The capacity of soccer players to acquire and maintain a good level of physiological fitness throughout the season is paramount. This demands the capacity to perform sport-specific activities that have been identified as requiring aerobic fitness, anaerobic power, speed, agility, and flexibility (30). Most semiprofessional soccer teams do not perform any off-season training; therefore, it is important to determine what affects this may have on preseason and seasonal fitness levels in this athletic competition.
Previous research has identified that short periods of detraining (8 weeks) can result in reduced aerobic fitness (30), anaerobic power and sprint ability (32), and increased percent body fat in professional soccer players (18,25). Reilly and Williams (30), Brady et al (6), and Bangsbo (4) suggest that players in this deconditioned state at the start of preseason have to work extremely hard in an effort to try to reach the high levels of fitness required for the start of the season in such a short space of time, resulting in mental and physical fatigue during latter stages of the season, potentially with an associated high catabolic status (19). Previous research has shown that in professional players, aerobic fitness increases from preseason to midseason and then decreases (13). Anaerobic power remains unchanged during the season (7), whereas agility and sprint performance deteriorate in the off-season and then improve as a result of preseason training (14). Flexibility does not change in the preseason period (22), and percent body fat increases significantly in the off-season and then decreases throughout the season (2,25).
Although seasonal changes have been investigated in professional soccer players (7,13), and in collegiate teams pre- and postseason (34) and in relation to hormonal concentrations over the season in starters and nonstarters (19), there is a paucity of research involving semiprofessional players who experience considerable additional employment demands that may limit their capacity for more regular training and conditioning. The aim of this study, therefore, was to investigate the seasonal variations in physiological fitness of semiprofessional soccer players over a 12-month period.
Approach to the Problem
A battery of fitness assessments was performed 5 times over a 12-month period. Fitness variations were analyzed by comparing the test results of each successive fitness test in sequence from one stage of the season to the next. This type of intermittent testing can monitor and track physiological fitness and can help to identify fluctuations in each fitness parameter throughout the season. The tests chosen were selected because they provide valid and reliable data, are soccer-specific, and have high test-to-test sensitivity, which is important to highlight changes in fitness.
Before testing, institutional ethical procedures were satisfied and informed consent of the club was given to perform the project. Participants were 13 semiprofessional male soccer players from a team in the English Nationwide Conference North League (mean age, 24 ± 4.4 years; mean height, 178.0 ± 6.08 cm; mean body mass, 75.6 ± 6.09 kg). Before testing, the participants completed written informed consent forms and pretest health screening questionnaires.
The schedule of seasonal fitness testing was performed as follows over a 12-month period: (a) time 1, end of season one (April); (b) time 2, before preseason training for season 2 (July); (c) time 3, postpreseason training for season 2 (August); (d) time 4, midpoint of season 2 (February); and (e) time 5, end of season 2 (April).
Before each testing session, participants complied with the following pretest guidelines: (a) not to take part in any strenuous exercise 2 days before testing; (b) not to consume any energy/performance-enhancing drinks or supplements 48 hours before testing; (c) not to take caffeine, alcohol, or tobacco at least 3 hours before testing; and (d) not to consume food at least 2 hours before testing. Participants wore heart rate monitors to monitor heart rate at rest and during aerobic testing to calculate work rate to determine if participants were working to their maximum. All testing sessions took place at the same time of the day, between 6 pm and 9 pm, to prevent the effects of circadian variation on the variables measured (29). All participants wore nonslip footwear, shorts, and t-shirts. Before every test, the protocol was explained and the test administrator demonstrated the correct test technique to ensure the participants understood how to carry out the test. All testing was carried out at the same indoor sports hall. During all testing sessions, the ambient temperature was kept between 22° C and 24° C. Order of testing was as follows, as recommended by the American College of Sports Medicine (2). Resting heart rate measurements, height and weight measurements, body composition (bioelectrical impedance), aerobic power (20-m multistage fitness test), anaerobic power (countermovement jump with arm-swing), sprint ability (15-m sprint test), agility (Illinois agility test), and flexibility (sit and reach).
Aerobic fitness was assessed using the Multi-stage Fitness Test (MSFT) CD (Multistage Fitness Test AA1CD; National Coaching Foundation, Leeds, UK). All participants performed the MSFT following the set procedures and protocols (21). Participants' scores were recorded as the level and number of shuttles immediately before the beep on which they were eliminated. Participants' scores (level and shuttle number) were then converted to predicted VO2 max values using the conversion table of Leger and Lambert (21).
