Soccer is an intermittent sport that requires different physiological components. In modern football, physiological considerations are increasingly essential to optimal performance not only in adults, but also in children. The capacity of soccer players to produce varied forceful and explosive actions, such as sprinting, jumping, tackling, kicking, turning, and changing pace, highly influences soccer match performance, as suggested by others (30). The capacity to repeat explosive bouts is an important determinant of player performance (35) and is associated with high aerobic power (O2max) (31). However, the ability to produce a powerful single-bout effort (i.e., explosive actions) is as important as aerobic power for success in soccer (30). For instance, high-speed sprinting only contributes up to 3% of the total distance covered in children's games (6), yet most crucial moments of the game such as winning ball possession, scoring, or conceding goals depend on it (30). Initial acceleration, jumping, and agility are various explosive actions that are crucial when the player is involved in fast game play. Initial acceleration can be referred to as short sprint (0-10 m) (23), and agility can be recognized as the ability to change direction, start, and stop quickly (25,34). Game analyses have demonstrated the importance of these qualities in soccer since a mean sprint time of 2.3 seconds (10-12-m sprint) (6) and a mean of 50 turns per game have been recorded (38). Such explosive actions are integral elements for success in soccer and have to be trained independently from aerobic power with an optimal training program (18).
Training for maximal strength has been suggested to play a major role in improving explosive actions as a result of an increased force availability (11). Conversely, explosive high-velocity training has demonstrated greater improvements in rate of force development and explosive actions in comparison with traditional weight training methods for maximal strength (17,36). The literature related to children's strength training reaffirms such controversy. Some studies have found that changes in strength had a significant effect on explosive actions (7,19), whereas others have reported no significant improvements (14,16). The absence of several stimuli during strength training could explain this inconsistency: (a) segmental coordination, in regard to power transport by biarticular muscles, and neural control mechanisms for optimal movement patterns (32); (b) specificity, according to joint angle and angular velocities (13); and (c) eccentric overloading (15).
Plyometric training does provide such training stimuli and has shown evidence to improve explosive actions in pubertal (5,26) and prepubertal (12,22,23) populations. Previous concerns regarding the safety of plyometric training for children have been dispelled by the American College of Sports Medicine (1). To minimize the risk of injury, close supervision, proper technique, and progressive training programs have to be undertaken. Because plyometric training requires similar movements encountered in usual playing activities of children, no specific strength level is required to commence a plyometric program (9). Off-season and pre-season plyometric programs may reduce the instance of injuries and improve sport performance in children by strengthening the supporting structures (i.e., ligaments, tendons, and bone) and enhancing muscular performance (muscular strength, muscular endurance, and muscular power). An inactive off-season period of 8 to 12 weeks is likely to result in a state of detraining in children (22). Plyometric programs, in collaboration with other training regimens, are often implemented only during the pre-season to bring the children to a certain fitness level for the beginning of the season. In such training program design, it is difficult to allocate the improvement in explosive actions to the plyometric training only or the combination of the return to activity after a detraining period and the plyometric training. Previous studies (5,26) demonstrated the benefits of a plyometric training program in pubertal boys during a basketball season, suggesting that in-season specific training might be necessary to maintain or increase the explosive actions ability. However, in previous studies investigating the effect of plyometric training on explosive actions of pre-pubertal and pubertal children (5,12,22,23,26), plyometric training was always an additional load to the regular sport activity, raising the question of whether the improvement was a result of the new training regimen or only because of the additional training load. To the author's knowledge, it is unknown if a short-term plyometric program implemented as a substitute for some soccer drills within regular soccer practice in-season would enhance specific explosive actions of early pubertal players compared with soccer training alone. It was hypothesized that the combination of soccer drills and specific power training with no additional training time in-season would enhance explosive actions to a greater extent than soccer drills only.
Experimental Approach to the Problem
This study examined the ability of a short-term plyometric training program implemented as a substitute for some soccer drills within regular soccer practice to improve explosive actions compared to soccer practice only during the in-season. Two groups were formed from early pubertal male soccer players; 1 followed the modified soccer practice (training group, TG) and the other followed the regular soccer practice (control group, CG). All participants completed a battery of 6 tests before and after an 8-week period. Tests were related to different jump variables, initial acceleration, and agility, which were regarded as essential components to be successful in soccer (31). All participants attended 2 practices per week lasting for 90 minutes.
