Several time–motion analysis studies have detailed the activity profiles of professional soccer match play (1,12). The consensus of reports shows outfield players to cover an average distance of 9 to 12 km during a game, according to the different positional roles (9). Although the prevalence of low-intensity activity experienced during match play elicits a greater reliance on aerobic metabolism (32), this is often interspersed with important bouts of high-intensity activity and sprinting (3,9) that coincide with decisive moments of the game (i.e., tackles, passes, and shots) (25).
Competing within the elite level of the sport, professional soccer players are expected to possess well-developed physical capabilities complementing the technical and tactical demands of contemporary soccer (21). In particular, superior aerobic capacity (4), muscular strength, power (41), and repeated sprint ability (RSA) could be critical components to combat the limited ball contacts encountered during match play, as reported for central defenders and central attacking midfielders competing in the English and Spanish first division (9). However, a congested schedule, as is often found among soccer's elite clubs, often makes it problematic for coaches attempting to simultaneous integrate these different training parameters (25).
The supposition that small-sided games (SSGs) may simulate the physiologic workloads and intensities commensurate of actual match play while also developing technical and tactical proficiency has led to its popularity as a training modality in the applied and scientific domain within recent years (7,10,11,20,28). Specifically, from an applied perspective, the potential to increase aerobic capacity with regular ball involvement may satisfy the sport scientist, coach, and players' demands, thus highlighting its advantages over generic training methods such as interval running training. However, manipulating the pitch size, number of games played, duration, coach encouragement, and technical restrictions have been shown to severely alter the physical and technical demands associated with SSG (7,10,11,20,31). In relation to these findings, it is of paramount importance that session design and SSGs implementation should be performed with precision and careful consideration of the training objective (18,34).
Conversely, the scientific evidence supporting SSG as a useful training modality has shown cardiovascular stress and training adaptations comparative to generic short-duration intermittent running training (8,26). Previous research conducted with Norwegian first division players concluded that SSGs could induce a steady-state exercise intensity of 91% of maximal heart rate (HRmax), corresponding to about 85% of maximal oxygen uptake (V[Combining Dot Above]O2max) (24). Studies have also shown SSG to elicit similar effects on aerobic capacity than 7 weeks of generic training in preseason (18) or 6 or 12 weeks of interval training in youth players during a competitive season (26,33). However, there seems a paucity of data pertaining to the effects of SSG on physical parameters other than maximal aerobic capacity. Moreover, claims of SSGs do not simulate the high-intensity repeated sprint demands of elite-level soccer that have been confined to female soccer players (14) and warrants further scrutiny to whether SSG can actually improve RSA test performance in elite male soccer players.
The aim of the present study was to examine the effects of a 4-week SSG (3 vs. 3 + GKs) training intervention over 7 sessions on the physical performance (i.e., speed, aerobic performance, and repeated sprint ability) of elite male soccer players during the in-season break. It was hypothesized that an SSG training intervention would induce a greater increase of the repeated sprint ability and sprint performance than increases on aerobic capacity. The findings could potentially provide valuable information to coaches for the design and promotion of the use of SSGs as part of a periodized conditioning program within elite-level soccer, especially during the in-season break.
Experimental Approach to the Problem
To examine the changes in physical performance after the 4-week periodized SSG training intervention, all players were tested during 2 sessions, 4 weeks apart (i.e., pre- and posttests) (Table 1). The study was conducted during the in-season break with players not involved within competitive fixtures over this period. The shutdown in fixtures over the intervention training period (4 weeks from start to finish) was because of a combination of poor weather, international fixtures, and a reserve team fixture break (for the nonplaying squad players). The structured periodized training intervention was able to be controlled at this specific period of the season to a greater level than normal because of this nonscheduled fixture break. This period was not designed as a recovery period as the domestic league does not have a scheduled mid-season break but because of the cancellation of fixtures/international breaks and nonselected players, it was an opportunity to run the intervention.
Testing sessions took place over a 2-day period at the same time of day to eradicate the potential effects of any circadian variation on the participants (day 1: anthropometry and repeated sprint ability assessments and day 2: running economy (RE) and lactate assessments). After the pretest, 7 SSG training intervention sessions (each lasting 45–90 minutes incorporating warm-up, low-intensity technical work, SSG sessions, and cooldown) were performed within the 4-week periodized program. In addition to SSGs, the players performed technical tactical sessions and some injury prevention exercises during the study period. However, these sessions (heart rate [HR] < 85% of HRmax) were not deemed as intense enough to induce any significant changes on the player's physical profile over a 4-week period. All players were fully familiarized with the experimental procedures and the requirements of the games before the present study because of the testing and training protocols being used within the club as part of its sports science and conditioning structure.
