Research indicates that performance in soccer depends on various physical qualities and skills including tactical and technical skills as the 2 most import factors affecting performance in soccer (3,4,25,28). Other studies not only support this assumption but also claim that physical capabilities such as aerobic endurance, strength, and running speed must be well developed to reach a high performance level in soccer (13,14,19,27). These physical abilities do not have to be extremely developed, but they must be of a high level (28,29). Exactly how high, depends on the competition level and the player's position in the field. At the international level, aerobic endurance is the most required quality among midfield players, whereas strength and running speed are of great importance for attackers (19,28,38,40). High running speed makes the players able to use their technical and tactical skills efficiently. Fast wingers may challenge the back players by well-practiced feints, whereas fast forwards can reach the ball before the defense player.
Soccer game analyses on male elite players indicate that the players sprint between 1-11% of the total game time on high speed (37). Furthermore, the duration of the sprints is normally between 2 and 4 seconds, but the duration varies according to the role and position of the player (28,29,38). Usually, midfielders have the shortest sprint duration, whereas the wingers and attackers on average have the longest sprinting duration (2). The duration of the sprints indicates that there is a large demand on acceleration speed and less demand on maximum speed. Studies show that sprint starts mainly while the players are already running, which indicates that the demand for maximum speed is larger than what the duration of the sprinting indicates (37).
Analysis of soccer games at an elite level shows that the majority of the players conduct short sprint runs (2-4 seconds) every 60-90 seconds, which equals about 60-90 sprints during a game (28,37). Wingers and forwards run significantly more sprint accelerations than do central defenders and midfielders, whereas the midfielders run the longest total distance during a game (3,28,37). Furthermore, fast sprint runs with relatively short breaks indicate that the demand for endurance running speed is high during the season. This indicates that the players need to practice on repeated acceleration with short breaks (30-120 seconds) to be able to maintain their speed over time. Such training may cause metabolic changes and delay fatigue within muscle (36). Studies have shown that performance reduction in 36.7-m sprint was observed in college soccer players toward the end of the season (18). Furthermore, the cortisol and testerone, which has been identified as reliable markers of training stress, were found to be within the normal range throughout the season but low after preseason conditioning (18).
Speed is believed to be a genetic quality skill and less dependent on training (31). Furthermore, it is believed that specialized training on running speed may result in a significant improvement in soccer players with little speed training experience. Harrison and Bourke (12) have reported a significant improvement in rugby players' running speed after specialized training. Several studies have shown that training on running speed combined with strength and Plyometrics training could significantly develop strength, jump ability, and running speed after only 8-13 weeks (7,9,17,23,34,35). This could be because of both muscular and neural responses (21,31). An improvement of 0.1 second on a 40-m sprint equals around 0.7-m in distance. In soccer, this can be the difference between winning and losing an important duel.
Research has pointed out the demands for speed among male soccer players. At present, there are no studies available in which one has studied soccer players at a high national level with only sprinting training stimuli similar to that model used with elite athletes in track and field. Based on our role in the Norwegian Olympic center as a training facility for a large number of teams at different performance levels, including national squads, we decided to investigate the effect of 10 weeks' 40-m repeated sprint training program that does not involve strength training on maximum sprinting speed and endurance running speed. Because jumping height and aerobic capacity are also important skills in soccer, we have chosen to examine what effect the training would have on these skills.
Experimental Approach to the Problem
To test the effect of 40-m repeated strength training on repeated sprint ability (RSA), jumping height and aerobic capacity, the 20- and 40-m maximal sprints and repeated sprint were measured at the Norwegian Olympic Committee and Confederation of Sports using a start mat and 2 pairs of double infrared photocells, which were connected via cables and connected to a computer (PC Pentium 3) that measures time to the nearest 0.001 seconds. The photocells were mounted on a 50-m sprint running track. Jump height was estimated in the laboratory of the Norwegian Olympic Committee and Confederation of Sports using force platform-based determinations of impulse and thus velocity at take-off. The force platform used was an AMTI model OR6-5-1. The data were amplified (AMTI Model SGA6-3), digitized (DT 2801), and saved to a computer (PC Pentium 3) with the aid of the special software program Biopack MP 100. Aerobic capacity was measured using the Beep test; the Beep test was conducted on an indoor artificial grass pitch following a procedure that was developed by Ramsbottom et al. (26). A JVC Boomblaster (RVNB51WEN) was used to play the Beep-test CD that came with the test package.