All testing procedures and risks were fully explained, and both parents and participants were asked to provide their written consent prior to the start of the study. The study was approved by the Institutional Review Board for use of human subjects of the university. Fourteen soccer players belonging to the same team were enrolled as the TG and 11 soccer players from a different team were defined as the CG. All players had a 2- to 4-year background of systematic soccer training and competition experience and had just followed a 4-week pre-season training after 2 months of off-season training. Randomized assignment of participants was not possible because of practical limitations. However, both teams played in the same league and age group and trained twice a week for 90 minutes using the same soccer drills. The anthropometric characteristics of the participants are presented in Table 1.
Standardized tests of explosive actions were performed before and immediately after training under the same weather and field conditions. Testing sessions were scheduled >48 hours following a competition or hard physical training to minimize the influence of fatigue. Participants followed a familiarization session to reduce any learning effects. Prior to testing, each subject underwent a 15-minute progressive standard warm-up on the field. All tests were performed on the same day and supervised and recorded by the same investigators. Test order was the same on both testing occasions and the better score of 2 trials was recorded for further analysis. Two minutes of rest was accorded between each trial to reduce fatigue effects. While waiting, participants performed low-intensity activity to maintain physiological readiness for the next test.
Vertical Jump Tests
All vertical jump heights were calculated from flight time (3). Both flight time and contact time were measured with a jumping mat (Ergojump, Globus Italia, Codogne, Italy). Three different types of vertical jumps were performed to assess specific parameters of the performance: squat jump (SJ), countermovement jump (CMJ), and contact test (CT). For all jump assessments, participants were asked to keep their hands placed on their hips to eliminate arm swing and to ensure that the back remains upright to reduce angular displacement of the hip. Participants also had to leave the ground with the knees and ankles extended and land in the same position and location to minimize horizontal displacement and influence on flight time. The SJ consisted of jumping vertically from a static squatting position with a knee angle of 120 degrees, assessing concentric power of the lower limbs. The CMJ test was comparable to SJ except that subjects started the jumps with a rapid downward movement (approximately 120-degree knee angle) to activate the slow stretch-shortening cycle (SSC ≥250 ms). The CT consisted of jumping over a 20-cm hurdle, and, on contact mat landing, immediately jumping as high as possible. The objective was to maximize the ratio between height and ground contact time (GCT), called reactive strength (40). These instructions required the participants to quickly reverse the downward motion of the body to an upward movement induced by a fast SSC (<250 ms) muscle action of the leg extensors. Such technique has previously been found to produce small knee flexion and short GCT (2). Jumping height after rebound and GCT were recorded for CT. Reactive strength was calculated as previously reported in the literature (40): Height (cm)/GCT (s).
Multiple 5 Bounds Test
The multiple 5 bounds test (MB5) was started from a standing position. The participants tried to cover the longest distance by performing a set of 5 forward jumps with alternative left- and right-leg contacts. Because of its specificity, especially for soccer players, the MB5 test is often used instead of the vertical jump as a measure of muscle power and coordination (12). The distance of the MB5 was measured to the nearest 0.5 cm using a tape measure.
Infrared photoelectric cells with polarizing filters and a handheld computer were used to measure sprint times to 1/100th of a second (Globus Italia, Codogne, Italy) and were placed at the start and at 10 m. The starting position was standardized for all participants. They started in a standing position (split stance) with the toe of the preferred foot forward 0.3 m behind the starting gate. This was intended to allow some forward lean and cause triggering of the timing system as soon as the subject moved. The photocells were set approximately 0.6 m above the floor, which was typically around hip level to capture the trunk movement rather than a false trigger from a limb. The participants were not permitted to use a “rolling” start, to eliminate momentum, and were instructed to sprint with maximum effort when they were ready. All sprints were performed with running shoes on a hard surface outside.
The agility test was performed on the field, with soccer shoes, and consisted of four 60-degree changes of direction over 10 m (Figure 1). The timing system and start procedure were the same as the 10-m sprint. Poles of approximately 1.5-m high were placed on the floor to indicate the change of direction. The participants were not allowed to touch the poles as they sprinted and changed direction. This test was selected because it required acceleration, deceleration, and balance control, which are facets of agility (34). Its relative simplicity minimized the role of learning effects.
For a period of 8 weeks in-season, the TG performed various plyometric drills for 20 to 25 minutes as a substitute for some soccer drills within the usual 90-minute practice twice per week. All plyometric sessions 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. Taking into consideration the stress of the plyometric training on the musculotendon unit, exercise intensity was progressively increased from a level classified as low to moderate, an appropriate intensity for children (1,8). Exercise intensity was determined as low to high with a scale of 1 to 5 (8), and exercise volume was determined by the number of ground contacts. The weekly load was calculated with the following formula: intensity × volume. The program was periodized in 2 progressive macrocyles of 3 weeks and 1 final progressive macrocyle of 2 weeks (Figure 2).
