Soccer (football) is considered as the most popular sport in the world across both sexes and all age groups with respect to active players and also to spectators (34,35). In fact, approximately 265 million people play soccer, and the highest proportion of participants is under the age of 18 years (33).
During latest years, it has been shown that playing soccer can induce considerable beneficial effects on cardiovascular risk profiles and bone health from childhood to older age (9,56,72,82). In contrast to the beneficial effects, playing soccer may also induce an inherent risk of injuries (27,86). In fact, soccer is a high-intensity sport with frequent changes in movement, velocity, and direction with high impacts and many situations of contacts between players. Thus, soccer players have a substantial risk of injury, which has been estimated to be 1,000 times greater than other occupations (20). The widely accepted consensus for epidemiological studies (38) shows that soccer injuries come in a wide variety, but most of them affect the lower extremities, including the upper leg, knee, and ankle (24,49). Therefore, injury prevention is utmost importance, and it is necessary to implement preventive measures to reduce the risk of injury and, thus, to support the health benefits associated with playing soccer (22,23). Because of the frequency of injury, the resulting costs, and not least the personal suffering of the injured players, several studies have focused on injury prevention measures in soccer to keep players on the pitch (64,65,80,81). To prevent soccer injuries, several strategies have been developed by the teams. These strategies are ranged from protective equipment to warm-up and cool-down (80). Significant reductions of soccer injuries have been achieved by implementing intervention programmes focusing on intrinsic risk factors for specific injuries (80). Some studies demonstrated that eccentric strength reduced the risk of hamstring injury in different populations of soccer players (4,5,70). Other studies showed that neuromuscular training and exercises focusing on balance, strength, flexibility, and stability might also reduce the risk of injury (80). However, it is important to note that most of these studies were conducted in adult soccer players, and little data exist concerning the effects of preventing programmes on injury rate in elite adolescent soccer players.
Optimizing the physical potential of young soccer players is one of the main objectives of youth soccer academies. Indeed, elite soccer player must be prepared to perform and sustain high loads of training observed at elite level (36,66). The injury prevention aspects represent an important consideration for young elite soccer players and their academies. Strengthening muscles through resistance training will increase the forces they are capable of sustaining, making them more resistant to injury, whereas improved motor control and coordination will also improve balance and joint stability. For adolescent soccer players in particular, structural adaptations to resistance training may represent the key of injury prevention. These effects include strength enhancement of supporting connective tissues and passive joint stability, and also increased bone density and tensile strength, which are particularly useful in collision sports such as soccer (3,29).
Literature studies indicate that regular participation in an appropriately designed exercise program inclusive of resistance training can enhance bone mineral density and improve skeletal health (2,11) and likely reduce injury risk in young athletes (67,79). The benefits have been identified and related to training in sport disciplines such as long-distance running, gymnastics, swimming (13), and tennis (51). However, the development of muscle strength and power through resistance and strength training in adolescent soccer players in relation with performance enhancement and injuries prevention is, to the best of our knowledge, still uninvestigated.
Consequently, the first aim of this study was to examine the effect of 12 weeks with combined plyometric and resistance training on anthropometric and physical fitness parameters of elite young soccer players (13–15 years), during one season. The secondary aim was to examine whether such a training regime could reduce injuries in young soccer players.
We hypothesize that such kind of strength training when it is incorporated to soccer training program may increase physical performances and reduce injuries in the experimental group (EG).
Experimental Approach to the Problem
Physical fitness is a major component in soccer performance and an important health determinant. We aimed in this study to provide an update on the role of strength training in elite young soccer players and offer new perspectives for promoting resistance exercise as part of a long-term approach to youth physical development. Concerted efforts by practitioners and researchers are needed to raise awareness about the importance of enhancing muscular strength and motor skill proficiency in the early years especially in young soccer players. Indeed, data from recent meta-analyses indicate that strength training may enhance the muscular strength and motor performance of children and adolescents, and the effects of resistance training on motor performance skills seem to be more pronounced in the early years. Adolescents who do not develop sufficient levels of muscular strength and movement skill competency may be less-efficient “movers” on the playground and the sport field.
