Soccer Performance Research Articles

Effect of FIFA 11+ intervention on change of direction performance in soccer and futsal players: A systematic review and meta-analysis

This study systematically reviewed scientific literature regarding the effects of the FIFA 11+ intervention on the change of direction (COD) performance in soccer and futsal players. We conducted a keyword search on June 30, 2020, in SPORTDiscus, PubMed, Web of Science, Academic Search Complete, Scopus, CINAHL, and Google Scholar. We performed a meta-analysis to estimate the pooled effect size of FIFA 11+ interventions on COD performance. Sixteen RCTs studies met the eligibility criteria and were included in this review. Compared to the control group, the FIFA 11+ intervention yielded a 0.87 standard deviation (95% CI = 0.52, 1.21; I2 = 96.2%) decrease in the time of COD task completion in soccer and futsal players. In addition, there were no differential effects of the FIFA 11+ intervention compared to the control group in COD performance enhancement among age, gender, athletic level, sport, intervention frequency, intervention duration, and measure covariates. In conclusion, the FIFA 11+ intervention is effective in improving the COD performance of soccer and futsal players. Future studies should recruit large representative samples from professional soccer or futsal teams and examine the effectiveness of the FIFA 11+ intervention on the COD performance among population subgroups (e.g., by age and gender). Studies focusing on different COD tests are warranted to deepen the understanding of the FIFA 11+ intervention effects on the COD performance.

Specific physical performances among male elite youth soccer players: effect of maturity status.

There is a lack of studies that investigated the relationship between anthropometric profile, biological maturity and specific soccer performances. Therefore, the aims of the present study were to establish normative data of anthropometric and specific physical performances and to determine the impact age and somatic maturation on young soccer players (U13-U19, n=487). Measurements include anthropometric variables to determine the age of peak height velocity (PHV), leg muscle volume (LMV) and soccer specific test (SST); Squat jump (SJ), Counter Movement jump (CMJ), sprint 10 meter (T<inf>10m</inf>), sprint 20 meter (T<inf>20m</inf>), sprint 30 meter (T<inf>30m</inf>) and intermittent-endurance tests. Reference values showed a significant difference between anthropometric variables, LMV and SST according to PHV categories. (M)ANOVA analysis showed a significant age using maturity interaction effect of all anthropometric variables. A significant result was reported in the majority of SST performances for the age, the T<inf>10m</inf> and for T<inf>20m</inf> performances. The full model of multiple regressions and the multiple equations was used to determine the best predictors of physical performances according to anthropometric variables. This study provides normative data for anthropometric characteristics and physical performances according to chronological age (U13-U19) and maturity groups of young soccer players.

Does aerobic performance define match running performance among professional soccer players? A position-specific analysis

ABSTRACT Aerobic performance is considered an important determinant of match running performance in soccer, but studies have rarely investigated this issue in top-level players. This study examined the possible associations between direct measures of aerobic performance and match running performance in elite soccer players. Aerobic performance was tested at the beginning of the season in laboratory settings. The match-running performance was measured by a global positioning system over a competitive half-season for a total of 82 match performances in professional players from Croatia (age: 23.76 ± 2.64; body height: 181.62 ± 7.09 cm; body mass: 77.01 ± 6.34 kg) and clustered as central player (n = 57) and side player (n = 25) performance. No significant differences in aerobic performance were noted between central and side players. The anaerobic threshold was correlated with high-speed running (19.8–25.1 km/h), sprint running (>25.1 km/h), and high-intensity running (>19.8 km/h) among side players (r = 0.52, 0.53, and 0.59, respectively; p < 0.01). For central players, the aerobic threshold was correlated with the total distance covered, low-intensity running (<14.3 km/h), and distance covered in the zone of running (14.4–19.7 km/h) (r = 0.47, 0.49, and 0.39; p < 0.01, 0.01, and 0.03, respectively). Conditioning for central players should include activities with intensities corresponding to aerobic thresholds, while conditioning of side players should be focused on the development of anaerobic thresholds.