Standing Vertical Jump
Standing vertical jump (SVJ) height was assessed by performing the countermovement vertical jump (with arm swing). A digital jump meter was used (Takei A5106; Takei Scientific Instruments, Tokyo, Japan). The vertical jump mat was laid down flat on the floor. The participant wrapped the jump belt around their waist and reset the digital display clock to zero. Using the cord wheel on the belt, the test administrator wound up the belt cord attached to the mat until taut. When landing, the participant was instructed to land on the mat in an upright extended position. When ready, the participant performed takeoff with 2 feet. They jumped for maximum height using arm swing and executing a dip or countermovement immediately before the upward propulsion. The vertical displacement was then read from the digital display screen on the belt and recorded. After each jump, the score attained was conveyed back to the participant with the aim to help motivate the participant and help them try to improve on their previous score. Each participant had 3 attempts with the highest score taken as their countermovement jump score. The participant performed the 3 jumps in succession with 10 seconds rest between each jump.
Sprint speed was assessed by a 15-m sprint test. A distance of 15 × 2 m was measured out for the test area. The 2-m sides of the area were marked out clearly with duct tape and the 15-m sides marked with cones. One set of electronic sprint timing gates (Newtest 2000; Newtest Powertimers, Oulu, Finland) accurate to 0.01 seconds were set up at either side of the 2-m ends representing the start and finishing lines. The timing gates were positioned at the start and finish lines to ensure the timing beams were directly above the lines measured. Another 2-m line was then marked out at the start position, half a meter behind the start line. This was to indicate to the participant where to place their foot to ensure that they did not break the beam when getting set to run. The participants were instructed to place the heel of their leading foot until it was just touching the back line. The distance of a half meter was chosen to ensure no speed was gathered before the participant crossed the line. Before beginning, participants were instructed to sprint as fast as possible through the course toward the finish line making sure not to slow down before the finish gate. One participant was tested at a time. When lined up correctly at the start, each participant began when ready, thus eliminating reaction time. Participants were tested 3 times. When finishing each sprint, participants returned to the start and waited in turn for their next sprint. Their 15-m sprint times were recorded to the nearest 0.01 second. The participant's fastest time was taken as their 15-m sprint time. In this test, a standing start was important because this mimics soccer players' actions during a game, ie, running short distances from a standing start.
All participants completed the Illinois agility test to assess agility. Participants were instructed to complete the course in as fast a time as possible. Each participant began in a lying position and was face down underneath the starting gate. Forearms were flexed at the elbows and hands outside the shoulders. The vertex of the participant's head was level with the starting line. Each participant had 3 attempts to complete the course and their fastest time was taken as their agility score.
Percent Body Fat
Percent body fat was measured using a bioelectrical impedance analyser (Bodystat BS1500; Bodycare Products, Warwickshire, UK). Participants removed their right shoe and sock. The participant then laid on their back on a physiotherapy couch in the supine position with legs apart and arms away from their body. The participant lay in this position for 5 minutes before testing. A pair of electrodes was then placed on the right hand of the participant, one below the knuckle at the bottom of the middle finger and one on the wrist. Another pair of electrodes was placed on the right foot, one on top of the foot (bottom of the leg) and another at the base of the second toe. The clips on the black lead were then attached to the electrodes on the front of the wrist and the top of the foot. The red lead clips were attached to the electrodes below the middle finger and the electrode placed at the start of the second toe. The participants' gender, age, height, weight, and daily physical activity level (standardized as high for these players) were entered into the analyzer, which then measured percent body fat.
A sit and reach box (Cranlea LSRBS1; Cranlea and Company, Birmingham, UK) was used to measure flexibility. Participants were instructed to remove their shoes and sit on the floor with their legs fully extended with the bottoms of their feet against the sit and reach box. The participants then placed their hands on top of each other and slowly bent and reached forward as far as possible sliding their fingers along the top of the sit and reach box and holding their final position for 2 seconds. Each participant had 3 attempts at the test with the furthest measurement reached taken as their sit and reach score.
Playing Schedule, Preseason, and Seasonal Training and Conditioning
To contextualize the fitness testing within the demands of the competitive season and, in so doing, acknowledging the nature of training and conditioning programs throughout the season, data were collected regarding the frequency and relative emphasis on coaching and fitness of training sessions throughout the 12 months.