The participants were matched according to their pretest results in the 40-m sprint test. They were then randomly assigned into 1 of 2 groups, a Training group (TG) and a control group (CG). The study was conducted in the precompetition phase of the training program for the participants and ended 13 weeks before the start of the season; the period of the precompetition period was 26 weeks. The length of the mesocycle was 10 weeks. The pretests and the posttests were conducted on 2 separate days with 2 days with light training in between. On test day 1, 40-m maximal sprint, 10 × 40-m repeated sprint and CMJ were measured, and on test day 2, aerobic capacity (Beep test) was assessed.
Twenty young well-trained elite male soccer players of age (±SD) 16.4 (±0.9) years, body mass 67.2 (±9.1) kg, and stature 176.3 (±7.4) cm volunteered to participate in this study. The participants were all highly committed to training and trained 14.1 ± 2.5 h·wk−1 (5-7 training sessions a week). The participants play for among the best 3 junior teams in Norway. Furthermore, 10 participants were chosen to be part of the national team of their age group. All participants and their parents gave their written voluntary informed consent, and the local ethics committee at the Norwegian School of Sport Sciences approved the study. The participants did not have any systematic strength training in the form of weight training, but they had aerobic endurance training in the form of interval training and long run (2-4 times a week), and enduring strength training of the abdominal, back, and torso muscles. Nordic hamstring was the only strength training performed by the subjects (2-4 sets with 10 repetitions per week). The duration of the soccer trainings was 1.5 hours, where about 30 minutes was used for warming up and cooldown. Approximately 1 hour was spent on pure soccer training. Warming up was often in the form of short passing exercises or coordination exercises with the ball (<75% of maximum heart rate), followed by more intensive exercises such as cuts, moves with and without ball, turns with and without ball, and feint with and without ball. Most often, the main soccer practice consisted of a variation of playing using small and large fields. The practice was performed with 3 vs. 3, 4 vs. 4, and 7-11 vs. 7-11. Such training was carried out 3-4 times per week. On training days with light training loads, the exercises were focused on defense tactical drills, attack tactical drills, and dead balls.
To make the participants familiar with the testing procedure, they were asked to complete a full training session in the testing procedure 1 week before pretest 2. To measure reliability, the participants were tested during this week, and this was considered pretest 1. On pretest 1 and before the testing took place, the participants' stature was measured.
On pretests 1 and 2 (test day 1), the participants started with a 15 minutes' general warm-up, which consisted of running at 60-70% of maximum heart rate. After the general warm-up, the participants were asked to do 4-5 accelerations over 50 m. Then, maximum running speed was tested by sprinting 3 × 40 m with 4 minutes' recovery between trials. The best result was retained for analysis. The time was measured for 0-20 m (acceleration speed) and for 20-40 m (maximum sprinting speed). The participant started from a standing-up position placing the front foot on a starting mat; when the test leader gave the signal, the participant started the sprint using the shortest time possible to finish the 40 m. The timer was started automatically when athletes left the starting mat and stopped when they passed the photocells at both 20 and 40 m.
After the maximum running speed test, the participants took a 5-minute recovery and then completed the countermovement jump test (CMJ). The CMJ test was performed by standing on the force platform mat with the plantar part of the foot contacting the ground, and with hands on hip, and from an erect standing position on the force platform with a knee angle of 180°, a countermovement down until the knee angle around 90° was performed. Then, immediately the participant jumped. Three trials were allowed. The best result was retained for analysis. A greater than 3-minute recovery between trials was provided.
After another 5 minutes' recovery, the participants were asked to complete the 40-m repeated sprinting test by sprinting 10 maximum sprints with 60-second recovery between each sprint. The participants were asked to sprint as fast as possible on each sprint. The participant started from a standing-up position placing the front foot on the starting mat; time started automatically when the athletes left the starting mat and stopped when they passed the photocell placed on the 40-m mark. The mean time for 10 runs was used for analysis.