The undervalued set piece: Analysis of soccer throw-ins during the English Premier League 2018–2019 season

Set pieces in soccer (i.e., free kicks and corners) have been examined in detail and are a common focus for coaches during training and performance preparation. However, limited evidence is available on the impact of throw-ins on soccer performance and if coaches should dedicate time in training towards this specific set piece. Therefore, this research aimed to firstly examine if throw-in performance is linked with soccer performance, and secondly the effect throw-in direction and length has on first contact success rate, possession retention, mean time in possession and shot creation. 16,154 throw-ins from 380 English Premier League matches during the 2018–2019 season were analysed. Higher final league position was correlated to increased throw-in first contact success and possession retention. 83% of throw-in’s resulted in a successful first contact, 54% resulted in possession being retained and 8.8% of throw-ins led to a shot at goal from the possession achieved after a successful first contact. Throw-in’s which went backwards or laterally in direction resulted in increased first contact success, retaining of possession, and shot creation. The least efficient throw-in was forwards and long, which resulted in both reduced first contact success and possession retention. Findings highlight, that throwing the ball laterally or backwards should be a focus for coaches and players during attacking training. In contrast, a team’s defensive strategy should reduce the opportunities to throw backwards or laterally with a higher press and look to force a long forward throw-in, therefore, increasing the likelihood of winning possession and counter attacking.

Fluid Balance, Sweat Na+ Losses, and Carbohydrate Intake of Elite Male Soccer Players in Response to Low and High Training Intensities in Cool and Hot Environments.

Hypohydration increases physiological strain and reduces physical and technical soccer performance, but there are limited data on how fluid balance responses change between different types of sessions in professional players. This study investigated sweat and fluid/carbohydrate intake responses in elite male professional soccer players training at low and high intensities in cool and hot environments. Fluid/sodium (Na+) losses and ad-libitum carbohydrate/fluid intake of fourteen elite male soccer players were measured on four occasions: cool (wet bulb globe temperature (WBGT): 15 ± 7 °C, 66 ± 6% relative humidity (RH)) low intensity (rating of perceived exertion (RPE) 2–4, m·min−1 40–46) (CL); cool high intensity (RPE 6–8, m·min−1 82–86) (CH); hot (29 ± 1 °C, 52 ± 7% RH) low intensity (HL); hot high intensity (HH). Exercise involved 65 ± 5 min of soccer-specific training. Before and after exercise, players were weighed in minimal clothing. During training, players had ad libitum access to carbohydrate beverages and water. Sweat [Na+] (mmol·L−1), which was measured by absorbent patches positioned on the thigh, was no different between conditions, CL: 35 ± 9, CH: 38 ± 8, HL: 34 ± 70.17, HH: 38 ± 8 (p = 0.475). Exercise intensity and environmental condition significantly influenced sweat rates (L·h−1), CL: 0.55 ± 0.20, CH: 0.98 ± 0.21, HL: 0.81 ± 0.17, HH: 1.43 ± 0.23 (p =0.001), and percentage dehydration (p < 0.001). Fluid intake was significantly associated with sweat rate (p = 0.019), with no players experiencing hypohydration > 2% of pre-exercise body mass. Carbohydrate intake varied between players (range 0–38 g·h−1), with no difference between conditions. These descriptive data gathered on elite professional players highlight the variation in the hydration status, sweat rate, sweat Na+ losses, and carbohydrate intake in response to training in cool and hot environments and at low and high exercise intensities.

Effects of high-carbohydrate versus mixed-macronutrient meals on female soccer physiology and performance

Athletes are often told to minimize intakes of fiber, fat, and protein in pre-competition meals to avoid gut distress, but this guidance is based on scant direct evidence. This study evaluated the physiological and perceptual effects of pre-competition mixed-macronutrient (MM) and high-carbohydrate (HCHO) meals in collegiate female soccer players. Fifteen players participated in this randomized, investigator-blinded, crossover study involving two ~ 1000-kcal meals (HCHO and MM) consumed 4h prior to 70-min scrimmages. Assessments included global positioning system (GPS) tracking, heart rate (HR), perceived exertion (RPE), ratings of fatigue (ROF), gut symptoms, and perceptions of satiety, hunger, and fullness. Differences between conditions for HR, RPE, ROF, and gut symptoms were evaluated with Wilcoxon signed-rank tests. GPS data and hunger, satiety, and fullness scores were compared using within-subjects repeated measures ANOVAs. No statistically significant differences were found between the conditions at any time point for HR, RPE, ROF, or gut symptoms. Significant time effects were found for two GPS variables (total distance covered and high-speed running), indicating that participants covered less distance during the second half of the scrimmages in comparison to the first half. However, there were no significant condition or condition × time interactions for GPS data. Finally, there were no condition or condition × time interactions for hunger, fullness, and satiety, though significant time effects were observed. A MM meal consumed 4h prior to 70min of soccer competition does not increase gut symptoms and can be similarly ergogenic as a HCHO meal.

