sports performance, but optimal transfer from football-specific activities also requires football-specific exercise programs [ 29 ] in which the biomechanical and neuro-coordinative patterns of sport-specific motor tasks are taxed.

In summary, the analyzed literature suggests that the training of neuromuscular function and its combination with soccer-specific endurance results in improvements in non-specific (e.g., anthropometric characteristics, relative strength, and VO 2 max) and soccer-specific endurance and neuromuscular parameters (e.g., YYIER, RSA, and sprint).

Acknowledgments

No funding was used to assist in the preparation of this review.

Additional file

The gains in strength and sprint performance of high-level players after 5 to 10 weeks. Squares represent the 10-m distance [ 2 , 22 , 37 , 38 ]; circles represent the 20-m distance [ 37 ]; rhombi represent the 40-m distance [ 1 , 2 , 6 ]; + symbols represent the average of all distances; triangles represent the average of the 10-m distance; and lines represent the average of the 40-m distance. Figure S2. The gains in strength and jump performance of high-level players after 6 to 10 weeks. Squares represent the squat jump performance (SJ) [ 1 , 6 , 14 , 22 ]; triangles represent the countermovement jump (CMJ) performance [ 2 , 22 , 37 ]; rhombi represent the four bounce test (4BT) performance [ 6 ]; lines represent the five jump test [ 14 ]; circles represent the average CMJ; x symbols represent the average SJ performance; and + symbols represent the average 4BT performance. Figure S3. The gains in strength and change of direction ability of high-level players after 5 to 6 weeks. Squares represent the t -test performance [ 2 , 38 ]; circles represent the Zig-Zag test performance [ 2 ]; and rhombi represent the Illinois agility test performance [ 2 ]. Red-filled triangles represent average of all tests. Figure S4. The gains in strength and overall sprint performance of high-level players following traditional resistance exercise programs (TRE; 6 to 10 weeks) and combined programs (COM; 5 to 7 weeks). Filled circles represent the TRE results; empty circles represent the COM results; red-filled circles represent the average TRE [ 1 , 2 , 37 ]; empty red circles represent the average COM [ 6 , 22 , 38 ]. Figure S5. The gains in strength and overall jump ability of high-level players following traditional resistance exercise programs (TRE; 6 to 10 weeks) and combined programs (COM; 6 to 7 weeks). Blue-filled and unfilled triangles represent the countermovement jump (CMJ) results after TRE and COM, respectively; red-filled and unfilled triangles represent the squat jump (SJ) results after TRE and COM, respectively; green-filled and unfilled triangles represent the four bounce test (4BT) results after TRE and COM, respectively; yellow-filled triangles represent the five jump test (5JT) results after TRE; blue-filled and unfilled circles represent the average CMJ results after TRE [ 2 , 37 ] and COM [ 22 ], respectively; red-filled and unfilled circles represent the average SJ results after TRE [ 1 , 14 ] and COM [ 6 , 22 ], respectively; black-filled and unfilled circles represent the average overall jump ability increases after TRE [ 1 , 2 , 6 , 14 , 37 ] and COM [ 6 , 22 ], respectively.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

All authors read and approved the final manuscript.

Contributor Information

João R Silva, Email: [email protected] .

George P Nassis, Email: [email protected] .

Antonio Rebelo, Email: tp.pu.edaf@latana .

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