Fifteen, elite, male, professional soccer players (age: 24.5 ± 3.45 years; height: 181.1 ± 5.78 cm; body mass: 78.7 ± 7.67 kg; V[Combining Dot Above]O2max: 54.88 ± 5.25 ml·kg−1·min−1; and sum of 8 skinfold sites: 55.79 ± 15.15 mm) from a Scottish Premier League team volunteered for the investigation. Written informed consent was received from all players after a brief but detailed explanation about the aims, benefits, and risks involved with this investigation. Players were told they were free to withdraw from the study at any time without penalty. The study was conducted according to the Declaration of Helsinki, and the protocol was fully approved by the Sports Science Department at Rangers Football Club before the commencement of the assessments. The players had refrained from vigorous high-intensity exercise 24 hours before the testing sessions. During the study, all players were instructed to maintain normal daily food and water intake and no dietary interventions were undertaken. The training sessions performed pre intervention were general football training sessions (inclusive of low-, moderate-, and high-intensity training sessions) depending on the training week and fixture calendar. The players involved within this study would have generally performed some strength (low-level prevention sessions), tactical, technical (low intensity sessions), and conditioning sessions (inclusive of RSA activities and aerobic intervals). During the 4-month buildup to the start of the intervention, the players involved within the study had played a combination of reserve team games, first team games, and generic football training.
The anthropometric measurements included height, body mass, and summation of 8 skinfold sites using Harpenden calipers (biceps, triceps, subscapular, iliac crest, supraspinale, abdominal, midthigh, and calf) to determine the body fat level (29).
Repeated Sprint Ability Assessments
After a standardized warm-up, which involved running at 10 km·h−1 at 70% HRmax for 10 minutes followed by 5-minute bursts of self-selected running and stretching, players performed the RSA assessment consisting of 6 × 20-m maximal sprints, with a 25-second active recovery period to walk back to the start (15). The sprint time was measured using photocells (0.01-second precision; Brower Timing Systems, UT, USA) placed at the start, 10, and 20 m at the height of 1 m. The players started 0.7 m behind the starting gate. Repeated sprint ability was analyzed by 3 methods: (a) the fastest sprint time (FST) among the sprints, (b) total sprint time (TST), and (c) percentage decrement score (%Decre). The TST was used as it has been recommended by previous study of RSA in soccer players (44). The %Decre was selected as it was recently reported as the most valid and reliable method of quantifying fatigue in RSA test (16). Concerning the FST of 20 m, the associated 10-m split time was also selected for analysis.
Submaximal Treadmill Test
Before commencing the treadmill test, each player performed a 5-minute jogging warm-up on the motorized treadmill (Technogym, Run 500 model, Italy) at a velocity that elicited approximately 60% of the player's HRmax, which was obtained from the player's previous maximal treadmill test. Thereafter, the players performed 5 minutes of individually selected stretching exercises. After this warm-up, each player performed a submaximal running test at 3% gradient for 3 individual 3-minute running stages (stage 1: 9 km·h−1; stage 2: 11 km·h−1; and stage 3: 14 km·h−1). HR responses (Polar Team System, Kempele, Finland), oxygen uptake (V[Combining Dot Above]O2) (Medgraphics, London, United Kingdom), respiratory exchange ratio (RER), respiratory rate (RR), and blood lactate samples were taken at the last 15 seconds of each exercise stage. Capillary blood samples were withdrawn from the players' thumb and analyzed for whole blood lactate using an Analox GM7 analyzer (Analox Instruments, London, UK). The V[Combining Dot Above]O2 value obtained at the end of each stage from this test represented the RE of the players.
Small-Sided Game Training Intervention
All games were preceded by a standardized warm-up of 12 minutes followed by a 3-minute passive recovery. During this period, players were informed to only consume water if needed. All games were carried out on an outdoor grass field with an average temperature of 16 ± 1.74° C. The SSGs consisted of teams of 3 outfield players plus a goalkeeper being played on a 30 × 25-m pitch (area per player = 125 m2) for a 3-minute duration for the selected number of games increasing over the intervention period (Table 2). No specific tactical conditions were placed on players within the games, and a large number of soccer balls were placed in each net with play always starting with the goalkeepers when the ball went out of play to aid in a rapid continuation of play.
