Soccer is an intermittent sport, which requires different physiological components. The capacity to produce varied powerful actions during a 90-minute game is associated with high aerobic capacity (41). However, the ability to produce an explosive single-bout effort is as important as aerobic power for success in soccer (9). This includes movements such as sprinting, jumping, changing direction, throwing, or kicking frequently occurring in soccer (41). Many of these activities not only require maximal power but also a high rate of power development considering the short period spent on the ground to produce power, such as sprinting or changing direction (<100 milliseconds) (1,26). Various studies demonstrated that youth elite and subelite players were found to be faster, more agile, and more powerful than nonelite (16,45), whereas future international and professional players had superior explosive characteristics (i.e., speed, power) at youth level than future amateur players (21). These results support the fact that soccer-related explosive activities requiring power may not only be important qualities at youth level (16,45) but also at a later stage of a player's career (21) and need to be developed from a young age.
Plyometric exercises are commonly used to increase explosive actions in pubertal (42) and prepubertal (8,28,29) soccer players, with the advantage of being easy to integrate in soccer practice (space, time, equipment) and replicating the neuromuscular stimulus encountered in explosive soccer activities such as sprinting and jumping (12). Previous studies demonstrated that high intensity plyometric exercises, such as drop jumps, can be used safely and effectively from the beginning of training in young population (4) and soccer players (42). Generally, the high intensity requirements of drop jump training imply a reduced volume in training (4,7) and therefore may require less time to complete than other plyometric modes, while inducing comparable training adaptations to slow stretch-shortening cycle (SSC) training (42). The ability to continue improving explosive action during in-season is a challenge because of the limited time available for isolated training, when the emphasis is mostly placed on technical development in youth soccer (27). Meylan and Malatesta (28) demonstrated that in-season high volume plyometric training (∼100–120 ground contact/session) can increase explosive performance, but it remains unknown if a low-volume high-intensity training may induce similar changes in sprinting, jumping, and change of direction. Such approach may appear relevant to the time constraint that coaches may encounter for both physical and technical development of players.
Apart from sprinting, jumping, and change of direction, explosive training may also be beneficial to other soccer-specific athletic requirements such as ball kicking or endurance. Previous studies (29,48) investigating kicking velocity or distance demonstrated the efficiency of explosive training to improve this quality, but improvement in aerobic performance remained controversial. Some studies in explosive training in youth soccer players did not demonstrate any improvement in V[Combining Dot Above]O2max (29) or lactate thresholds (14), whereas others (48) demonstrated the efficiency of explosive training to improve Yo-Yo intermittent recovery test and submaximal running cost. Similarly, research in adult runners demonstrated that plyometric training may improve running time trial and economy but not V[Combining Dot Above]O2max and lactate threshold (32,40,44). It is therefore of interest to identify if plyometric in youth players have a positive influence on middle-distance running time trial, considering its multiple facet requirement (V[Combining Dot Above]O2max, lactate threshold and running economy) (32), likely to affect aerobic performance in soccer (41).
Given the limitations of the current literature, the purpose of this study was to determine the effect of replacing some soccer drills with low-volume high-intensity plyometric training exercises on explosive actions and middle-distance time trial of young soccer players during in-season. It was hypothesized that the replacement of some soccer drills with plyometric exercises, with no additional training time in-season, would enhance explosive actions and aerobic performance to a greater extent than soccer training alone.
Experimental Approach to the Problem
We examined the ability of an in-season short-term plyometric training program, implemented as a substitute for some soccer drills within the regular soccer practice, to improve physical performance compared with soccer practice alone. Two groups were formed from young male soccer players; one followed the modified soccer practice (training group [TG]) and the other followed the regular soccer practice (control group [CG]). Before and after a 7-week period, all players executed a battery of 8 tests related to explosive and endurance performance. This was a randomized controlled trial. The assigned groups were determined by a chance process (a random number generator on a computer) and could not be predicted. This procedure was established according to the “CONSORT” statement, which can be found at http://www.consort-statement.org.