Plyometric drills included multiple jumps (ankle hop, vertical and lateral hurdle jump), horizontal and lateral bounding, skipping, and footwork (speed ladder). Each plyometric session was composed of 4 different exercises and 2 to 4 sets of 6 to 12 repetitions. All exercises were executed on the grass to reduce landing impact. Because most children did not have any history of plyometrics, particular attention was paid to demonstration and execution. Four basic techniques were stressed: (a) correct posture (i.e., spine erect, shoulders back) and body alignment (i.e., chest over knees) throughout the jump; (b) jumping straight up for vertical jumps, with no excessive side-to-side or forward-backward movement; (c) soft landings including toe-to-heel rocking and bent knees; and (d) instant recoil preparation for the next jump. Phrases such as “shock absorber” and “recoil like a spring” were used as verbal and visualization cues for each phase of the jump (20). Finally, some principles were respected for optimal benefits of the plyometric sessions. Every week, 1 session focused on vertical power (movement in a vertical direction) and the other on horizontal power (movement in a horizontal direction) with a minimum of 48 hours separating each session and games to ensure the players were always fresh to compete (8). All exercises were adapted to the coordination capacity of the children and performed at full speed. No drill lasted more than 10 seconds to ensure that muscular energy was mainly produced by intramuscular phosphagen degradation (28), and a 90-second rest period was given between each set of exercises to allow for resynthesis of phosphagens (29).
All values are reported as mean ± standard deviation (SD). Two-way [time (before vs. after) × group (TG vs. CG)] repeated measures analysis of variance (ANOVA) tests were used to determine differences in jumping and sprint performance variables. Significance was located with Tukey test post-hoc analysis. Relationships between post-training variables of the TG were investigated with Pearson's product moment coefficient. The level of significance was set at p < 0.05.
Over the training period significant changes were observed in both groups in stature and body mass (p < 0.001). No significant difference between the 2 groups was recorded before or after training (Table 1).
Vertical Jump Tests
Changes in SJ and CMJ height are shown in Table 2. At baseline, jump heights in SJ and CMJ were significantly higher in the TG than in CG (p < 0.05). After the training period, TG demonstrated a significant increase of 7.9% in CMJ (p = 0.004), whereas the change in performance of the CG in this test remained nonsignificant (p = 0.15). Figure 3A shows responders and nonresponders to training and mean scores of CMJ performance for the TG (10 responders vs. 4 nonresponders). No significant modification in SJ performance was recorded in any of the groups (p > 0.05).
Table 3 illustrates the changes in CT variables. At baseline and post-training, GCT of CT was lower in CG than in TG (p < 0.05). The plyometric training followed by the TG had a beneficial impact on fast SSC movement of the lower limbs because jumping height after rebound increased by 10.9% (p = 0.01) Figure 3B shows responders and nonresponders to training and mean scores of jump height after rebound for the TG (11 responders vs. 3 nonresponders). However, no significant influence was observed on GCT (p > 0.05), resulting in a nonsignificant change in reactive strength (p > 0.05). The CG exhibited no significant changes in any of the CT variables (p > 0.05).
Multiple 5 Bounds Test
The intervention had no impact on 5 rebound jumps in either group (TG: 9.9 ± 0.9 to 10.3 ± 0.6 m; CG: 9.3 ± 1.0 to 9.6 ± 0.6 m; p > 0.05). Still, a significant difference between CG and TG was observed (p = 0.007) after training, induced by a 4% increase in this test for the TG (p = 0.06). In addition, 5MB performance and reactive strength were positively correlated (r = 0.66; p < 0.01).
At baseline, the TG was significantly faster than the CG in the 10-m sprint (p = 0.02). Training led to a significant decrease of 2.1% (1.96 ± 0.07 to 1.92 ± 0.07 seconds; p = 0.004) in sprint time for the TG, whereas no significant alteration of the CG performance was recorded (2.06 ± 0.12 to 2.01 ± 0.07 seconds; p = 0.15). Figure 3C shows responders and nonresponders to training and mean scores of 10-m sprint time for the TG (12 responders vs. 2 nonresponders). After the training period, a significant relationship between CMJ performance and 10-m sprint time was found (r = −0.67; p = 0.007) (Figure 4).
The training program had a beneficial impact on agility of the TG, resulting in a significant decrease of 9.6% (4.69 ± 0.16 to 4.24 ± 0.17 seconds; p < 0.001) in the agility test time for the TG. In contrast, agility test time significantly increased by 2.8% in the CG (4.58 ± 0.22 to 4.70 ± 0.25 seconds; p = 0.03). Figure 3D shows responders and nonresponders to training and mean scores of agility test time for the TG (13 responders vs. 0 nonresponders).