Association of the vertical and horizontal force-velocity profile and acceleration with change of direction ability in various sports

This study aimed to assess the association of the mechanical variables derived from the force-velocity (FV) profile (i.e. theoretical maximal force [F 0], velocity [V 0] and maximal power output [P max]) with change of direction (COD) performance in soccer, basketball and tennis players. Fifty-four male athletes (soccer n = 23; tennis n = 16; basketball n = 15) were assessed for the vertical (Vrt) and horizontal (Hzt) FV profiles, COD with the dominant (D) and nondominant (ND) legs, using the modified 505 test, and sprint. Hzt FV profile parameters showed stronger associations with performance than Vrt FV profile in the three sports. Specifically, the Hzt parameter most strongly associated with COD performance was F 0 in tennis (r = −0.83; p<0.001) and P max in soccer and basketball (r = −0.79; p<0.001). Associations between sprint times and COD test ranged from (r = 0.73–0.82) in soccer players, (r = 0.74–0.87) in tennis players and (r = 0.62–0.85) in basketball players, respectively (p<0.05). Considering the whole sample and the random effect of the type of sports, an improvement in sprint acceleration (i.e. one N/kg increase in F 0 and one W/kg in P max) was associated with −0.15 s and −0.04 s to complete the 505 test, respectively. In conclusion, our results suggest the potential usefulness of assessing the Hzt FV profile to maximize acceleration capabilities through training interventions which, in turn, may translate into improved COD performance. However, further longitudinal and experimental research is needed to confirm this hypothesis.

The Importance of In-Season Strength and Power Training in Football Athletes: A Brief Review and Recommendations

Football (soccer) performance requires a diversity of physical attributes and biomotor abilities, such as strength, power, jump mechanics, repeat sprint ability, change of direction and on-ball skills. In-season training focus is often based on metabolic conditioning activities such as small sided games, tactical and technical football drills and traditional running drills in order to further develop and maintain aerobic and anaerobic capacity. However, this often comes at the expense of strength training, which may be compromised for additional time on the pitch. Therefore, the purpose of this review is to provide an evidenced-based approach, to the periodisation of strength and power during a football season. Secondly, the increased popularity of incorporating on-pitch pre-rehabilitation and injury prevention programs that utilise unstable exercises during the entire season to maintain strength and power will also be discussed. Collectively, literature suggests that strength and power maintenance can be achieved with one strength session per week for football athletes. However, it is important for strength and conditioning coaches to continue the development of strength and power characteristics during a football season, which not only assists on-pitch performance but may also attribute to reduction of injury risk. Evidence supporting the effectiveness of on-pitch pre-rehabilitation programming centred on unstable training is lacking at this time.

Psychophysiological Responses and Cognitive Performance: A Systematic Review of Mental Fatigue on Soccer Performance

Background: Successful performance in soccer is associated with multiple factors such as physical, technical and perceptual-cognitive performance. In contrast to physical fatigue, nowadays one of the most popular affecting factors is mental fatigue, especially in soccer. Objectives: This systematic review aims to clarify the impact of mental fatigue intervention on psychophysiological responses and cognitive performance in soccer. Methods: A literature review was conducted by using the keywords of “mental fatigue and soccer” and “cognitive fatigue and soccer” in the content of confined space, psychophysiological and cognitive performance in soccer within the databases of Pubmed, Scopus, Web of Science (WOS) and Sport Discuss from the 1st of January 2010 to the 31st of January 2022. Systematic searches of six databases resulted in consist of 7 studies. The study was characterized based on PICO (Population, Intervention, Comparison and Outcome) criteria. Results: The current results showed that mental fatigue had a negative impact on psychophysiological responses, impaired cognitive performance, and decreased utilization of technical skills. Conclusions: According to this systematic review, mental fatigue reduces performance via impairing psychophysiological responses, cognitive performance, and technical skills in soccer.