Data are expressed as mean ± SD. The normal distribution of the data was checked using the Kolmogorov-Smirnov test. After confirming normal distribution, paired sample t-test was used to compare the difference between pre- and posttest. Significant level was defined as p ≤ 0.05. Effect size (Cohen's d) was calculated to determine the practical difference between SSG and large-sided games. Effect size values of 0 to 0.19, 0.20 to 0.49, 0.50 to 0.79, and 0.8 and above were considered to represent trivial, small, medium, and large differences, respectively (5).
There was a trivial effect of SSG training on skinfold thickness (Table 3). Concerning the RSA, 4 weeks of SSGs induced significant improvement in RSA as indicated by faster 10-m FST (p < 0.05, small effect, Table 3), TST (p < 0.05, medium effect), and smaller %Decre (p < 0.05, medium effect).
Concerning the submaximal aerobic performance (Table 4), 4 weeks of SSG training significantly reduced the V[Combining Dot Above]O2 at the running speed of 9 (p < 0.05, large effect), 11 (p < 0.05, large effect), and 14 km·h−1 (p < 0.05, medium effect). Moreover, it also significantly lowered the HR responses at the running speed of 9, 11, and 14 km·h−1 (all p's < 0.05, large effects). There were small to trivial effects on RER, RR, and blood lactate (La) after 4 weeks of small-sided training games.
The aim of the present study was to investigate the effect of a 4-week SSG training intervention on physical fitness performance measures in elite, adult, professional, soccer players during the in-season break. The main findings revealed that the training intervention significantly improved players' repeated sprint ability, sprint performance (TST and %Decre, Table 3), and RE, which is presented as a reduced V[Combining Dot Above]O2 and HR when running at submaximal levels of 9, 11, and 13 km·h−1 (Table 4). It appears from our findings, that the periodized SSG training intervention could have a positive effect on both the anaerobic and aerobic system during the in-season break.
The findings from the present study add to the emerging paradigm of research that has identified SSG as an alternative training modality to generic drills (e.g., interval running training) capable of improving physical fitness characteristics in elite senior soccer players. Corroborative studies have shown a 7-week preseason training period of SSG to significantly improve Yo-Yo intermittent recovery test performance, although not V[Combining Dot Above]O2max (18), while a 3% improvement in RE has been found in youth players after 12 (4 weeks preseason and 8 weeks in-season) weeks of SSG (26). Despite similar trends being shown, it would seem futile to compare the results from the present study to those previously conducted, particularly given that metabolic and physical stressors have shown to vary greatly when manipulating the SSG training variables (e.g., pitch dimensions, number of players, coach encouragement) and the subsequent differences in research methodologies innate within the literature.
The inclusion of an RSA test as a measure of performance in the applied and research domain is substantiated by reported similarities of its physiological characteristics with high-speed sprinting performed during actual match play (31). Performing repetitive sprint efforts, change of direction, kicking, tackling, and dribbling are characteristics likely to severely exacerbate the physiologic strain during SSG. Yet this may offer an auxiliary physical stimulus because the ability to perform technical and tactical requirements under fatigued conditions is considered important for soccer (25). In addition, despite repeated sprinting drills and interval training (15 × 15 seconds at 120% of maximal aerobic speed) shown to concurrently improve sprint, vertical jump, V[Combining Dot Above]O2, and RSA during preseason and in-season training (13,43), such drills do not offer the motivation or enjoyment for players compared with those including a ball and failing to imitate the unorthodox movements commonly associated with SSG (20), further limiting any comparisons. As a result, the present study is unique as it revealed a 4-week periodized program of SSGs (3 vs. 3) that improved measures of RSA and RE during the mid-phase of the season at an elite level of professional soccer.
The ability to recover from intense bouts of repeated sprint effort is deemed critical during intermittent sports such as soccer and can be facilitated by a superior aerobic energy system (2). During the recovery phase, V[Combining Dot Above]O2 is elevated to restore metabolic processes to preexercise conditions. The adaptations associated with an increased level of aerobic fitness may facilitate the recovery process and subsequent sprint performance by providing aerobically derived energy at a faster rate during the recovery period (38,39). Possessing an elevated aerobic capacity is also associated with adjunct higher glycogen stores necessary for energy release during intense bouts of activity (21). The benefits of an 11% improvement of V[Combining Dot Above]O2max found in youth soccer players has shown to culminate in greater involvement with the ball, total distance covered, and a 100% increase in the number of sprints performed during match play (36). Research has also shown a high aerobic capacity to be correlated with RSA (2) and team success (42), thus further advocating the advantages of a superior V[Combining Dot Above]O2max.