Initially 121 male soccer players between 10 and 16 years of age fulfilled the inclusion criteria to participate in the study. Subjects were recruited from 4 different soccer teams with similar competitive schedule (1 official competitive game per week) and soccer drills used during their 2 weekly training sessions. Soccer players fulfilled the following inclusion criteria: (a) more than 2-year background of systematic soccer training and competition experience, (b) continuous soccer training in the last 6 months, (c) no plyometric training experience in the last 6 months, (d) no background in regular strength training or competitive sports activity that involved any kind of jumping training exercise during the treatment. To be included in the final analyses, participants were required to complete all the familiarization sessions, training sessions, and complete all tests, which resulted in 76 players included for the final analyses. Subjects were randomly divided into a CG (N = 38) and plyometric TG (N = 38). Subject characteristics are provided in Table 1. Institutional review board approval for our study was obtained, and all subjects (and their parents or guardians) were carefully informed about the experiment procedures and about the possible risk and benefits associated with the participation in the study, and an appropriate signed informed consent/assent document has been obtained pursuant to law before any of the tests were performed. We comply with the human and animal experimentation policy statements guidelines of the American College of Sport Medicine. Sample size was computed according to the changes observed in plyometric (i.e., reactive strength index) performance (d = 0.3 mm·ms−1; SD = 0.35) in a group of young adolescents submitted to the same training program applied in this study (4). A total of 8 participants per group would yield a power of 80% and a = 0.05.
Subjects followed a familiarization session of 90 minutes before testing to reduce any learning effects. Standardized tests were scheduled >48 hours after a competition or hard physical training to minimize the influence of fatigue and performed under similar weather, time, and field conditions before and immediately after the 7-week period over 2 days. On day 1, players characteristics (height, body mass, and self-assessed Tanner pubic hair and genital stage), and performance test were conducted in the following order: countermovement jump (CMJ), 20- (RSI20) and 40- (RSI40) cm drop jump reactive strength index, 5 alternated leg bounds test (MB5), 20-m sprint (20 m), and Illinois agility test. On day 2, maximal kicking test for distance (MKD) followed by a 2.4-km time trial were performed. All tests were administered in the same order before and after training in the same sporting clothes and recorded by the same investigators. In addition, all participants (and their parents or guardians) were instructed to have a good night's sleep (≥9 hours) before each testing day, to avoid drinking, or eating at least 2–3 hours before measurements. All participants were motivated to give their maximum effort during performance measurements. At least 2 minutes of rest was allowed between each trial to reduce fatigue effects. While waiting, the participants performed low-intensity activity to maintain physiological readiness for the next test. The best score of 3 trials was recorded for all performance tests apart for the single 2.4-km time trial. As in previous studies from our laboratory (4), high intraclass correlation coefficients were obtained for the different performance test varying between 0.81 and 0.98.
Height was measured using a wall-mounted stadiometer (Butterfly, Shanghai, China) recorded to the nearest 0.5 cm, body mass was measured to the nearest 0.1 kg using a digital scale (BC-554 Ironman Body Composition Monitor; Tanita, Illinois, USA), and body mass index (BMI) was calculated (kg·m−2). Maturity was determined by self-assessment of Tanner stage (46).
Vertical Jump Tests
Testing included the execution of maximal CMJ, RSI20, and RSI40. All jumps were performed on a mobile contact mat (Ergojump; Globus, Codogne, Italy) with arms akimbo. Take-off and landing was standardized to full knee and ankle extension on the same spot. The participants were instructed to maximize jump height and minimize ground contact time during the RSI20 and RSI40 after dropping down from a 20- and 40-cm drop box, respectively. The reactive strength index was calculated as previously reported (49).
Multiple 5 Bounds Test
The multiple 5 bounds test (MB5) was started from a standing position and performed a set of 5 forward jumps with alternative left- and right-leg contacts to cover the longest distance possible. The distance of the MB5 was measured to the nearest 0.5 cm using a tape measure (28).
Twenty-Meter Sprint and Illinois Agility Test
The sprint time was measured to the nearest 0.01 seconds using single beam infrared reds photoelectric cells (Globus Italia, Codogne, Italy). The starting position was standardized to a still split standing position with the toe of the preferred foot forward and behind the starting line. Sprint start was given by a random sound, which triggers timing. The photoelectric signal was positioned at 20 m and set ∼0.7 m above the floor (i.e., hip level) to capture the trunk movement rather than a false trigger from a limb. The Illinois agility test has been described, and its reliability addressed elsewhere (15). The timing system and procedures were same as the 20-m sprint, except that subjects started lying on their stomach on the floor with their face down.
