Modern association football imposes on players more and more demanding requirements related to their pre-competitive training. The preparation of soccer players before the competitive season affects to a great extent the practical implementation of players’ tactical and technical skills during matches. The proper selection of training strategies and technologies in players’ preparation is only possible when the structure and character of players’ match effort and the energy costs of particular motor activities are well known (4,16,22,28,32,40). The lack of such information prevents any application of soccer-specific loads of optimal character, volume, and intensity to develop soccer-specific motor abilities. Only game-specific loads have a positive effect on structural and functional changes in the player’s body and ensure the development of sport-specific motor skills (6).
Match analyses show that elite soccer players cover an average distance of between 9 and 14 km and vary their running pace between 1.4 and 10 m·s−1, 800–1,000 times during a match (7,10,23,34). The distance covered during a match depends on the player’s training experience, fitness, and position of play (3,9,19,28,32). Soccer is a non-cyclical and intermittent sport in which short-duration maximum-intensity activities, for example, sprint runs over a distance of 10–20 m, and high-intensity actions, such as counterattacks, are intertwined with activities of low and moderate intensity (marching and jogging) and with pauses, for example, standing (9,19,28,31).
During the overall soccer match effort (90 minutes), a player’s energy is produced by aerobic processes, whereas during high-intensity and maximum-intensity match phases, it is produced by anaerobic processes (23). Thus, soccer players make use of all energy sources involved in ATP resynthesis. Bangsbo (6) showed that more than 90% of a soccer player’s energy during a match comes from aerobic processes that supply the energy to run at low and moderate speeds and to payoff oxygen debt run up during match phases of high and maximal intensity.
The total distance covered by soccer players during a match is a fairly superficial method of game assessment (37). A comprehensive game analysis must account for the number and frequency of sprint runs and other activities of maximal and submaximal intensity performed by players. Running around the pitch at such levels of intensity requires the highest energy expenditure and thus, from the standpoint of physiological and motor assessment, is highly significant in modern soccer (14).
Sprinting is one of the most important activities in soccer, although it merely constitutes between 1 and 12% of the mean total distance covered by a player during a match, that is, from only 0.5 to 3% of playing time (37,42,45). During a competitive game, players perform 2- to 4-second long-sprint runs, every 90–180 seconds on average. It is assumed that players of higher ability cover longer sprinting distances with higher intensity (27). With regard to the player’s position of play, training experience and use of various methods of monitoring and classification of particular distances in individual phases of intensity, the average sprinting distance covered during a match is between 200 and 1,200 m (9,28,32,33).
For several years, different authors have studied the sprinting activity of soccer players using statistical data from national league games, for example, English (12,13,20), Italian (28), and Spanish (19), and combined data from league competitions and cup playoffs (19,28,32). Di Salvo et al. (18) carried out a comprehensive sprinting analysis of soccer players during the European Champions League and UEFA Cup (now Europa League) competitions. However, no comprehensive independent research has been conducted on sprinting activities of soccer players during UEFA Cup matches only.
The aim of this study was a detailed analysis of sprinting activities of professional soccer players during UEFA Cup matches with regard to their position of play and sprint duration.
Experimental Approach to the Problem
The authors assumed that the application of the Amisco Pro–computerized tracking system enabled an objective analysis of sprinting performance and total sprinting distance covered by professional soccer players. It is argued that the player’s position on the pitch affects the duration of performed sprint runs and the length of sprinting distance covered during a match. Statistically significant differences were assumed to exist between the player’s position and the following variables: total sprinting distance, total number of performed sprint runs, and the number of sprint runs in particular ranges according to the sprinting distance and duration.
Ten European Football Association (UEFA) Cup matches from the 2008–09 and 2010–11 seasons were examined. The analysis involved the activities of 147 players participating in entire matches, excluding the goalkeepers. Each outfield player was assigned to 1 of 5 positional groups: central defenders (CD, n = 39), external defenders (ED, n = 35), central midfield players (CM, n = 35), external midfield players (EM, n = 20), and forwards (F, n = 18). The profile of different playing positions was based on players’ activities on the pitch and the primary area in which these activities were carried out, as in Di Salvo et al. (19). Five of the examined matches ended in a draw, the home team won in 3 matches, whereas the visiting team won in 2.