On attaining a good aerobic base, any further changes in fitness level experienced during the season may be better detected through submaximal indices of aerobic fitness, such as RE and corresponding blood lactate concentration (26). Running economy may differ as much as 20% between individuals with similar V[Combining Dot Above]O2max (6), attributed to, among other factors, mechanical and neuromuscular skill, storage of elastic energy, and anatomical traits (35). Impellizzeri et al. (26) found SSG to be equally as effective as training without the ball in improving RE and V[Combining Dot Above]O2max. Also, the potential training effect of SSG and interval training has shown to lower HR (by 9 b·min−1) at 7 km·h−1 and subsequently improve RE by 14% at this speed (4). The greater gains found in the study by Chamari et al. (4) might be because of the different baseline fitness levels of the participants. Decreasing the energetic cost at a sustained workload may culminate in a reduced oxygen demand that may allow players to either exercise at a lower HR or similar HR but with greater intensity (19). Theoretically, an approximately 5% improvement in RE could decipher an extra 1,000 m covered during a soccer match (37). All of which could be considered desirable characteristics during 90 minutes of physically exertive match play. Nevertheless, in the present study, the significant improvement in RE (4.27–5.24%) shows that SSGs are comparable to results achieved through strength training (22,23) in the development of RE. Yet despite the present findings advocating RE as a sensitive measure of fitness during the mid-stage of the season, it is questionable whether these improvements are transferable to work economy and specific soccer movements such as high-speed sprinting, arced runs, or change of direction (17). Furthermore, it would be injudicious to suggest that the improved RE would equate to a concomitant increase in the players' V[Combining Dot Above]O2max.
Because the intensity of SSG can be severely altered by manipulating the exercise type, field dimensions, coach encouragement, and number of players involved (31,40), the present study reaffirms the necessity for fitness coaches to carefully consider the training objective when implementing SSG drills. For example, the influence the number of players can have on intensity is highlighted by research showing configurations of 3-a-side to elicit a lower blood lactate response comparable to SSGs comprising even fewer players (e.g., 1 vs. 1 and 2 vs. 2) (27). Rampinini et al. (31) found intraparticipant variability to be much greater for blood lactate concentration than mean HR during high-intensity bouts of SSG. This emphasizes the need to standardize coach encouragement to improve the reproducibility between different bouts and sessions of SSG. The same study by Koklu et al. (27) also showed HR and %HRmax to be greater during 3- and 4-a-side than games of 1- and 2-a-side. Correspondingly, greater HR values have been shown during SSG of 3 vs. 3 compared with 5 vs. 5 (30). Platt et al. (30) suggested that this may be because of the greater total distance covered, high-intensity activity, tackling, dribbling passing, and goal attempts encountered during the 3-a-side game. Regardless, it seems that exercising at intensities close to or above 90% of HRmax is required to improve aerobic fitness in highly trained soccer players (37).
The present study was conducted during the mid-phase of the season and comprised players not involved in competitive fixtures but still engaged in training. Admittedly, the lack of control group debilitates the findings and it could be argued that the gains in physical performance, as measured by RSA and RE, were attributed to the technical and tactical elements of training during the intervention period. However, it is worth noting that the SSGs were the only high-intensity sessions (e.g., >90%HRmax) performed during this period. Therefore, it can be inferred that physical improvements exhibited during a 4-week within-season SSG training intervention may culminate in greater fitness levels of physical qualities displayed during actual competitive match play.
The present study demonstrates that implementing a 4-week periodized SSG training intervention can improve physical fitness characteristics of elite professional soccer players during the in-season break. Being able to develop the physical profile of players, within a relatively short period, while encompassing technical and tactical elements, makes SSG an appealing proposition for fitness coaches, players, and coaches alike. Subsequently, the ability to perform repeated sprints is considered important during intermittent sports such as soccer, and the ability to improve repeated sprint ability during soccer-specific games can positively promote the need to integrate SSGs as part of elite soccer clubs' in-season conditioning program instead of generic nonspecific drills or sessions, when there is no official match scheduled.