The team played a total of 54 games (11 cup games and 43 league games) with the competitive season lasting 38 weeks. There was no midseason break and the team played matches and trained every week.
Training during preseason consisted of one session per training day of approximately 120-150 minutes in duration. Throughout the training conditioning sessions in weeks 1-4 (n = 13) of the preseason, the training emphasis was: (a) 60% aerobic/endurance conditioning consisting of distance running (aerobic interval training, hill running, and Fartlek running) and small-sided indoor soccer games; (b) 15% anaerobic game-related training consisting of high-intensity short sprint drills and agility-based drills; (c) 10% muscle strength training: (basic)-leg and upper body weight training (free weights and machine weights) and (functional) plyometrics and elastic rope training); and (d) 15% tactical training/set plays (corners, free kicks, etc).
During all preseason training sessions, flexibility (stretching) exercises were performed regularly after a warmup (5-minute light jog) and at the end of training sessions (5 minutes). Stretching exercises concentrated on the muscles of the lower body, including the hamstrings, calf muscles, quadriceps, and leg adductor muscles (groin). Other stretches included torso, shoulder, and lower back muscles.
The conditioning training sessions for preseason weeks 5-7 (n = 11) was slightly modified and consisted of (a) 45% aerobic conditioning; (b) 20% anaerobic high-intensity shuttle-running (sprint and agility-based); (c) 20% basic and functional muscle strength training (as stated previously); and (d) 15% tactical training/set plays (corners, free kicks, etc).
Preseason friendly games (n = 7) were also played to help improve the players' match fitness and their match skills.
Seasonal team training took place twice a week on Tuesday and Thursday nights lasting 90-150 minutes each. Training on Tuesday and Thursday of weeks 1-20 was comprised of: (a) 35% aerobic conditioning-low-intensity distance running and high-intensity aerobic 5 vs. 5 football matches; (b) 35% anaerobic shuttle running (sprint and agility-based drills); (c) 10% functional leg muscle strength training (plyometrics and elastic rope training); (d) 20% tactical training/set plays (corners, free kicks, etc); and (e) flexibility exercises were also performed (5-minute warmup/5-minute cool-down).
From week 20 to the end of the season, team training on most weeks was only performed on a Thursday as a result of rescheduled fixtures being played on Tuesdays. As a consequence of this, the team played on average 2 matches per week during this period. To help keep the players fresh and prevent fatigue, the intensity and duration of fitness training was reduced in the second half of the season. Prioritization of aerobic and anaerobic training in these sessions was reduced. This decision was made by the team coach because the team was already performing these types of activities at a high intensity during the games throughout the week. As a result, training between weeks 20-38 was reduced to 90 minutes through reducing the duration of aerobic and anaerobic training both by 10 minutes each.
Statistical analysis was performed using SPSS version 14.0 (SPSS Inc, Chicago, IL). While repeated-measures analysis of variance was initially performed to identify significant group differences, paired samples t-tests were preferred to analyze variation between each successive testing session (rather than between all testing sessions). Bonferroni correction was applied to reduce type I error (reducing the required α level for significance to p < 0.01 (0.05/5 pairs). Results are displayed in Table 1. Reliability of the dependent measures was assessed using single-measures intraclass correlations for the 5 trials (height 1.00; weight 0.99; VO2 0.79; standing vertical jump 0.90; sprint time 0.80; agility time 0.78; percent body fat 0.99; flexibility 0.98).
From Table 1, it can be seen that aerobic fitness at time 2 was significantly lower than time 1 (p < 0.001); time 3 was significantly higher than time 2 (p = 0.000); time 4 was significantly higher than time 3 (p = 0.004), and time 5 was significantly lower than time 4 (p = 0.007). Aerobic fitness at time 5 remained unchanged compared with time 1 (p = 0.777).
Standing vertical jump performance at time 2 was significantly lower than time 1 (p = 0.002); time 3 was significantly higher than time 2 (p = 0.000); time 4 was significantly higher than time 3 (p = 0.005) and time 5 remained unchanged compared with time 4 (p = 0.837). Standing vertical jump at time 5 remained unchanged compared with time 1 (p = 0.786).