The authors received written consent from the authorities of the KKS Lech Poznan football club to use statistical data provided by the club. The players were informed about all experimental procedures, and informed consent was obtained from them to participate in the study. To ensure club and players’ confidentiality, all performance data were anonymous. The study was conducted in compliance with the Declaration of Helsinki and was approved by the local ethics committee (No. 339/02). The study protocol was also approved by the Board of Ethics of The University School of Physical Education in Poznan.
The physical performance of players during the matches was examined using a computerized, semiautomated multicamera tracking system (Amisco Pro, version 1.0.2, Nice, France). Movements of all 20 outfield players from 2 competing teams were recorded during the game by 8t fixed synchronized cameras with a frame rate of 25 Hz, positioned at the top of the Municipal Stadium in Poznań, Poland (3). Signals and angles registered by the cameras were sequentially converted into digital data and stored on 6 computers for post-match analysis. Zubillaga et al. (46) have recently evaluated the reliability and validity of Amisco Pro for quantifying displacement velocities during match-related activities relative to data obtained using timing gates.
For the purpose of the study, only players’ sprinting activity was analyzed. Data on the covered sprinting distances, time spent performing sprint runs of different duration and distance category, and frequency of occurrence of each activity for players in different positions were obtained with the aid of specially developed software (Athletic Mode; Amisco Pro). Because of its technical specifications, the Amisco Pro System allowed registration of players’ activities as sprint runs at the speed of ≥24 km·h−1 that were no shorter than 1 seconds. The total number of sprints, total sprint distance covered, and the percentage of each sprint type were assessed. As far as sprint duration was concerned, 2 types of sprints were distinguished: S, short-duration sprint (below 5 seconds) and L, long-duration sprint (above 5 seconds). In addition, these sprints were classified by distance category: 0–10, 10.1–20.0, and >20 m.
All parameters were checked for their conformity to normal distribution. The conformity assessment was carried out with the Lilliefors test (p < 0.01). For all the parameters, the following descriptive statistics were calculated: arithmetic means, medians, interquartile ranges, and SDs to assess the total sprinting distance and the number of sprint runs in terms of their distance and duration, performed by players in different positions on the pitch (Figures 1 and 2)
Multifactor analysis of variance (ANOVA) was used to compare players’ positions and sprint categories in terms of covered distance and duration. Tukey’s honestly significant difference was used for the analysis of significant differences between mean values. The significance of differences between mean total sprinting distance and number of performed sprints was checked with the Kruskal-Wallis 1-way ANOVA. In the case of significant differences between mean values, the post-hoc test for multiple comparisons was used.
The level of statistical significance was set at p < 0.05. The number of sprint runs by distance category (0–10, 10.1–20.0, and >20 m) performed by players in 5 different positional groups on the pitch followed a pattern of conformity; however, the absolute intraclass correlation was not high (rICC = 0.5917). The analysis of the number of sprints performed by players in 5 different positional groups (CD, ED, CM, EM, and F) during the match with regard to sprint duration (0–5 and >5.01 seconds) showed, however, weak conformity with the absolute intraclass correlation of rICC = 0.3502. All statistical calculations were made using the STATISTICA 9.1. software package.
Total Sprint Distance
The statistical analyses revealed that the mean total sprint distance (≥24 km·h−1) covered by all examined players (n = 147) amounted to 237 ± 123 m. Statistically significant differences were found between all players’ positional groups (p < 0.05), in particular, between the forwards, external midfielders, and external defenders and between central defenders and central midfielders (p < 0.05).