Consequently, young soccer players who are not exposed to an environment with opportunities to enhance their muscular strength early in the training process may not develop the prerequisite abilities that would allow them to participate at high level. Moreover, by periodically varying the training stimulus with periods of low-intensity, moderate-intensity, and high-intensity training, it is likely that long-term performance gains will be optimized, boredom will be reduced, and the risk of overuse injuries will decrease. Therefore, we designed an experimental randomized controlled trial to monitor, in young soccer players, anthropometric, physiological, physical measures throughout 12 weeks of strength training and then to evaluate its effect on injuries incidence.
The sample consisted of young male athletes ranging in age from 13 to 14 years, who are part of a player development program of regional level soccer's players, member of sector-based training centers, selected, and integrated toward the age of 13 years for a training formation that will last 3 years. During the selection process, hundreds of boys from different parts of Tunisia, especially, are evaluated and selected by coaches of the soccer club.
The boys selected during this phase present the adequate biological characteristics to play in all soccer positions: goalkeeper (GK), defender, midfielder, or forward.
After the selection process, the boys were enrolled in a training program developed by the soccer center. Fifty-two young soccer players (6 GKs, 16 defenders [Def], 16 midfielders [Mid], and 14 attackers [Att]) agreed to participate in the study. All players and their parents were properly informed of the nature of the study without being informed of its detailed aims. Each player and his parents or guardians were informed of the experimental risks, and both signed an informed consent form before the investigation. The study protocol was conducted in accordance with the ethical standards and the guidelines of the Ethical Committee of the University of Manouba, Tunisia, which had approved the experimental protocol and the procedures involved.
These players practice soccer 9–10 months a year, at a rate of 4–5 sessions a week with 1 competitive game per week, in addition to their school physical education. These players were divided on a randomized order according to the playing position into 2 groups: experimental group (EG, n = 26) and control group (CG, n = 26). Each group was composed by 3 GKs, 8 Def, 8 Mid, and 7 Att. Experimental group participated to a strength training program during 12 weeks (4 × 3 weeks separated by 1-week recovery) (Figure 1). The players from EG participated 2 to 3 sessions per week in a resistance training program aiming at preventing injuries and enhancing physical performance. The duration of a resistance training session was 90 minutes.
When fit, all players trained at the soccer center for 100 minutes each day from Monday to Friday during the season (32 weeks). Fit players also participated in up to 22 matches during the season, and these generally took place on the weekends for their home clubs. Each soccer training session generally consisted of a 15-minute warm-up, 20-minute technical training, 20-minute tactical training, 30-minute simulated competition, and a 15-minute cool-down. Within the team, players of all the different positions trained together.
It is for importance to note that all the players belong at the soccer center during the soccer season except the weekends when they return to their family and their soccer teams for the match. Consequently, the nutrition and hydration habits were well controlled by the nutritional staff of the soccer center. Nutritional guidelines were provided to the players for the weekends. Hence, the 2 groups (EG and CG) followed the same nutritional and hydration protocol during the soccer season.
Experimental group and CG were evaluated at 3 different time points (Figure 1): at the baseline (T0), at the start of the investigation, which took place during the first part of the season (September–October), at the middle of the experiment period (T1) (e.g., after 2 times of 3-week training +1-week recovery), and at the end of the experiment period (T2) (e.g., after 4 times of 3-week training +1-week recovery).
The measurements were made each time over a 2-day period. On the first testing day, after a standardized warm-up of 15 minutes, consisting of low-intensity running, followed by a series of exercises (high knee lift, butt kicks, straight line skipping), and then short accelerations, participants performed the running speed tests, agility test, and vertical jumping tests. On the second testing day, the players performed after a standardized warm-up a Yo–Yo intermittent recovery test level 1 (YYIRTL1).
Before beginning our protocol, the participants were thoroughly familiarized with all testing equipment and procedures. For all testing days, subjects were asked to refrain from any exhaustive physical activity during the 48 hours preceding the experiment and to eat a standardized meal 1.5 hours before measurements (10 kcal·kg−1, 55% of which came from carbohydrates, 33% from lipids, and 12% from proteins) as determined by the nutritionist. To minimize any effects of diurnal variation, the 3 testing sessions were conducted within 2 hours of the same time of the day.