On pretest 2, day 2, the participants started with the same warm-up procedure as described on test day 1. The Beep test started after the test leader measured and marked a distance of 20 m with cones and a marked line. Then, the CD (the soundtrack) and the CD player were checked to make sure the soundtrack would be played at the right speed between the sound signals (Beep). Afterward, the participants were informed of the test procedure. Four experienced test leaders were responsible for making sure that the participants fulfill the testing criteria.
The Training Intervention
The CG was instructed to continue the teams' original training plan. The TG completed 1 extra training session with repeated speed training. The speed training was planned and carried out by an expert on running speed. The training expert has a Ph.D. in training methodology and has been a coach for the Norwegian national team and for some of the best female sprinters in Norway. The training program completed by TG is described in Table 1.
Before the speed training, the participants completed both a general and specific warm-up. The general warm-up consisted of 15 minutes of jogging at a low intensity. During the special warm-ups, the athletes ran 5-7 accelerations over 40-50 m, with a recovery of 2-3 minutes between each run. The participants had to complete at least 90% of the training period and had to be able to complete all the tests for their results to be included in further analysis.
Raw data were transferred to SPSS 16.0 for Windows and Microsoft Excel for analysis. Intraclass correlation coefficient (ICC) was assessed on the data from pretest 1 and pretest 2 to examine reliability of performance tests. To detect differences in measures between pretest 2 and posttest, paired t-test was performed to test for a difference in central location (mean) between the paired samples (within group). To test for a difference in central location (mean) between groups, the independent sample t-test was applied, and the effect size was calculated according to Rosnow and Rosenthal (30) to determine how effective the applied repeated sprint training was. To determine if the effect size was trivial, small, moderate, large, or very large, the scale used is based on the argument presented in Batterham and Hopkins (5); Hopkins et al. (15). Differences were considered significant at p ≤ 0.05, and the results are expressed as mean and SD. The 95% confidence interval (95% CI) was also calculated for all measures.
Differences within groups and between groups of a variety of physiological measures are shown in Table 2. The results indicate that there was a notable improvement within the TG group from pre to posttest on 40-m sprint, 10 × 40-m repeated sprint speed, 20-m top speed, and CMJ.
The results indicate that there was a notable improvement within the CG group in 10 × 40-m repeated sprint speed only. A comparison between groups indicates that there were notable differences between the 2 groups on 10 × 40-m repeated sprint with a moderate effect size of d = 1.0- and 20-m top speed with d = 0.9. Furthermore, the effect size was moderate in the 40-m sprint, CMJ, and Beep-test results even though there was no marked statistical significance, whereas a small effect was observed on 20-m acceleration speed and body weight (Table 2).
The day-to-day reliability of measurements gave an ICC of r = 0.99 for mean 40-m maximum sprint speed, r = 0.95 for mean 10 × 40-m RSA, r = 0.94 for mean 20-m acceleration speed, r = 0.97 for mean 20-m top speed, r = 0.91 for mean CMJ, and r = 0.85 for the mean Beep test.
The main finding in this study indicates that both the TG and the CG had a marked improvement in the 10 × 40-m RSA by 0.12 and 0.06 seconds, respectively. When between-groups results were examined, the TG showed a statistical significant progress than did the CG (p < 0.05) on RSA and 20-m top speed with a moderate effect (Table 2). No other studies have documented a similar effect on RSA. The improvement of performance on the RSA is substantial, especially when the athletes worked out only with specific speed training once a week over 10 weeks. Because the participants in the study trained in soccer 14 h·wk−1 on average, there was a slight concern if a 1 speed training a week would be enough to develop RSA. However, the participants did not perform any sprint training beyond this 1 exercise per week in the form of sprint drills. The participants performed interval training during their regular soccer training, but the interval training implemented was not in the form of sprint drills. The Interval training was aerobic endurance training that was carried out with short intervals and was under 75% of the maximum sprinting speed. This training could have a positive effect on both TG and CG performance on RSA only (Table 2). Furthermore, the RSA improvement noticed in both CG and TG can be because of the athletes' daily soccer training and (or) the learning benefits from the training between the pre to posttest. This could be a result of improving technique and the participants' ability to use their capacity better (10). Nevertheless, the notable and moderate improvement of TG over CG could be explained by the weekly extra speed training.