As far as players’ positions of play were concerned, the longest sprint distance was covered by the forwards 345 ± 29 m, followed by external midfielders (314 ± 123 m), external defenders (265 ± 121 m), and central defenders (186 ± 82 m). The lowest total mean sprint distance was covered by the central midfielders (167 ± 87 m) (Figure 1).
Total Number of Sprints
The total number of sprints performed by all players (n = 147) was 11.2 ± 5.3 sprints per match. Statistically significant differences in terms of total numbers of performed sprints were noted between positions of play (p < 0.05), that is, between the forwards, external midfielders, central defenders, and central midfielders (p < 0.05) (Table 1).
Number of Sprints by Sprint Duration Categories: Short-Duration Sprint (S) and Long-Duration Sprint (L)
With regard to the total number of short-duration sprints (S), statistically significant differences were found between all positions of play (p < 0.05). The forwards and external midfielders performed a far greater number of short-duration sprints (S), statistically, than the central midfielders and central defenders (p < 0.05). There were statistically significant differences between all positions of play as far as long-duration sprints were concerned (L) (p < 0.05). The external defenders ran the highest number of long-duration sprints (L) and only differed significantly from the central midfielders, who performed fewest sprints (L) (p < 0.05) (Table 1). The ratio between the numbers of short-duration sprints (S) and long-duration sprints (L) displayed no significant differences (p ≥ 0.05) between the positions of play, with the exception of external defenders vs. central midfielders (p > 0.05).
Number of Sprints by Distance Categories
Figure 2 presents the positional differences for each of the 3 sprinting distance categories: 0–10, 10.1–20.0, and >20 m. In the 0- to 10-m category, the highest number of sprints was performed by external defenders and external midfielders (0.9 ± 1.0). There were no statistically significant differences between players’ positions on the pitch. In the 10.1- to 20.0-m category, central defenders and central midfielders performed fewer sprints (3.9 ± 2.5 and 4.5 ± 2.6, respectively) than external midfielders 7.1 ± 2.4 and forwards 7.8 ± 2.9 (p < 0.05). In the category of sprints longer than 20 m, most sprints were performed by the forwards (7.4 ± 3.7) and external midfielders (6.9 ± 3.2) vs. central midfielders (3.4 ± 2.5) and central defenders (4.1 ± 1.9), whereas the external defenders (5.5 ± 2.7) ran significantly more sprints than the central midfielders (p < 0.05) (Figure 2).
The percentage distribution of the total number of sprints run by all studied soccer players (n = 147) in the 3 distance categories was: 7 ± 9% for distances between 0 and 10 m, 48 ± 16% for distances between 10.1 and 20.0 m, and 45 ± 17% for distances longer than 20 m.
For the last few decades, there has been an increasing research interest in match analysis in association football (2,3,8,9,18,19,21,39,41). Match analysis data describing physical, technical, and tactical parameters allow soccer coaches to identify positive and negative aspects of preparation of the team and individual players. Match analysis also provides important information about the physiological requirements of soccer players during real competition. Thanks to such data, comprehensive training plans can be devised for individual players in relation to their respective positions on the pitch.
In recent years, time-motion analysis for assessment of players’ physical (motor) parameters has gained significance in game monitoring processes. Studies have revealed that some of the most important activities of players during a match are those of maximal intensity (13,17,18). The present study aimed to assess the sprinting activity of elite soccer players participating in the UEFA Cup competition, with regard to their position of play.
The obtained results show that the mean total sprinting distance (≥24 km·h−1) covered by 147 professional players was 237 m (± 123 m), regardless of their position on the pitch. These results correspond to those of other authors (9,13,18), who carried out their studies with the aid of various match analysis techniques for assessment of players at different levels of competitive experience. Authors who have used the Amisco Pro–computerized match analysis system include Di Salvo et al. (19), Dellal et al. (17), Lago et al. (25), and Andrzejewski et al. (3). In their studies, the total sprinting distances covered by examined players amounted to 271 ± 137, 237 ± 74, 275 ± 101, and 255 ± 138 m, respectively, that is, similar to the results of the present study (Figure 1).