Puberty Stage Assessment
The puberty stage was the indicator of biological maturity status. It was determined and recorded by a pediatrician experienced in the assessment of secondary sex characteristics according to the method of Tanner (76). Children at pubertal development stages 1–5 were evaluated. According to their pubescent status, the young soccer players and the CG belonged to Tanner stage (2–3).
Each participant came to the medical center of the training center for a medical examination and anthropometric measurements performed by a pediatrician at each period (T0, T1, and T2). Body height and body mass were measured with standard techniques to the nearest 0.1 cm and 0.1 kg, respectively, for each subject. To estimate the adiposity, skinfold thickness was measured at 4 sites on the left side of the body (triceps, biceps, subscapular, and suprailiac) using a Harpenden skin-fold calliper (British Indicators Ltd., Luton, England) for calculation of percent body fat according to the equations described by Durnin and Rahaman (21). All measurements were taken in the morning at 07.30 hours by the same investigator at each period (T0, T1, and T2).
Physical Fitness Characteristics
Physical fitness characteristics were determined using the following tests: (a) Running speed test (sprint test 10, 20, and 30 m), (b) agility test: shuttle run, and (c) vertical jumping: squat jump (SJ), countermovement jump (CMJ), and drop jump (DJ). All the players were familiarized with such tests.
Assessment of 10, 20, and 30 m Sprint Times
Linear sprint speed was evaluated over 30 m. Infrared timing gates (Cell Kit Speed Brower, Draper, UT) were positioned at the start line (0 m) and at 10, 20, and 30 m at a height of approximately 0.5 m off the ground. Athletes performed 3 repetitions with the best (fastest) times used for statistical analysis. A minimum of 4 minutes of recovery was provided between repetitions.
Assessment of Agility
The T-test was administered using the protocol outlined by Semenick (73). Participants performed 3 trials and the fastest time from 3 trials was used as the T-test score. Previous research showed that T-test was highly reliable with a intraclass correlation coefficient (ICC) and SEM of 0.96 and 0.1, respectively (14).
Assessment of Vertical Jumping
Each subject performed 3 kinds of maximal jump: (a) the SJ, (b) the CMJ, and (c) the DJ. The ground reaction force generated during these vertical jumps was estimated with an ergo jump (Opto Jump Microgate, Bolzano, Italy). In addition, the players performed a 5 jump test (5JT); each subject performed 3 jumps interspersed with 1-minute rest between each jump, and the best was used for analysis.
Yo–Yo Intermittent Recovery Test Level 1 Performance
The YYIRTL1 was used to estimate V[Combining Dot Above]O2max. Based on the equation from Bangsbo et al. (10), V[Combining Dot Above]O2max (ml·min−1·kg−1) = YYIRTL1 distance (m)* 0.008,4 + 36.4.
All tests were made by the same investigators, scheduled at the same time of day, performed in the same order and using the same apparatus at each period (T0 and T1).
Assessment of Injuries Rate During the Soccer Season
The medical staff reported and validated each injury in accordance with the Fédération Internationale de Football Association (FIFA) consensus statement (34). It was used to record the type, location, and severity of injuries. The database was checked weekly by the researchers and the medical staff (71).
Baseline data for the players were collected yearly, at the start of each season. Individual player participation in training and matches was registered. We also received a monthly standard injury form, on which the team medical staff recorded player injuries. Recorded injuries included any event resulting in the player being unable to train fully or to play matches (time-loss injuries), and the player was considered injured until the team's medical staff allowed full training and declared him available for match selection.
Location of Injuries
This study used the following 12 categories of injuries, which have been used in previous studies (38,45,85): foot, ankle, lower leg, knee, thigh, hip/groin, upper extremities, shoulder/clavicle, lumbar/sacrum/pelvis, head/face/neck/cervical, abdomen, and sternum/rib/dorsal.
Types of Injuries
Injuries were classified into 7 categories in accordance with the consensus statement for football (38): fractures and bone stress, joints (nonbone) and ligaments, muscles, and tendons, contusions, lacerations and skin lesions, central/peripheral nervous system, and other injuries.
In addition, injuries were also classified as traumatic (those with an acute onset) or overuse injuries (those without any known trauma).