The current study indicated that 8 weeks of plyometric training within soccer practice induced positive effects on explosive actions of early pubertal soccer players. Significant improvements were observed in 10-m sprint, agility test, jump height (CMJ), and jump height after rebound (CT). Figure 3 highlights these results, showing that in most cases not only mean group improvement occurred, but also individual improvement (responders vs. nonresponders). No significant changes in any test variables were observed in the CG, demonstrating the importance of specific power training to enhance explosive actions of soccer players. In addition, because baseline data showed a significant difference between the 2 groups in CMJ and 10-m sprint, the window for improvement in these variables was smaller for the TG. Still, the TG demonstrated a statistically significant performance improvement in these 2 tests, in contrast to the CG. This observation reinforces the value of an independent power training program to enhance explosive actions of soccer players. Such improvement could have a positive influence on game performance because the ability to win challenges and score goals is related to this type of physical demand.
The 10-m sprint (initial acceleration) and agility test had the particularity to assess the specific sprint ability of early pubertal soccer players. To the authors' knowledge, no study has looked at the relevance of a plyometric program incorporated within regular soccer practice, in either an adult or children's population, to improve these qualities. Because an earlier study (25) demonstrated that agility and acceleration are independent qualities in soccer, it was necessary to assess them with specific testing. Both initial acceleration and agility have been found to be powerful discriminators between elite junior players and regional junior players and therefore should be used as descriptive tests for soccer performance (31). The distance of 10 m appeared to be the most relevant to assess the specific quality of acceleration in soccer because of the high frequency of short, high-intensity sprints during a game (6). A significant decrease in 10-m sprint time (−2.1%) in the TG demonstrated the efficiency of a plyometric program to improve specific explosive actions of young soccer players. The percentage of change in performance after a training period in the current study is in accordance with previous findings on initial acceleration in children (7,23). However, none of the aforementioned studies have reported significant improvement.
Initial acceleration has been shown to be more difficult to enhance than maximal velocity, probably because of the smaller margin for improvement and the different forces involved (7,23). Therefore, even if the improvement in the current study was small, it offers insight on the possibility for greater improvements over a longer training period that should increase the capacity to win challenges in game situations. Several studies (10,27,39) were designed to determine the important factors for short-distance sprints. Murphy et al. (27) reported that GCT was the biggest discriminator between fast and slow sprinters at more than 15 m. The current study did not find any significant change in GCT after a vertical preload (CT), but change in GCT during acceleration could have occurred. Further studies should use force-plate or video analysis to determine possible decreases in GCT after plyometric training. Others (10,39) found a relationship between CMJ and the 10-m sprint. These results were confirmed by the present study (Figure 4) and can be explained by the specificity of the acceleration phase where the center of mass is lower and GCT is longer when compared to the maximal velocity phase, resulting in a slow SSC of the muscle in similar motion to CMJ. This relationship verified the validity of an acyclic vertical jump to predict field performance and the role of vertical velocity and forces during initial acceleration.
The agility test was selected for its short duration (4 to 5 seconds) and its ability to test different qualities than the straight sprint (40). Previous studies in early and pubertal soccer players (7,22,31) used different agility tests, lasting from 7 (31) to 19 seconds (7), over a distance of 40 (22,31) to 50 m (7). These large differences in test selection did not allow comparison with the current study. The goal of the present study was to remain specific to the explosive bouts encountered during a soccer game. For this reason, the agility test was short (10 m) with multiple changes of direction. The significant change in agility time performance (−9.6%) demonstrated that a plyometric program can have a positive influence on a field test similar to game play and therefore may have an impact on true soccer performance. The plyometric drills selected contained many powerful lateral movements, which had an impact on the capacity to change direction faster. In addition, the plyometric training program may have improved the eccentric strength of the lower limb, a prevalent component in changes of direction during the deceleration phase (34).
The plyometric program was also effective in significantly increasing jump height of CMJ (+7.9%) and CT (+10.9%) but not SJ (+0.6%). The lack of improvement in the SJ performance can be explained by the study design. The plyometric training exclusively stressed the SSC of the muscles; consequently, pure concentric contraction, assessed by the SJ, was not stimulated during training. Previous studies (12) investigating plyometric training in children have reported the same tendency of stronger improvement in jump tests involving SSC, compared to pure concentric jump tests. In contrast, resistance training, involving minimal SSC of the muscle, has been shown to be more effective in improving SJ performance as compared to CMJ (7,24). Therefore, the training mode must be chosen carefully in regard to the field performance targeted. Because explosive actions in soccer mainly require muscular contractions involving the SSC, the current study did focus on improving such quality. The significant improvement in jump height in CMJ and CT tests confirms the effectiveness of the application of plyometric training in achieving this goal, which may improve game performance. Post-training CMJ performance results by the TG are in accordance with previous results reported in the literature after a plyometric training followed by children (12). In addition, the current results of the CMJ after the plyometric training program also appear to be greater than mixed training methods composed of resistance training and plyometric training followed by children despite a greater training load (22).