Development of the “interactivelab” platform for network analysis in soccer

Aims: To introduce a platform called “InteractiveLab” (ILab) for collecting and analyzing ball passing networks during soccer games. Methods: The software was organized to collect data through a mobile interface and touch screen and simultaneously access that data from a remote database, allowing the automated acquisition, storage, and processing of data during games through an application from the web. The analysis is based on the concept of social networks, characterized by the interaction of players through passing exchanges. Results: This descriptive study presents the construction architecture and functioning of the developed software. It also presents the results of intra- and inter-rater reliability and a comparison with the manual collection method. Data were extracted and viewed according to the attacking unit classifications, with the following four outcomes: (a) interception, (b) lost ball, (c) incompletion, and (d) completion. This classification allows for the configuration of the data for a more precise analysis. Some limitations were highlighted, as well as future projections for the improvement of applications and analysis of the interactions network in the context of soccer. Conclusion: It is concluded that the InteractiveLab platform is a viable and beneficial tool that offers new possibilities for analysing performance in soccer. Moreover, given the lack of solutions that work similarly, this product also has market potential.

Relationship between training load and match running performance in men's soccer

Objectives: The main aim of this study was to test the relationship of volume and weekly training intensities with match-running demands of professional male soccer players. Methods: The training volume and intensity load and match demands of 18 professional soccer players (age: 20.7 ± 1.8 years) were monitored daily for 15 weeks. Results: Match high-speed running (mHSR) and match accelerations (mACC) were found to be moderately positively correlated with weekly HSR (wtHSR) and weekly ACC volume (wtACC) (r = 0.497; p &lt; 0.01; r = 0.367; p &lt; 0.01), respectively. Moderate positive correlations were also found between mHSR, mACC and mHSR and mean training intensity of ACC (mtACC) (r = 0.366; p &lt; 0.01). Conclusions: Weekly HSR and ACC training volume and intensity have moderate associations with match HSR and ACC loads. Coaches should prioritize weekly HSR and ACC during the training process, for ensuring that players perform more HSR and ACC actions during the subsequent match.

Putting the player first: A method to analyse and develop expert players performance in professional soccer

ABSTRACT The purpose of this paper is to present a player-centred performance analysis method as an effective mechanism to enhance expert players’ performance in professional soccer. Data were collected through an application of a developed performance analysis method and subsequent unstructured interviews to explore participants’ experience of the applied methodology. Grounded by an enactive perspective of human activity, the applied methodology foregrounds the player’s intrinsic experience and places the player at the centre of the analysis and interpretation process. The sample included thirty professional soccer players with professional playing experience ranging from 2 years to 19 years and representing three professional teams. Using an interpretive data analysis approach results were considered from a methodological perspective concerning the core functions of a performance analysis method. Categories regarding performance analysis and performance development were highlighted. Findings demonstrate that adopting a player-centred approach to performance analysis in professional soccer provides advancement of the understanding of the collective performance of the expert player and may increase the opportunity for sustained learning.

Implementing High-speed Running and Sprinting Training in Professional Soccer.

High-speed running and sprinting training play an important role in the development of physical capabilities, sport-specific performance and injury prevention among soccer players. This commentary aims to summarize the current evidence regarding high-speed running and sprinting training in professional soccer and to inform its implementation in research and applied settings. It is structured into four sections: 1) Evidence-based high-speed running and sprinting conditioning methodologies; 2) Monitoring of high-speed running and sprinting performance in soccer 3) Recommendations for effective implementation of high-speed running and sprinting training in applied soccer settings; 4) Limitations and future directions. The contemporary literature provides preliminary methodological guidelines for coaches and practitioners. The recommended methods to ensure high-speed running and sprinting exposure for both conditioning purposes and injury prevention strategies among soccer players are: high-intensity running training, field-based drills and ball-drills in the form of medium- and large-sided games. Global navigation satellite systems are valid and reliable technologies for high-speed running and sprinting monitoring practice. Future research is required to refine, and advance training practices aimed at optimizing individual high-speed running and sprinting training responses and associated long-term effects.