The severity of each injury was defined according to the number of days elapsed from the date of injury to the date of the player's return to full participation in team training or availability for competition. The injury severity was classified into 4 categories that have been used in previous studies (33,38,85): minimal (≤3 days), mild (4–7 days), moderate (8–28 days), and severe (>28 days). In addition, recurrent injuries were recorded. Recurrent injuries were defined as injuries of the same type and location that occurred after the player recovered and returned to full participation. Recurrent injuries were classified as less severe, equally severe, or more severe in comparison with the original injury (15).
Calculation of Training and Match Exposure
Data of training and match exposure were collected on a weekly basis. For team match exposure, the total player match exposure time in hours for a team is given by (NMPMDM)/60, where NM is the number of team matches played per week, PM is the number of players in the team (normally 11), and DM is the duration of the match in minutes (normally 90) (38).
For team training exposure, the total training exposure time in hours is given by the sum of the values for (PTDT)/60 for every training session throughout the study, where PT is the number of players attending a training session and DT is the duration of the training session in minutes (38).
Injury rate was calculated as the number of injuries per 1,000 hours of exposure.
Strength Training Program
The intervention consisted of 12 weeks of progressive resistance training divided into 3 phases: familiarization phase, progression phase 1, and progression phase 2.
The players were supervised by the physical coaches for ensuring that exercise prescriptions were properly performed and achieved during a particular workout (e.g., velocity of movement, appropriate spotting and technique, and also safety considerations, intensity of the training, and prescribed rest periods). At every training session, the players noted the number of sets, reps, and training load for each exercise in a training diary.
Baseline information relating to the players in the EG and CG was collected at the beginning of the intervention season.
The program was periodized, starting with a 2 weeks of familiarization phase with 3 training sessions a week. During this phase, the players performed exercises with simple low load (50–60% of 1 repetition maximum [1RM]). The type of program design was recommended when resistance training is introduced, and the more advanced the athletes become in performing the exercises, the more variation may be necessary to avoid performance plateaus (55).
The familiarization phase consisted of a basic program with 10 exercises, which were given to players in EG. The familiarization phase focused on the correct technique and form, and adaptation of the load. After the familiarization phase, the players in EG were given an individual training program and included most of the exercises from the basic program. The individual training programs were partly based on the pretest results, but they also took into account of the players' capacity. For progression phase 1, the aim of the training was progression in strength and power with 3 training sessions a week for 3 weeks. The training load was approximately 30–50% of 1RM (15–20 repetitions) and was increased, if possible, in a progressive manner every 2 weeks to maintain the 70% level. Progression phase 2 consisted of 6 weeks interspersed with 1-week recovery of high-intensity resistance training, with 3 training sessions a week, focusing on maximum strength and power gains. The individual program for each player did not change during this phase, but the training intensity increased. The players increased the training load to 80% of 1RM in some of the multiple-joint exercises, such as the squat and the bench press. The players were instructed to complete all repetitions of all sets, even if assistance was required for the last few repetitions of a particular set (so-called forced reps).
The players in EG were tested, in week 3 and in week 6 in squat, bench press, push-ups, and sit-ups to determine any flaws and to evaluate performance enhancement.
Results are expressed as mean ± SDs. Significant differences were assumed when p ≤ 0.05. The data were analyzed using SPSS for Windows (version 16.0; SPSS, Inc., Chicago, IL, USA). All variables used in the study were checked for normality of distribution before the analyses; Kolmogorov–Smirnov tests were used for each variable. A 2-way repeated-measures analysis of variance (2 × 2) was performed to determine whether significant differences existed between groups (EG and CG) and testing throughout the time (T0, T1, and T2). If significant main effects or interactions were present, a Bonferroni post hoc analysis was conducted. A chi-Square test was used to assess injuries occurrence (percentage). The measurement reliability was assessed by ICCs.
Anthropometric and Physiological Characteristics
The baseline measures were not significantly different between EG and CG (Table 1). Height and weight increased significantly (p ≤ 0.05) during the 12-week training period in both groups with no difference between the 2 groups, except for the body height of which the increase was significantly higher (p ≤ 0.05) between T1 and T0 in training group (TG) compared with CG (3.6 ± 1.9 cm and 2.0 ± 1.7 cm, respectively) (Table 1). No significant changes were observed for lower limb length for neither group.