Maximal Kicking Distance Test
After a standard warm-up, each player kicked a new size 5 soccer ball (Nike Seitiro, FIFA certified) (10) for maximal distance on a soccer field. Two markers were placed on the ground side by side to define the kick line. Participants performed a maximal instep kick with their dominant leg after a run up of 2 strides. A 75-m metric tape was placed between the kicking line and across the soccer field. An assessor was placed near the region where the ball land after the kick to mark the point of contact and to measure the distance kicked. The distance was measured to the nearest 0.2 m. All measurements were completed with a wind velocity <20 km·h−1 (Chilean Meteorological Service, Santiago, Chile). Previous studies have reported a high reliability of similar soccer kicking test (23).
A 2.4-km Time Trial
After a warm-up of 2 laps and 4-minute rest, players performed 6 laps of a 400-m outdoor dirt track timed to the nearest second using a stopwatch. The wind velocity at all times was less than 9.9 km·h−1, the relative humidity was between 50 and 70%, and the temperature was between 15 and 20° C (Chilean Meteorological Service). Motivation was considered maximal, as this test was conducted as part of the selection process.
The study was completed during the mid-portion of their competition period. Before the competitive period, subjects completed 2 months of summer preseason training. The control group did not perform the plyometric training but performed their usual soccer training. A detailed description of the usual soccer training applied during the competition period is depicted in Table 2. To know the training load during the intervention, the session rating of perceived exertion (RPE) was determined (Table 1) by multiplying the soccer training duration (in minutes) by session RPE, as described previously in young soccer players (18). We used the Chilean translation of the 10-point category ratio scale (CR10-scale) modified by Foster et al. (11).
Before the initiation of the training period, the TG subjects were instructed on proper execution of all the exercises included in the program. During intervention, the TG removed the technical drills (i.e., ball control, ball pass, ball conduction and dribbling, ball kicking, ball heading exercises) and replace them with plyometric drills within the usual 90-minute practice twice per week for 7 weeks. This time frame or number of sessions are higher (3,22,42,43) or very similar (5,28) to those previously reported to induce significant explosive adaptations in young soccer players and youths (4). All plyometric sessions lasted 21 minutes and were performed just after the warm-up to ensure that the players were in a rested state and gain optimal benefits from the specific program, according to the training principle of priority (19). Plyometric drills included 2 sets of 10 repetitions of drop jumps from 20, 40, and 60 cm (i.e., 60 contacts) performed on a grass soccer field. Exercise intensity was determined as high (28) and exercise volume low (i.e., total ground contacts) (4). Although we did not increase the training volume during the 7-week period, as we used high intensity plyometric exercises performed with maximal effort, an adequate training stimulus was applied during each plyometric session, as previously demonstrated in young boys (4) and soccer players (22).
The rest period between repetitions and sets was of 15 (34) and 90 seconds (4), respectively, as previous research had demonstrated that this is an adequate rest interval for this type of training. The subjects were instructed to place their hands on their hips and step off the platforms with the supporting leg straight to avoid any initial upward propulsion or sinking, ensuring a drop height of 20, 40, and 60 cm. Participants were instructed to jump for maximal height and minimum contact time, every jump to maximize reactive strength (i.e., bounce drop jumps). As players did not have any history of formal plyometrics, all exercises were supervised, and particular attention was paid to demonstration and execution, giving maximal motivation to athletes during each jump. Training sessions were separated with a minimum of 48 hours (including games) to ensure that the players were always fresh to train (28). Aside from the formal training intervention, all participants attended their regular physical education classes.
All values are reported as mean ± SD. Relative changes (%) in performance and standardized effects (SEs) are expressed with 90% confidence limits. Normality and homoscedasticity assumptions for all data before and after intervention were checked respectively with Shapiro-Wilk and Levene's tests. To determine the effect of intervention (i.e., plyometric training) on explosive strength adaptations, a 2-way variance analysis with repeated measurements (2 groups × 2 times) was applied. When a significant F value was achieved across time or between groups, Tukey's post hoc procedures were performed to locate the pairwise differences between the mean values. The α level was set at p ≤ 0.05 for statistical significance. All statistical calculations were performed using STATISTICA statistical package (Version 8.0; StatSoft Inc., Tulsa, OK, USA). In addition to this null hypothesis testing, these data were also assessed for clinical significance using an approach based on the magnitudes of change. Threshold values for assessing magnitudes of SEs (changes as a fraction or multiple of baseline SD) were 0.20, 0.60, 1.2, and 2.0 for small, moderate, large, and very large, respectively (17). The effect was deemed unclear when the chance of benefit (a standardized improvement in performance of >0.20) was sufficiently high to warrant use of the intervention, but the risk of impairment was unacceptable. Such unclear effects were identified as those with an odds ratio of benefit to impairment of <66, a ratio that corresponds to an effect that is borderline possibly beneficial (25% chance of benefit) and borderline most unlikely detrimental (0.5% risk of harm). The effect was otherwise clear and reported as the magnitude of the observed value (17).