The Hero Myth in Popular Culture: The Hong Kong Action Hero in Shaolin Soccer and the Hollywood Kung Fu Champion in The Matrix

Cinema basically blends art and economics, and the film industry presents the hero figure in a fascinating image and action. The film visualizes a brave man or woman’s physical actions in the service to mankind. The action hero in his or her physical actions with rigor and passion to serve his or her nation and community exemplifies what a man or woman should be doing irrespective of personal interests. In the Wachowski brothers –directed The Matrix (1999), Neo Anderson, featuring Keanu Reeves, uses kung fu techniques and a rigorously trained body to save humans from the invasion of machines. Similarly, Steel Leg Sing in Stephen Chow’s Shaolin Soccer (2001), projecting Chow himself as an athletic champion, leads the Shaolin brothers team to a sweeping victory. The computer-generated graphic effects of the soccer hero’s exceptional performance in Shaolin Soccer and that of Neo’s virtual kung fu actions in The Matrix replicate a simulated body through electronically created computer graphics. Mighty Steel Leg Sing’s soccer game, enhanced by his acrobatics, embodies a perfect blend of the body, mind, and spirit. During a prestigious tournament, Sing sports soccer in such a spectacular feats that blends the athletic body and the twentieth-century popular sport. Like the digitized Neo in The Matrix, the soccer hero in Shaolin Soccer exposes magnificent feats of swirling movements and swift physical actions in the soccer field. The computer-generated graphics reposes the dynamics of the hero’s spiritual journey in the modern time human-computer interface brought to audiences through popular media and digital technology, shared by the Hong Kong and Hollywood action cinemas.

Tactical performance in soccer: The importance of choosing and properly applying tactical assessment tools

BACKGROUND: Several tactical assessment tools have been proposed to evaluate players’ tactical performance in more representative contexts. AIM: Investigate the associations between the results from different tactical assessment tools, TSAP and FUT-SAT, in youth soccer players. METHOD: Twenty-seven male U-11 soccer players participated in the study (mean age = 11.02 ± 0.78 years). The players' technical-tactical performance was analyzed using the Team Sports Performance Assessment Procedure - TSAP. The players' tactical performance and behavior were assessed using the Football Tactical Assessment System - FUT-SAT. Pearson's correlation test was used to analyze the association between the performance indexes obtained through both assessment tools. RESULTS: With the results obtained, it was possible to verify that both assessment tools' performance indexes did not present a significant correlation. CONCLUSION: Through these results, it is possible to conclude that it is extremely necessary for the correct choice of the assessment tools to be used, considering their characteristics and limitations, as well as their relation with tactical content to be emphasized during the learning process.

Diachronic analysis application for the detection of soccer performance standards: a case study

Abstract The purpose of this study is to analyse the interactions established by Gerard Piqué, one of the world’s top defenders, within the general context of the team. The type of interactions (prospective and retrospective) that the player establishes with his team mates and opponents; the parts of the pitch where his actions are set in motion and later finished; his preferred skill or technical moves and finally his influence on set pieces are taken into account. His output throughout the 2012 UEFA Euro, where he played 100% of the game. The instrument of observation proposed by Maneiro &amp; Amatria (2018) has been used. Polar coordinate analysis, a powerful tecnique capable of reducing the amount of data without compromising its analytical capacity, was used. The results showed significant interactions with players from the same and different lines thanks to technical actions of control, pass and carrying of ball. The analysis of the player’s interactions within the general context of the team will provide other teams with tactical alternatives and allow themto organise special training programmes.

Decomposing the variability of match physical performance in professional soccer: Implications for monitoring individuals

The aims of this study were to establish sources of variability in match physical performance of professional soccer players and provide a method for monitoring individual between-match changes. Eleven players meeting the final inclusion criteria were monitored through an entire in-season competition phase (n = 240 individual match observations). Ten Hertz global positioning systems were used to measure match total distance (TD), total high-speed running distance (≥ 21 km·h−1; HSRD), total accelerations (TAcc) and maximum running velocity (V max). Between-player, between-position, between-match and within-player variability were determined through linear mixed effects models. These data were then used to establish the practical significance of individual changes using a Minimum Effects Testing framework. All sources of variability were greater for HSRD (13–36%) when compared with all other metrics (<6%). Using combined between-match and within-player variability along with the smallest worthwhile change (0.2 × between-player SD), between-match individual changes of ±∼10–15% in TD, TAcc and V max were established as practically significant. For HSRD, these thresholds were considerably higher (≥60%). In conclusion, the ability for soccer practitioners to identify meaningful changes in match physical performance can aid decision making around player management following competition. Our study provides a method to flag changes beyond the normal match-to-match variability and by a substantial magnitude. This may have implications for recovery but should be combined with other sources of data (internal load and response) and used only as an adjunct to practitioner domain knowledge/experience.