Sprint/speed performance time at time 2 was significantly slower than time 1 (p = 0.005); time 3 was significantly faster than time 2 (p = 0.007); time 4 was significantly faster than time 3 (p = 0.000), and time 5 remained unchanged compared with time 4 (p = 0.139). Sprint/speed time at time 5 remained unchanged compared with time 1 (p = 0.835).
Agility performance time at time 2 was significantly slower than time 1 (p = 0.003); time 3 was significantly faster than time 2 (p = 0.008); time 4 remained unchanged compared with time 3 (p = 0.126) and time 5 remained unchanged compared with time 4 (p = 0.250). Agility performance at time 5 remained unchanged compared with time 1 (p = 0.062).
Body fat percent at time 2 was significantly higher compared with time 1 (p = 0.001); time 3 was significantly lower than time 2 (p = 0.005); time 4 was significantly lower than time 3 (p = 0.001), and time 5 was significantly lower than time 4 (p = 0.007). Body fat percent at time 5 remained unchanged compared with time 1 (p = 0.252).
Flexibility at time 2 was significantly lower than time 1 (p = 0.000); time 3 was significantly greater than time 2 (p = 0.001); time 4 was significantly greater than time 3 (p = 0.006), and time 5 remained unchanged compared with time 4 (p = 0.209). Flexibility at time 5 remained unchanged compared with time 1 (p = 0.700).
The purpose of this study was to investigate seasonal variations in the physiological fitness of semiprofessional soccer players over a 12-month period. The mean height and body mass of the participants was similar to those reported previously (7,34) as were percentage body fat values when both mean and SD from previous studies are examined (7,30,34).
The values obtained for the fitness tests at test 2 were, with prepreseason training, similar to those previously reported figures for VO2 max but lower for standing vertical jump (34), although differences in the SVJ may be attributable to the loaded warmup provided in the study by Silvestre et al (34).
From the end of season one to immediately before preseason training in season 2 (the “off-season”), significant deconditioning was apparent in all fitness variables. Previous research that has studied this period of the season is limited. However, Reilly and Williams (30) support these findings identifying that short detraining periods (8 weeks) can result in reduced aerobic fitness. Deconditioned players returning to preseason training can be put under physical and mental pressure to try to reach the high levels of fitness required for the start of the season in such a short space of time, thus resulting in mental and physical fatigue during later stages of the season. Both sprint performance and agility performance also showed significant decreases during the off-season period, which is supported by Ross and Leveritt (32) who identified that detraining can cause a loss in speed and power over a distance of 10-20 m. Amigo et al (3) similarly found sprint performance slowed significantly after an 8-week off-season break which may be resultant from decreased muscle volume (8). Flexibility also decreased significantly during the off-season period. No studies to date have been carried out examining changes in flexibility during the off-season; however, Wilmore and Costill (36) state that flexibility performance decreases rapidly during periods of detraining and recommends stretching exercises during the off-season. Measurements of percent body fat also demonstrate significant increases; this is similar to previous research (18,25) in which no off-season training was undertaken. To avoid these significant reductions in sport-specific fitness during the off-season and to attempt to prevent the potential harmful impact of acute overtraining syndrome often associated with using this short time period to improve fitness quickly and greatly (19), players should undertake off-season “maintenance” training that incorporates all of the aforementioned sport-specific fitness components.