Despite the potential advantages of SSG, care should be considered when implementing this training method in a periodized program. An overreliance on this training method may mask specific weaknesses within a player's profile and requires stringent control and standardization of influential factors (e.g., duration, pitch sizes) to avoid potential overtraining effects. The practical implications may be further enhanced by quantifying the optimal load and intensities of SSG alongside other forms of training that more specifically represents the day-to-day activity within a professional soccer club. In addition, in-season training load should be monitored to prevent players from overreaching and overtraining. Coaches could use SSGs during the in-season break to continually develop the physical, technical, and tactical components of the game in conjunction to each other rather than in isolation.
Results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
1. Bangsbo J. The physiology of soccer—With special reference to intense intermittent exercise. Acta Physiol Scand Suppl 619: 1–155, 1994.
2. Bishop D, Edge J, Goodman C. Muscle buffer capacity and aerobic
fitness are associated with repeated sprint ability
in women. Eur J Appl Physiol 92: 540–547, 2004.
3. Bradley PS, Sheldon W, 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.
4. Chamari K, Hachana Y, Kaouech F, Jeddi R, Moussa-Chamari I, Wisloff U. Endurance training and testing with the ball in young elite soccer players. Br J Sports Med 39: 24–28, 2005.
5. Cohen J. Statistical Power Analysis for the Behavioral Sciences. Hillsdale, NJ: Erlbaum Associates, 1988. pp. 567.
6. Daniels JT. A physiologist's view of running economy. Med Sci Sports Exerc 17: 332–338, 1985.
7. Dellal A, Chamari K, Owen A, Wong DP, Lago-Penas C, Hill-Haas S. Influence of the technical instructions on the physiological and physical demands within small-sided soccer games. Eur J Sport Sci 11: 341–346, 2011.
8. Dellal A, Chamari K, Pintus A, Girard O, Cotte T, Keller D. Heart rate responses during small-sided games and short intermittent running training in elite soccer players: a comparative study. J Strength Cond Res 22: 1449–1457, 2008.
9. Dellal A, Chamari C, Wong DP, Ahmaidi S, Keller D, Barros MLR, Bisciotti GN, Carling C. Comparison of physical and technical performance in European professional soccer match-play: The FA premier league and La LIGA. Eur J Sport Sci 11: 51–59, 2011.
10. Dellal A, Hill-Haas S, Lago-Penas C, Chamari K. Small-sided games in soccer: amateur vs. professional players' physiological responses, physical and technical activities. J Strength Cond Res 25: 2371–2381, 2011.
11. Dellal A, Lago-Penas C, Wong DP, Chamari K. Effect of the number of ball contacts within bouts of 4 vs. 4 small-sided soccer games. Int J Sports Physiol Perform 6: 322–333, 2011.
12. 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.
13. Dupont G, Akakpo K, Berthoin S. The effect of in-season, high-intensity interval training in soccer players. J Strength Cond Res 18: 584–589, 2004.
14. Gabbett TJ, Mulvey MJ. Time-motion analysis of small-sided training games and competition in elite women soccer players. J Strength Cond Res 22: 543–552, 2008.
15. Girard O, Mendez-Villanueva A, Bishop D. Repeated-sprint ability-part I: Factors contributing to fatigue. Sports Med 41: 673–694, 2011.
16. Glaister M, Howatson G, Pattison JR, McInnes G. The reliability and validity of fatigue measures during multiple-sprint work: An issue revisited. J Strength Cond Res 22: 1597–1601, 2008.
17. Helgerud J, Engen LC, Wisloff U, Hoff J. Aerobic
endurance training improves soccer performance. Med Sci Sports Exerc 33: 1925–1931, 2001.
18. Hill-Haas SV, Coutts AJ, Dawson BT, Rowsell GJ. Time motion characteristics and physiological responses of small-sided games in elite youth players: the influence of player number and rule changes. J Strength Cond Res 24: 2149–2156, 2010.
19. Hill-Haas SV, Dawson B, Impellizzeri FM, Coutts AJ. Physiology of small-sided games training in football
: A systematic review. Sports Med 41: 199–220, 2011.
20. Hill-Haas S, Rowsell GJ, Dawson BT, Coutts AJ. Acute physiological responses and time-motion characteristics of two small-sided training regimes in youth soccer players. J Strength Cond Res 23: 111–115, 2009.