Improvement in the fast SSC (<250 ms) capacity of the muscle (32) after the plyometric training was assessed with the CT. Jump height was smaller in the CT compared to CMJ as a result of the requirement to reduce ground contact time (2), but a larger improvement was observed (+10.9%). Such results demonstrated that the plyometric program was more efficient in improving fast SSC capacity of the muscle (<250 ms) with small angular displacement, as compared to slow SSC (>250 ms in CMJ), probably because the movement pattern of the training program was similar to the CT. To the authors' knowledge, the present study is the first to report a significant change in a preloaded jump after a training program of any kind in children. The increased powerful concentric force after the SSC could have been induced by various neuromuscular adaptations involved during the stretch reflex and the storage of elastic energy in the SSC of the muscle: greater muscle stiffness at ground contact resulting in a fast recoil of the muscle (33) and subsequent better use of the elastic energy (4); greater muscle activity as a result of an earlier activation of the stretch reflex (4); and desensitization of the Golgi tendon organs, allowing the elastic component of muscles to undergo greater stretch (21). Because no physiological measurements (e.g. electromyography, motor units activation, muscle stiffness) were taken in the current study, the underlying adaptations induced by the plyometric training remain hypothetical. If such adaptations may enhance jump capacity, they had no significant influence on GCT (−2.3%) of CT. GCT was already below 250 ms (fast SSC) (32); therefore, the margin for improvement was small and an 8-week period was probably insufficient to influence this variable.
The ratio between jump height and GCT of the CT, referred as reactive strength, was calculated because it has been reported to be a predictor of running ability (e.g., sprinting, changes of direction) (40). No significant improvement was observed in this variable (+17.6%) in the TG, which can be explained by the lack of improvement in GCT. However, a significant relationship was found between the reactive strength and MB5 (r = 0.66), giving insight to the role of reactive strength in multiple SSC movement, such as maximal running velocity.
Some methodological limitations exist in the current study and need to be addressed. First, the subjects were defined as early pubertal, yet their biological maturation was not assessed. The maturation of the participants can vary considerably when the participants are 12 to 14 years old, which may affect neuromuscular adaptations and athletic performance. Therefore, it would be of interest to report the maturation level of the children and investigate if different neuromuscular adaptations and athletic performance occur after plyometric training depending on their maturation level. Second, the current study did not quantify the neuromuscular changes after plyometric training and no ground reaction force measurements were collected during the jump, sprint, or agility performance. Such methods should be used in further studies to provide a better understanding of the adaptations induced by plyometric training in child populations. Third, no methods were undertaken to determine whether the plyometric training program had a true effect on game play. Only speculation can be made that greater performance in descriptive tests will result in superior match-play performance, as suggested by others (37).
The practical implication of the current research would be that plyometric training combined with soccer training leads to greater improvements in numerous explosive actions than soccer training alone. Soccer-related athletic abilities such as vertical jumps involving SSC movement (CMJ, CT), acceleration, and agility performance significantly improved in the training group only. Such improvements can be beneficial to winning challenges and could be transferred into game-play performance. The training stimulus was not suitable to improve pure concentric movement (SJ) and predictive measures of field performance (5MB, reactive strength); therefore, strength and conditioning coaches must be aware of the specificity of plyometric training. Besides enhancing explosive actions of young soccer players, plyometric drills demonstrate the following advantages: simulation of explosive sport movements on the field, adaptability to a specific population, integration within practice, and inexpensive equipment requirements compared to a weight room. No training-related injuries were reported; however, coaches should consider progressive increases in the load and ensure exercises are performed on soft landing surfaces, reducing the probability of player injury. To improve the quality of every session, small technical drills activity between sets should be implemented to maintain awareness and discipline among the children who may become impatient when they have to remain inactive. Because there are noticeable athletic differences among children in this age group, it could also be worthwhile to set up 2 different levels of the same exercise to increase the personalization of the training program.
The authors wish to thank Jean-Sébastien Scharl and “Centre Analyse Sport et Santé” (CASS, University of Lausanne) for their technical assistance; the participants for their contribution to the study; and Ken Nosaka, Michael McGuigan, Joseph Maté, and Travis McMaster for their suggestions and English language editing.
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