The analysis of sprinting distances covered by players with regard to their positions of play during the matches revealed that the forwards, external midfielders, and external defenders covered statistically longer sprinting distances than the central defenders and central midfielders. These results are similar to Lago et al. (25).
The total number of performed sprints is another parameter of maximal intensity activity performed by players during a soccer match (21,41,45). The kinematic analysis of 10 UEFA Cup matches showed that the mean number of sprint runs performed by examined players amounted to 11.2 ± 5.3 sprint runs per match. This is similar to the findings of Rey et al. (36) who recorded 12.5 ± 3.7 sprints per match in their study of players from the Spanish Soccer League. The analysis of the number of sprints per match with regard to players’ positions on the pitch shows that the highest sprinting activity was characteristic of the forwards (15.9 ± 5.1), who ran almost twice as many sprints per game as central midfielders and central defenders. The same observations were made by Di Salvo et al. (19) in their match analysis with the use of Amisco Pro, who also revealed that the fewest sprints were performed by central midfielders and central defenders.
In the present study, sprint runs were analyzed with regard to their duration. Amisco Pro enables an analysis of short-duration sprints (S) and long-duration sprints (L). It is interesting that no significant differences were found between the number of short- and long-duration sprints performed by elite players participating in UEFA Cup matches with regard to their position of play. The results show clearly that about 90% of all sprinting activities of professional soccer players were shorter than 5 seconds (S) and that only 10% were longer than 5 seconds. The greatest number of long-duration sprints (L) was performed by the external defenders. This is certainly indicative of the growing intensity of defensive play in soccer and the frequently used match play system, which relies heavily on wingers and their tactical tasks both in attack and in defense.
One of the most significant problems in speed training practice is choosing the right proportions between training load duration and rest. The proportion should follow the rule: the longer the running distance, the longer the cool-down phase (1,15,26,30).
During repetitive speed exercises, the contribution of phosphocreatine hydrolysis to the meeting of energy demand of working muscles increases after each loading (5,11). The cool-down phase duration depends not only on the stimulation of the central nervous system but also on the rate of recovery of the autonomic nervous system functions related to the payoff of oxygen debt run up during physical exercise and on the rate of phosphocreatine resynthesis (6,24,28,43). The duration of rest and the length of running distances should be adjusted for individual players and their respective positions of play.
The present study showed that the dominant sprinting activities during UEFA Cup matches were sprints between 10 and 20 m and longer than 20 m. They constituted, respectively, 48 and 45% of the total number of sprints performed by players in analyzed matches (Figure 2). The most active sprinters in the analyzed distances were forwards and external midfielders who indeed ran more sprints than central defenders and central midfielders. However, no significant differences were observed between sprint runs performed by players in different positions over distances between 0 and 10 m. The Amisco Pro system does not permit an identification of players’ sprinting activity shorter than 1 second. Therefore, the actual number of recorded sprints over the distance between 0 and 10 m was low.
The obtained results concerning selected aspects of analysis of the sprinting activity of soccer players during UEFA Cup matches correspond to the results of other authors who revealed that the sprinting distances covered by elite soccer players were mostly between 10 and 20 m and longer than 20 m (8,29,35,38,44).
The aim of the present study was to examine differences between players’ positions on the pitch and to quantify demands placed on elite soccer players in individual positions during match play. The analysis of elite-level match play focusing on sprinting activity (≥24 km·h−1) and assessing the total number of sprints, sprinting distance covered and the percentage of each sprint duration category, revealed a number of statistically significant differences between different playing positions. There is a consensus in sports science that the most effective pre-competition training is the one that most closely replicates competitive performance conditions. Therefore, training prescriptions in soccer should also be based on the specific requirements of playing positions and thereby ensure that players are better prepared to fulfill their tactical responsibilities during the game.