21. Hoff J. Training and testing physical capacities for elite soccer players. J Sports Sci 23: 573–582, 2005.
22. Hoff J, Helgerud J. Maximal strength training enhances running economy and aerobic
endurance performance. In: Football
(Soccer). New Developments in Physical Training Research. Hoff J., Helgerud J., eds. Trondheim, Norway: Norwegian University of Science and Technology, 2003. pp. 37–53.
23. Hoff J, Helgerud J, Wisloff U. Maximal strength training improves work economy in trained female cross-country skiers. Med Sci Sports Exerc 6: 870–877, 1999.
24. Hoff J, Wisloff U, Engen LC, Kemi OJ, Helgerud J. Soccer specific aerobic
endurance training. Br J Sports Med 36: 218–221, 2002.
25. Iaia FM, Rampinini E, Bangsbo J. High-intensity training in football
. Int J Sports Physiol Perform 4: 291–306, 2009.
26. Impellizzeri FM, Marcora SM, Castagna C, Reilly T, Sassi A, Iaia FM, Rampinini E. Physiological and performance effects of generic versus specific aerobic
training in soccer players. Int J Sports Med 27: 483–492, 2006.
27. Koklu Y, Asci A, Kocak FU, Alemdaroglu U, Dundar U. Comparison of the physiological responses to different small-sided games in elite young soccer players. J Strength Cond Res 25: 1522–1528, 2011.
28. Owen A. Physiological & technical analysis of small-sided conditioned training games within professional football
. Soccer J 49: 5, 2004.
29. Owen AL, Wong DP, McKenna M, Dellal A. Heart rate responses and technical comparison between small- vs. large-sided games in elite professional soccer. J Strength Cond Res 25: 2104–2110, 2011.
30. Platt D, Maxwell A, Horn R, Williams M, Reilly T. Physiological and technical analysis of 3 v. 3 and 5 v 5 youth football
matches Insight: FA coaches association Journal 4: 23–24, 2001.
31. Rampinini E, Impellizzeri FM, Castanga C, Abt G, Chamari K, Sassi A, Marcora SM. Factors influencing physiological responses to small-sided soccer games. J Sports Sci 25: 659–666, 2007.
32. Reilly T. Energetics of high-intensity exercise (soccer) with particular reference to fatigue. J Sports Sci 15: 257–263, 1997.
33. Reilly T, Thomas V. A motion analysis of work rate in different positional roles in pro football
match-play. J Hum Mov Stud 2: 87–97, 1976.
34. Reilly T, White C. Small-sided games as an alternative to interval training for soccer players. J Sports Sci 22: 559, 2004.
35. Saunders PU, Pyne DB, Telford RD, Hawley JA. Factors affecting running economy in trained distance runners. Sports Med 34: 465–485, 2004.
36. Smaros G. Energy usage during a football
match. In: Proceedings of the 1st International Congress on Sports Medicine Applied to Football
. Vecciet L., ed. Rome, Italy: D Guanillo, 1980. pp. 795–801.
37. Stolen T, Chamari K, Castagna C, Wisloff U. Physiology of soccer: An update. Sports Med 35: 501–536, 2005.
38. Taoutaou Z, Granier P, Mercier B, Mercier J, Ahmaidi S, Prefaut C. Lactate kinetics during passive and partially active recovery in endurance and sprint athletes. Eur J Appl Physiol Occup Physiol 73: 465–470, 1996.
39. Tomlin DL, Wenger HA. The relationship between aerobic
fitness and recovery from high intensity intermittent exercise. Sports Med 31: 1–11, 2001.
40. Williams K, Owen A. The impact of player numbers on the physiological responses to small sided games. J Sports Sci Med 6: 99–102, 2007.
41. Wisloff U, Castagna C, Helgerud J, Jones R, Hoff J. Strong correlation of maximal squat strength with sprint performance and vertical jump height in elite soccer players. Br J Sports Med 38: 285–288, 2004.
42. Wisloff U, Helgerud J, Hoff J. Strength and endurance of elite soccer players. Med Sci Sports Exerc 30: 462–467, 1998.
43. Wong PL, Chaouachi A, Chamari K, Dellal A, Wisloff U. Effect of preseason concurrent muscular strength and high-intensity interval training in professional soccer players. J Strength Cond Res 24: 653–660, 2010.
44. Wong PL, Lau PW, Mao DW, Wu YY, Behm DG, Wisloff U. Three days of static stretching within a warm-up does not affect repeated-sprint ability in youth soccer players. J Strength Cond Res 25: 838–845, 2011.