Before and after training, no significant difference were observed between the intervention and control group in height, body mass, BMI, or maturity status (Table 1). Also, no significant change within the groups was observed after the training period.
There was no significant difference between groups at baseline in all performance measures. Differences between groups became significant in RSI20 (p < 0.01), RSI40 (p < 0.01), MKD (p < 0.01), and agility (p ≤ 0.05) after the 7-week period (Table 3).
After training, the TG demonstrated a significant (p < 0.001) and small increase in CMJ, RSI20, RSI40, and MB5, whereas no significant changes were observed in CG (Table 3). The training program did not induce a significant change in sprint performance for the TG, whereas the CG exhibits a significant (p < 0.001) and small increase in 20-m time (Table 3). The training program had a beneficial impact on the Illinois agility test time, resulting in a significant (p < 0.001) and small decrease for the TG. In contrast, the CG achieves a significant (p < 0.001) and small increase in the Illinois agility test time (Table 3).
After intervention, a significant (p < 0.001) and small change was observed in MKD performance for the TG, whereas no significant change was observed in the CG (Table 3). After intervention, a significant (p < 0.001) and small change was observed in 2.4-km performance time for the TG, whereas no significant change was observed in the CG (Table 3).
For the CMJ, RSI20, RSI40, MB5, 20-m sprint time, Illinois agility test time, MKD, 2.4-km time trial, 88, 93, 95, 81, 32, 95, 70, and 88%, respectively of subjects from the TG were responders to training.
This study indicated that 7 weeks of plyometric training induced significant and small to moderate improvements in CMJ, RSI20, RSI40, MB5, Illinois agility test time, MKD, and 2.4-km time trial performances. These results show that the combination of soccer drills and specific power training with no additional training time in-season optimize general and soccer-specific explosiveness and endurance performance in young soccer payers.
Although we used higher rigor to include subjects in the final analyses (i.e., completion of all training session) compared with the previous interventions in young subjects (13,47), the magnitude change in CMJ (SE = 0.20) and MB5 (SE = 0.28) in this study was smaller than previously reported for both the CMJ (SE = 0.50–0.87) (3,8,28,48) and MB5 (SE = 0.44–0.86) (8,28), after explosive training with young soccer players using interventions of similar duration or number of sessions as in this study. However, these discrepancy in training effect can be attributed to the training specificity, as the previous studies mentioned above used both slow and fast SSC (3,8,28,48), which the former being similar to CMJ and horizsontal stimulus (8,28). The greater magnitude in RSI20 and RSI40 (SE = 0.37–0.57) would support such contention considering the fast SCC of the current training program. In addition, Meylan and Malatesta (28), who did not include any drop jump training into their plyometric program, found no significant change in reactive strength. Considering the necessity to produce a high rate of power development in explosive actions (27), the improvement in RSI may have enhanced physical parameters of game performance. The improvement observed could have been induced by various neuromuscular adaptations, such as increased neural drive to the agonist muscles, improved intermuscular coordination, changes in the muscle-tendon mechanical-stiffness characteristics, changes in muscle size or architecture, and changes in single-fiber mechanics (24), but because no physiological measurements were made, only speculations are possibly.
The lack of improvement in 20-m sprint time after the current plyometric training demonstrated that other training stimulus may be necessary to enhance sprinting performance of young soccer players during the competitive period. A lack of change in 15-m sprint time after drop jump–based plyometric training has been previously reported in 17-year-old soccer players (42). As the training stimulus was only vertical in nature, this may had reduced the chances for soccer players to gain adaptations, considering the importance of horizontal force production and the application in sprint performance (30) and the principle of training specificity (33,36). Despite the lack of 20-m sprint improvement, a small reduction to complete the Illinois agility test was found in the TG. The current results are similar to those reported by Thomas et al. (42), where high intensity bounce drop jumps had a small positive effect on agility performance in young soccer players but only a trivial effect on 15-m sprint time. An increase in power development (31), reactive strength (49), and eccentric strength (39) may have contributed to the improvement in agility performance, whereas acceleration may be more dependent in a slower stretch-shorten cycle and rate of power production similar to the CMJ (6), which was not targeted in the current training program.