Explicit and implicit activation of gender stereotypes additively impair soccer performance and learning in women

Studies involving the manipulation of instructions regarding the negative characteristics of a group or comparisons with members of another group (explicit activation of stereotypes) have shown that age, weight, and gender stereotypes can be harmful to motor performance and learning. To date, however, no study has observed whether implicit stereotype threats, such as the sex of the coach or experimenter, can also influence the acquisition of motor skills. In the present study, the individual and combined impact of implicit and explicit influences of gender stereotype on women’s soccer performance and learning was examined. In a 2 × 2 design, 60 women were divided into four groups according to the presence or absence of explicit (ES) and implicit (IS) stereotypes: ES/IS, ES, IS, and control. The groups with implicit activation practiced in the presence of a male experimenter. The groups with explicit activation received instructions activating the gender negative stereotype. The control group practiced without stereotype activations. The results showed that both explicit and implicit activation additively impaired soccer performance and learning, with both main effects being significant for practice and retention. The ES/IS group showed lower scores on the task relative to the other groups, while the ES and IS groups showed worse scores compared with the control group. The findings suggest that stigmatised populations may be forced to cope with more than one social identity threat while learning sport motor skills and indicate the importance of further studies testing strategies to minimise the deleterious effects of negative stereotypes.

The validity of small-sided games in predicting 11-vs-11 soccer game performance.

Predicting performance in soccer games has been a major focus within talent identification and development. Past research has mainly used performance levels, such as elite vs. non-elite players, as the performance to predict (i.e. the criterion). Moreover, these studies have mainly focused on isolated performance attributes as predictors of soccer performance levels. However, there has been an increasing interest in finer grained criterion measures of soccer performance, as well as representative assessments at the level of performance predictors. In this study, we first determined the degree to which 7-vs-7 small-sided games can be considered as representative of 11-vs-11 games. Second, we assessed the validity of individual players' small-sided game performance in predicting their 11-vs-11 game performance on a continuous scale. Moreover, we explored the predictive validity for 11-vs-11 game performance of several physiological and motor tests in isolation. Sixty-three elite youth players of a professional soccer academy participated in 11 to 17 small-sided games and six 11-vs-11 soccer games. In-game performance indicators were assessed through notational analysis and combined into an overall offensive and defensive performance measure, based on their relationship with game success. Physiological and motor abilities were assessed using a sprint, endurance, and agility test. Results showed that the small-sided games were faster paced, but representative of 11-vs-11 games, with the exception of aerial duels. Furthermore, individual small-sided game performance yielded moderate predictive validities with 11-vs-11 game performance. In contrast, the physiological and motor tests yielded small to trivial relations with game performance. Altogether, this study provides novel insights into the application of representative soccer assessments and the use of continuous criterion measures of soccer performance.

Importance of anthropometric features to predict physical performance in elite youth soccer: a machine learning approach

ABSTRACT The present study aimed to determine the contribution of soccer players’ anthropometric features to predict their physical performance. Sixteen players, from a professional youth soccer academy, were recruited. Several anthropometric features such as corrected arm muscle area (AMAcorr), arm muscle circumference (AMC) and right and left suprapatellar girths (RSPG and LSPG) were employed in this study. Players’ physical performance was assessed by the change of direction (COD), sprint (10-m and 20-m), and vertical jump (CMJ) tests, and Yo-Yo Intermittent Recovery Test level 1 (Yo-Yo IRT1). Using an extra tree regression (ETR) model, the anthropometric features permitted to accurately predict 10-m sprint, 20-m sprint and Yo-Yo IRTL 1 performance (p < 0.05). ETR showed that upper-body features as AMAcorr, and AMC affected 10-m and 20-m sprint performances, while lower-body features as RSPG and LSPG influenced the Yo-Yo IRTL 1 (Overall Gini importance ≥ 0.22). The model predicting COD and CMJ presented a poor level of prediction, suggesting that other factors, rather than anthropometric features, may concur to predict their changes in performance. These findings demonstrated that the upper- and lower-body anthropometric features are strictly related to sprint and aerobic fitness performance in elite youth soccer.