During the preseason period, all fitness components showed a total reversal from off-season results. Percent body fat decreased significantly at postpreseason training compared with before preseason training. This is similar to previous studies (22,24-26). In this study, the loss in body fat during the preseason period may have been a direct result of the high levels of aerobic, anaerobic, and muscle strength training performed during this period coupled with preseason training games. This is supported by Reilly (27) who states that low body fat is a direct reflection of high training intensities, whereas Bangsbo (5) states that match play places high demands on the aerobic energy system (90%) and causes large energy expenditure; therefore, the body uses fat stores as energy. A significant increase in preseasonal aerobic fitness is similar to previous studies (14,22), which attribute these increases to high levels of high- and low-intensity aerobic training performed during the preseason (7 weeks), which is identical to training undertaken by participants in this study. This coupled with the preseason matches would have helped to increase their aerobic capacity and VO2 max. Anaerobic power increased significantly from prepreseason, whereas no previous research analyzing anaerobic power during this period has been performed. Wisloff et al (37) and Hoff and Helgerud (17) agree that vertical jump performance can show signs of improvement during a soccer season when basic (weight training) and functional (plyometric and elastic rope training) leg muscle strength training is performed regularly. This could explain the increases observed in this study because during the last 2 weeks of preseason training, prioritization of “basic” and “functional” muscle strength training was rated highly by the coach. Sprint performance was also significantly faster during this period. This is similar to Helgerud et al (14) and Hoff (16) who found sprint performance of professional soccer players to be significantly faster at postpreseason compared with prepreseason. The improvements in the present study could have been a result of the friendly matches played during the preseason; this is supported by Bangsbo (4) who states that friendly games enhance speed and acceleration as a result of the high-intensity nature of match play. Also, the faster times in the present study could have been the result of the gradual increase in speed training during the preseason period and a high prioritization of muscular strength training performed by the participants. Significant increases in agility performance at postpreseason compared with prepreseason are similar to previous research (22) that attributed these increases to game-related training and match play. This could also be the case in this study because anaerobic speed training, including shuttle running and small-sided indoor soccer games, was implemented regularly in preseason training. Mercer et al (22) report that 5 vs. 5 game play requires players to change direction approximately every 3 seconds helping to increase agility. These matches may also have improved acceleration and motor coordination, which Reilly (28) states are essential for good agility performance. A significant increase in flexibility at postpreseason compared with prepreseason conflicts with previous research conducted on professional soccer players (22). It is possible that the professional players may have been required to perform some form of off-season maintenance training. Significant increases in the present study may have been the result of the “moderate” levels of flexibility training during weeks 1-4 of preseason training and the “high” levels incorporated into weeks 5-7.
During the first half of the season, aerobic fitness continued to increase significantly from postpreseason to midseason and is similar to previous research by Haritonidis et al (13), Bangsbo (4), and Ekblom (10). However, Casajus (7) studied VO2 max levels in professional soccer players at the start of a season and in the middle of the season and found that although there was a slight increase in players' VO2 max, the difference was not significant. A possible explanation for this finding could be that the players may have nearly reached their VO2 max peak at the start of the season, therefore, making it difficult to improve their aerobic levels any further. This may have been caused by a very high aerobically based preseason training period designed to improve their aerobic fitness. No training details were disclosed in this study, which makes it difficult to examine the reasons for the unchanging VO2 max levels. Ekblom (10) suggests that increases such as these are the result of continuous aerobic training and match play during this period of the season. Again, this could explain the increases of VO2 max in the present study because aerobic training was given high priority during this period as well as the players regularly playing 2 matches per week. The players' percent body fat continued to decrease significantly at midseason compared with postpreseason. This is similar to research by Alberquerque et al (1) who also studied semiprofessional soccer players and may be a result of regular training and match play. Anaerobic power continued to increase significantly during this period. This is in contrast to previous studies (7,9) that found anaerobic power remained unchanged from postpreseason to midseason. However, these studies failed to report details of seasonal training of the players in their studies, therefore making it difficult to analyze why performance had remained unchanged. Continued high prioritization of leg muscle strength training during the first half of the season may have been a major factor in these increases in anaerobic power.
Significant increases in flexibility were achieved at midseason compared with postpreseason, which may be a reflection of the “high” prioritization of flexibility training during the first half of the season. These seasonal increases agree with previous research by Ekstrand and Gillquist (11) but disagree with research studied by Rupp and Kuppig (33) who state that hamstring flexibility decreases as the season advances. However, the reliability of this study is questionable because the sample tested was not the same on both testing occasions.
At midseason, sprint performance times continued to become significantly faster compared with postpreseason. This disagrees with previous research (25) that found sprint performance was unchanged. However, the distance sprinted in Ostojic's study was 50 m, a substantially longer distance than the 15-m test used in the current study. In the present study, “high” and “very high” prioritization of muscle strength training performed during this period coupled with regular match play and high prioritization of anaerobic speed training could explain these increases. In addition, significant decreases found in body fat percent throughout the season may also have contributed to these improvements, because competitive sports players with a lower body fat percent invariably have better sprint performance (25), because superfluous body fat acts as dead weight during activity (23,31). Further analysis of changes in percent body fat in relation to changes in sprint times indicates that there is a strong correlation between these 2 fitness components. During the off-season period, body fat percent increased along with increased sprint times and from prepreseason to midseason as body fat percent decreased, sprint times improved likewise. During the playing season from postpreseason to midseason to the end of season 2, agility performance remained constant. No comparative data exist regarding seasonal variation of agility performance of professional or semiprofessional players.