Running speed is a quotient of covered running distance and running time. With this formula, we have calculated that both TG and CG completed the 10 × 40-m sprinting pretest with 96% of maximum running speed. After calculating the equivalent percentage for the posttest, it was found to be 97% for both groups. This shows that the participants have the ability to complete repeated sprints with intensity closer to maximum capacity. Because both groups improved in this percentage, the results could be explained by their ordinary soccer training and the repeated sprint training program.
The within-group results indicate that TG had a marked progress of 0.06 seconds in the 40-m maximum sprinting speed test (Table 2). The split time of the 40-m maximum test shows that the improvement had occurred in the top running speed phase (20-40 m). The results show, however, that TG had a notable improvement in performance in the 20-m top speed when compared to CG (Table 2). However, there was no significant progress in the athletes' ability to accelerate (0-20 m), and the effect size (d = 0.2) of the repeated sprint program was small in the acceleration phase. This was surprising, because a previous study of rugby players reported significant progress in acceleration speed after similar training (12). One explanation could be that these studies have not been carried out on elite athletes or that 1 session per week in this study gave too little stimulant. Another explanation could be that participants in the Harrison and Bourke (12) study completed several maximum sprints up to 20 m in both regular Rugby training and games. Consequently, this could have stimulated and improved their ability to accelerate. Repeated sprinting over a longer distance (40 m) can be a new and unaccustomed stimulant for soccer players, which again can result in muscular and neural responses (21,31). Improvement in running performances in 60, 100, and 200 m was noticed in track and field training (phosphate training) (11,39). The results in this study indicate that the RSA seems to be as trainable as the ability to develop maximum speed. Spencer et al. (36) indicate that this is because of metabolic conditions such as energy system contribution, adenosine triphosphate depletion and resynthesis, phosphocreatine degradation and resynthesis, glycolysis and glycogenolysis, and purine nucleotide loss. Komi and Bosco (16) and Mero and Komi (22) suggested that it could be because of better use of stored elastic energy in leg extensors of the sprinters.
The results of this study indicate that speed endurance training can have a positive effect on the athletes' leaping power. The within-groups results show that the TG made notable progress on CMJ. This is further clear when examining the effect size of the training on the CMJ ability (Table 2). On examining the effect size, we noticed that even though there were no statistically notable differences between the groups, there was a moderate effect of the repeated sprint program, and this improvement was not unexpected because previous studies have documented that speed, leaping power, and strength can affect each other positively (7,9,34,35).
The progress made by the TG on the beep test was close to being statistically significant. The CG was not close to experiencing a statistically significant improvement. However, the comparison between groups shows that the effect size of the repeated sprint training program had a moderate effect on Beep-test performance, and this was surprising because there are certain studies, which imply a connection between repeated sprinting ability and maximum oxygen uptake (1,6,8,20). However, other studies have not found such a connection (2). Another possible explanation for the improved beep-test performance could be that the participants' work economy has developed through the repeated sprint training. Furthermore, several studies have documented that strength training, bounding, and speed training as a supplement to endurance training can improve work economy in running (24,32,33). Increased stiffness in muscles, better ability to store elastic energy, and improved rate of force development could be an explanation of the progress in work economy and the observed results in this study (24,32).
The results indicate that the RSA training implemented in this study had a positive effect on several of the measured variables including RSA performance. It is necessary to repeat the study on more participants is needed to generally draw a conclusion. Furthermore and according to the results of this study, we can advice the use of repeated sprint training similar to the one in this study only in periods in which the players have no speed training included in their program. Furthermore, because the sample size in this study is 20, the results are valid only for those who took part in this study. Furthermore, strength training could be essential in soccer, however, benefits were observed even without strength training is most likely to be caused by the training specificity.
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