The improvement in kicking performance demonstrated that soccer-specific explosive actions of young male soccer players can be enhanced during the competitive period with a short-term plyometric training program implemented as a substitute for some soccer drills. An improvement in kicking performance after plyometric training has been previously reported in preadolescent (29) and adolescent soccer players (35). As players had more than a 2-year background of systematic soccer training and competition experience and given the lack of improvement in the CG, the positive change in kicking performance are unlikely to be related to the technical training over the short-term period of 7-week in this study. It had been suggested that an increased strength and power of legs' extensor muscles because of plyometric training may increase kicking performance in young soccer players, and these changes could be attributed solely to neuromuscular adaptations (29). It may be that these neuromuscular adaptations had an effect on the biomechanical factors related to kicking performance, such as maximum linear velocity of the toe, ankle, knee, and hip at ball contact (20), resultant in higher ball kicking velocity and hence MKD.
The TG exhibited a small reduction in 2.4-km time trial and became significantly fitter than the CG, despite no additional aerobic training. The change in neuromuscular ability in this study, especially the RSI, is likely to be transferred into a better running economy (37,38), which could potentially explain the positive change in the 2.4-km time trial of the TG (32,40). Previous explosive training in young soccer players did not induce improvement in V[Combining Dot Above]O2max (29) or lactate thresholds (14) but was efficient at enhancing Yo-Yo intermittent recovery test level 1 performance (48). This discrepancy is likely related to the fact that the change in neuromuscular power after an explosive training can contribute to the change of direction during an intermittent test (e.g., Yo-Yo or 30-15 intermittent fitness test) with change of direction (2) or running economy (25,40) but has a limited influence on V[Combining Dot Above]O2max or lactate threshold (40,44). Given the multi-directional nature of the game and necessity to cover long distances (41), explosive neuromuscular training should be considered as a complimentary method to aerobic conditioning in youth soccer players in addition to its anaerobic function.
The replacement of technical exercises with low-volume high-intensity plyometric drop jump exercises was effective at improving several explosive actions and endurance capacity in youth soccer players, which may have high transference into game play performance. Thus, a twice-weekly short-term high intensity plyometric training program implemented as a substitute for some soccer drills within the regular in-season soccer practice can enhance explosive and endurance performance in young soccer players compared with soccer training alone. The reduced volume of plyometric training ensured minimal time allocated to non–soccer-specific training while maintaining continuous physical development of young players during the season. Considering that some young soccer teams (especially amateur teams) had limited time to train (e.g., 90 minutes, 2 times per week), the current findings may be relevant to programming plyometric training in this context.
Although concern has been expressed by some researchers regarding the injury risk during plyometric training, to the best of the author's knowledge, when adequate controlled plyometric training intervention had been applied, no important injuries had been reported. More so, plyometric training had been advocated as a preventive injury strategy and even as a rehabilitation tool. In fact, during our intervention, the relative high intensity of the training program did not result in any injuries, and it is important to notice that in the present investigation, subjects reported little subjective muscle pain after the training sessions (data not shown). However, practitioners need to be mindful of the players' movement competency before introducing drop jump exercises and place a considerable emphasis on coaching.
Also, in accordance to the concept of training specificity, drop jump training was most effective at improving tests replicating the training stimulus (RSI) and transferred to performance measures, where vertical neuromuscular power and reactive strength were relevant (CMJ, 5 MB, MKD, 2.5-km time trial). However, another or a complimentary training stimulus should be implemented to improve 20-m sprint time in young soccer players. Future studies should include training programs with multi-directional and unilateral-bilateral exercises, given the nature of sprinting and other explosive movement on the field (e.g., tackling, change of direction). Finally, short-term plyometric training program can also be considered as an intervention strategy to increase kicking ability and endurance in youth soccer players, but we recommend that this training method must be adequately incorporated in a comprehensive training program that develop the specific technical abilities critical to achieve adequate kicking performance (especially at young ages) and with an adequate aerobic conditioning program, to optimize training adaptations.
The authors declare that they do not have any conflict of interest in accordance with the journal policy and disclose no professional relationships with companies or manufacturers who will benefit from the results of the present study.
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