During a soccer game, players undertake a variety of activities requiring rapid generation of power, for example, sprint runs and sudden changes of running directions. Such activities directly affect the match score, and speed is one of the main foundations of effective implementation of tactical and technical guidelines in matches. Pre-competition speed training of soccer players and maintaining a relatively high level of speed capabilities during the competitive season is one of the key issues in modern soccer training.
The results of the study provide important information on the role of sprinting activities of elite soccer players competing in UEFA Cup matches and reveal significant differences in sprinting activities with regard to the players’ respective positions on the pitch. This information can be used for appropriate planning and implementation of speed training, accounting for the total sprint distance and the number and category of sprints performed by players in different positions.
In coaching practice, even in professional soccer clubs, the same speed training loads are used for players in different positions on the pitch. If trainers follow this methodological uniformity, then speed training will be ineffective as it will fail to account for players’ specific abilities.
The study results clearly indicate the necessity to apply distances between 10 and 20 m and longer than 20 m in soccer speed training. Also, individual adjustment of speed training loads is very significant. It seems unfounded, therefore, to apply 40- or even 50-m running distances in soccer speed training. Training players to run such long distances can lead to the emergence of lactate in muscles and blood.
The planning of speed training should not merely be confined to performing exercises without the ball at the coach’s command. Match analysis shows that a player’s running speed is strictly related to rapid changes of running directions, stopping, changes of running pace, speed of perception, anticipation, reaction, and decision making. Thus, speed exercises with a partner and the ball involving coordination elements will greatly improve the training process as they develop the motor and cognitive speed components and make soccer training closer to real competition conditions.
In motor training aimed at the development of speed abilities, the individualization of training loads is perfectly justifiable. The crucial issue is whether the speed abilities of soccer players can be shaped regardless of their positions of play. It is argued that the training of forwards and external midfielders should involve sprinting speed components to a far greater extent than the loads for central defenders and central midfielders. The sprinting loads should be greater for the former in terms of number of sprint runs of short and long duration during particular training sessions. Sprinting distances in training sessions, in particular, in the ranges from 10.1 to 20.0 m and longer than 20 m, should also be adjusted for respective positions of play. The number of sprinting distances within these 2 ranges is, in fact, twice as large in forwards and external midfielders than in central defenders and central midfielders. The results of the present study show that soccer coaches can effectively apply comparable sprinting speed loads in the training of all players regardless of their position on the pitch, only in the range of 0–10 m.
The use of the Amisco Pro–computerized match analysis system permits the design of the most optimal speed training loads in the process of preparation of professional soccer players. The structure of such training loads should be individually adjusted in terms of sprinting distance and number and types of sprints with regard to the player’s position on the pitch. This will ensure training efficiency and will positively affect the development of players’ speed competences in the pre-competitive and competitive periods.
The authors wish to express their thanks to the authorities of KKS Lech Poznan football club for making available their statistical match data for research purposes. The results of the present study do not constitute endorsement of Amisco Pro or KKS Lech Poznan football club by the authors or the NSCA.
1. Abt G, Siegler JC, Akubat I, Castagna C. The effects of a constant sprint-to-rest ratio and recovery mode on repeated sprint performance. J Strength Cond Res 25: 1695–1702, 2011.
2. Ali A, Farrally M. A computer-video aided time-motion analysis technique for match analysis. J Sports Med Phys Fitness 31: 82–88, 1991.
3. Andrzejewski M, Chmura J, Pluta B, Kasprzak A. Analysis of motor activities of professional soccer players. J Strength Cond Res 26: 1481–1488, 2012.
4. Arnason A, Sigurdsson SB, Gudmundsson A, Holme J, Engebretsen L, Bahr R. Physical fitness, injuries and team performance in soccer. Med Sci Sports Exerc 36: 278–285, 2004.
5. Balsom PD, Soderlund K, Sjodin B, Ekblom B. Skeletal muscle metabolism during short duration high-intensity exercise: Influence of creatine supplementation. Acta Physiol Scand 154: 303–306, 1995.