Aerobic fitness was then shown to peak at midseason and decrease significantly toward the end of season 2. This agrees with research conducted by Haritonidis et al (13) and Heller et al (15). The decrease in aerobic fitness in the present study could be attributable to fatigue or fixture overload in the second half of the season. This meant that aerobic training was downgraded in the second half of the season to keep the players fresh, thus lowering VO2 max. Other reasons could include, as mentioned previously, player inactivity in the off-season followed by high-intensity preseason training putting undue stress on players resulting in fatigue during the latter stages of the season. Reilly and Williams (30), Brady et al, (6), and Bangsbo (4) all recommend that players maintain a low level of aerobic and anaerobic activity during the off-season period, which can ensure that training during preseason does not have to be as intensive and stressful than if no training was performed. Similarly, Gabbett (12), researching rugby league players, found reductions in preseason training loads (duration and intensity) resulted in an average improvement of 11.7% in maximal aerobic power throughout the season. Also, players' perceptions that they had achieved the required levels of aerobic fitness at midseason may have caused them to reduce their work intensity during their training sessions. In the second half of the season, anaerobic power remained unchanged from midseason to the end of the season, similar to previous research by Da Silva et al (9). The stable levels of anaerobic power found in the present study may have been expected because Reilly and Williams (30) found that professional soccer players who predominantly performed “functional” muscle strength training and little “basic” muscle strength training throughout the season, similar to this study, produced only very small decreases throughout the competitive season. As a result, it could be said that soccer training that only includes a “functional” type of muscle training may not be effective in developing vertical jump ability. Therefore, to help improve vertical jump and anaerobic power performance, weight training should be incorporated into team training programs; this is supported by previous studies on soccer players (17,37). This stabilization and cessation of significant increases in anaerobic power also could be the result of players playing on average 2 matches per week combined with low to moderate prioritization of muscle strength training (performing predominantly aerobic activity over anaerobic and strength training). Taylor (35) and Kraemer et al (20) suggest that this can interfere with gains in lean muscle mass, strength, and power in the legs. This may also explain the significant decrease in speed performance at the end of season 2 from midseason. From midseason to the end of season 2, flexibility performance remained stable and may indicate that the players had reached the required level of flexibility at midseason. Body fat percent continued to decrease toward the end of season 2 from midseason but not significantly. This is similar to previous research (24) that found that body fat levels remained constant from midseason to the end of the season in professional players. There were no differences in any fitness component between the end of season 2 and the end of season one, thus showing no overall improvement in physiological fitness over the 12-month period.
This is the first study of its kind to investigate changes in fitness variables over a complete 12-month cycle incorporating a full season of competition in semiprofessional players in England. As such, the sample group, although homogeneous in many respects to participants in other studies in relation to the changes observed from pre- to postseason, would appear to undertake a rigorous and extended competitive season and attempt to maximize training opportunities in addition to a normal full-time working life. As such, these participants may warrant further investigation, particularly in relation to the impact of overtraining syndrome at the start of the season and the impact this may have on subsequent physical and psychologic game performance. The apparent plateau of fitness variable in the midseason may well indicate attainment of the fitness levels required by the level of competition and the playing schedule, or it may reflect more long-term detraining or demotivation effects resulting from such a prolonged season with matches and training every week.
The “off season”appears to represent a period of significant reduction in sport-specific fitness in semiprofessional soccer players, which may cause fatigue in later stages of the season. Off-season light “maintenance” training (2-3 times a week) should therefore be used to attempt to minimize these reductions. In doing so, players would already be closer to midseason “peak” match fitness at the start of the season, which would reduce the pressure on players to quickly regain large fitness deficits lost during the off-season. Also, fitness training elements of preseason training could be refined for quality, permitting more time for engaging in match play and thus enabling players to adapt to the apparent unique demands of actual game play earlier in the season. It is also recommended that to help maximize speed performance, players should carry as little body fat as possible. Further longitudinal research over at least 2 consecutive seasons is recommended as well as the adoption of an interdisciplinary approach to identify the role of other variables such as perceived and actual success, motivation, training workload, and diet in seasonal variations in soccer-specific fitness.
We thank Philip Richards, Dave Mycock, and Nick Boot-Handford for their assistance with data collection in this study.
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