6. Bangsbo J. The physiology of soccer: With special reference to intense intermittent exercise. Acta Physiol Scand 151(Suppl. 619): 1–155, 1994.
7. Bangsbo J, Mohr M, Krustrup P. Physical and metabolic demands of training and match-play in the elite football players. J Sports Sci 24: 665–674, 2006.
8. Bangsbo J, Norregaard L, Thorso F. Activity profile of competition soccer. Can J Sport Sci 16: 110–116, 1991.
9. Barros R, Milton S, Misuta RP, Menezes PJ, Figueroa FA, Moura SA, Cunha RA, Neucimar JL. Analysis of the distances covered by first division Brazilian soccer players obtained with an automatic tracking method. J Sports Sci Med 6: 233–242, 2007.
10. Bloomfield J, Polman R, O’Donoghue P. Physical demands of different positions in FA Premier League soccer. J Sports Sci Med 6: 63–70, 2007.
11. Bogdanis GC, Nevill ME, Boobis LH, Lakomy KA. Contribution of phosphocreatine and aerobic metabolism to energy supply during repeated sprint exercise. J Appl Physiol 80: 876–882, 1996.
12. Bradley PS, Di Mascio M, Peart D, Olsen P, Sheldon BJ. High-intensity activity profiles of elite soccer players at different performance levels. J Strength Cond Res 24: 2343–2351, 2010.
13. Bradley PS, Sheldon BJ, Wooster B, Olsen P, Boanas P, Krustrup P. High-intensity running in English FA Premier League soccer matches. J Sports Sci 27: 159–168, 2009.
14. Carling C, Bloomfield J, Nelsen L, Reilly T. The role of motion analysis in elite soccer: Contemporary performance measurement techniques and work rate data. Sports Med 38: 839–862, 2008.
15. Clark P. Intermittent light intensity activity in English FA Premier League soccer. In J Perform Analysis Sports 10: 139–151, 2010.
16. Dellal A, Keller D, Carling C, Chaovachi A, Wong DP, Chamari K. Physiologic effects of directional changes in intermittent exercise in soccer players. J Strength Cond Res 24: 3219–3226, 2010.
17. Dellal A, Wong DP, Moalla W, Chamari K. Physical and technical activity of soccer players in the French First League—With special reference to their playing position. Int J Sports Med 11: 278–290, 2010.
18. Di Salvo V, Baron R, Gonzalo-Haro A, Gormasz C, Pigozzi F, Bachl N. Sprinting analysis of elite soccer players during European Champions League and UEFA Cup matches. J Sports Sci 28: 1489–1494, 2010.
19. Di Salvo V, Baron R, Tschan H, Calderon Montero FJ, Bachl N, Pigozzi F. Performance characteristics according to playing position in elite soccer. Int J Sports Med 28: 222–227, 2007.
20. Di Salvo V, Gregson W, Atkinson G, Tordoff P, Drust B. Analysis of high intensity activity in Premier League Soccer. Int J Sports Med 30: 205–212, 2009.
21. Ekblom B. Applied physiology of soccer. Sports Med 3: 50–60, 1986.
22. Hoff J, Wisløff U, Engen LC, Helgerud J. Soccer specific aerobic endurance training. Br J Sports Med 36: 218–221, 2002.
23. Krustrup P, Mohr M, Ellingsgaard H, Bangsbo J. Physical demands during an elite female soccer game: Importance of training status. Med Sci Sports Exerc 37: 1242–1248, 2005.
24. Krustrup P, Mohr M, Steensberg A, Bencke J, Kjaer M, Bangsbo J. Muscle and blood metabolites during a soccer game: Implications for sprint performance. Med Sci Sports Exerc 38: 1165–1174, 2006.
25. Lago C, Casais L, Dominguez E, Sampaio J. The effects of situational variables on distance covered at various speeds in elite soccer. Eur J Sport Sci 10: 103–109, 2010.
26. Little T, Williams AG. Specificity of acceleration, maximum speed and agility in professional soccer players. J Strength Cond Res 19: 76–78, 2005.
27. Mayhew SR, Wenger HA. Time motion analysis of professional soccer. J Hun Mov Stud 11: 49–52, 1985.
28. Mohr M, Krustrup P, Bangsbo J. Match performance of high-standard soccer players with special reference to development of fatigue. J Sports Sci 21: 519–528, 2003.
29. Müller E, Raschner C, Schwameder H. Analysis of perfomance in professional sports–significance, methods and fields of application. Spectrum Sportwiss 1: 47–70, 1998.
30. Osgnah C, Poster S, Bernardini R, Rinaldo R, Di Prampero PE. Energy cost and metabolic power in elite soccer: A new match analysis approach. Med Sci Sports Exerc 42: 170–178, 2010.
31. Rampinini E, Bishop D, Marcora SM, Ferrari Bravo D, Sassi R, Impellizzeri FM. Validity of simple field tests as indicators of match-related physical performance in top-level professional soccer players. Int J Sports Med 28: 228–235, 2007.
32. Rampinini E, Coutts AJ, Castagna C, Sassi R, Impellizzeri FM. Variation in top level soccer match performance. Int J Sports Med 28: 1018–1024, 2007.
33. Rampinini E, Impellizzeri FM, Castagna C, Coutts AJ, Wisloff U. Technical performance during soccer matches of the elite Italian Serie A league: Effect of fatigue and competitive level. J Sci Med Sport 12: 227–233, 2009.
34. Reilly T. The Science of Training—Soccer: A Scientific Approach to Developing Strength, Speed, and Endurance. New York, NY: Routledge, 2007.
35. Reilly T, Thomas V. A motion analysis of work rate in different positional roles in professional football match-play. J Hum Mov Stud 2: 87–97, 1976.
36. Rey E, Lago-Penas C, Lago-Ballesteros J, Casais L, Dellal A. The effect of a congested fixture period on the activity of elite soccer players. Boil Sport 27:181–185, 2010.
37. Rienzi E, Drust B, Reilly T, Carter JE, Martin A. Investigation of anthropometric and work-rate profiles of elite South American international players. J Sports Med Phys Fitness 40: 162–169, 2000.
38. Spencer M, Bishop D, Dawson B, Goodman C. Physiological and metabolic responses of repeated-sprint activities. Sports Medicine 35: 1025–1044, 2005.
39. Strøyer J, Hansen L, Klausen K. Physiological profile and activity pattern of young soccer players during match play. Med Sci Sports Exerc 36: 168–174, 2004.
40. Strudwick A, Reilly T, Doran D. Anthropometric and fitness profiles of elite players in two football codes. J Sports Med Phys Fitness 42: 239–242, 2002.
41. Tumilty D. Physiological characteristics of elite soccer players. Sports Med 16: 80–96, 1993.
42. Van Gool D, Van Gerven D, Boutmans J. The physiological load imposed on soccer players during real match-play. In: Science and Football. Reilly T., Lees A., Davies K., Murphy WJ., eds. London, United Kingdom: E & FN Spon, 1988. pp. 51–59.
43. Westerblad H, Allen D, Lannergren J. Muscle fatigue: Lactic acid or inorganic phosphate the major cause? News Physiol Sci 17: 17–21, 2002.
44. Winkler WA. A new approach to the video analysis of tactical aspects of soccer. In: Science and Football. Spinks W., Reilly T., Lees A., David K., Murphy A., eds. London, United Kingdom: E & FN Spon, 1988. pp. 368–372.
45. Withers R, Maricic Z, Wasilewski S, Kelly L. Match analysis of Australian professional soccer players. J Hum Mov Stud 8: 159–176, 1982.
46. Zubillaga A, Gorospe G, Hernandez A, Blanco A. Comparative analysis of the high-intensity activity of soccer players in top level competition. In: Science and Football VI. Reilly T., Korkusuz F., eds. United Kingdom: Routledge, 2009. pp. 182–185.