Mean V[Combining Dot Above]O2max value increased significantly (p ≤ 0.05) during the experimental period in both groups. Statistically significant difference (p ≤ 0.05) were observed at T2 between EG and CG. (71.2 ± 6.0 vs. 66.7 ± 4.3 ml·min−1·kg−1). The amount increase of V[Combining Dot Above]O2max was significantly higher (p ≤ 0.05) between T0 and T2 in EG compared with CG (Table 2). The improvement of the maximal aerobic speed was significantly higher (p ≤ 0.05) between T1 and T2 and between T0 and T2 in EG compared with CG (0.7 ± 1.2 vs. 0.1 ± 1.1 and 2.3 ± 1.6 vs. 1.5 ± 1.1 ml·min−1·kg−1).
Table 3 demonstrated worthwhile improvements in running times for 10 and 20 m in EG and CG. Compared to CG, EG performed significantly better in 10-m sprint running (p < 0.01) and 20 m (p ≤ 0.05) at T2. As shown in Table 4, the decrease in 10-m running time was significantly greater (p < 0.01) between T0 and T1 for CG compared with EG (−0.27 ± 0.19 seconds and −0.05 ± 0.13 seconds, respectively). Inversely, between T1 and T2, the 10-m running time improvement was significantly higher (p ≤ 0.05) in EG compared with CG. However, there were no significant changes in 30-m running times for the 2 groups between T0 and T1. Hence, Table 4 revealed greater decrease in 30-m sprint running in CG compared with EG (−0.42 ± 0.51 seconds and −0.14 ± 0.29 seconds, respectively), between T0 and T2.
As shown in Table 2, significant increases were observed in 5JT, SJ, and CMJ throughout the experimental period both for EG and CG. Significant differences (p ≤ 0.05) were observed in SJ at T2 and in CMJ at T1 between EG and CG (Table 3). In addition, the improvement amount was significantly greater (p ≤ 0.05) in 5JT between T0 and T1 and in SJ between T0 and T2, in EG compared with CG. Surprisingly, DJ performances were significantly improved during the 12-week training in CG and not in TG (Table 4).
Agility Test and Endurance Running Test
Similarly, both groups upgraded their performances throughout the training period in the T-test. Significant differences (p ≤ 0.05) were observed between EG and CG at T1 and T2 in T-test (Table 3). The T-test running time improvement was significantly (p < 0.01) better between T1 and T2 for EG compared with CG.
Only injuries that lasted for more than 3 days were taken into account for our analysis. As shown in Table 5, for our 2 groups, a total of 17 injuries were recorded over the soccer season (from October to April) and were higher in CG (13 injuries) than in EG (4 injuries). Most injuries were located at the lower limb. Regarding injury types, the most common injury occurred in CG during the soccer season was muscle strains and ankle sprain, which represent 46.2% and 23.1% of total injuries.
These 17 injuries were sustained by 32.69% of the total players during the season and were higher in CG (50%) than in EG (15%). Total exposure of time of CG was 5,590 hours (training: 4,732 hours; match: 858 hours), whereas for EG, total exposure of time was 5,700 hours (training: 4,842 hours; match: 858 hours).
The estimated total rate injuries per 1,000 hours of exposure were 2.32 and 0.70, respectively, for CG and EG. This difference was significantly higher in CG (P ≤ 0.05) (Table 5).
The 17 injuries recorded during the season accounted for a loss of 110 days, that is, absence from training and regular competitive season. For CG, 13 injuries recorded during the season accounted for a loss of 147 hours, that is, absence from training in the season and regular competitive season. Injuries by severity were as follows: 4 minimal (30.76%), 3 mild (23.07%), 5 moderate (38.46 %), and 1 severe (7.69%).
For EG, 4 injuries recorded during the season accounted for a loss of 18 hours, that is, absence from training in the season and regular competitive season. Injuries by severity were as follows: 1 minimal (25%), 1 mild (25%), and 2 moderate (50%). The nature and location of injuries are presented in Table 6.
The purpose of this study was to provide new perspectives for promoting strength training as part of a long-term approach to youth physical development in soccer. Data derived from fitness assessments have been systematically used over the last decade to identify the “pathway to success” and provide some explanations as to why academy football players are successful or not in acceding to higher echelons of play.
Understanding the process of elite development is a key element to attain positive results in any sport area. Several factors of a complex structure can contribute or inhibit elite development. Moreover, researchers agree that there is not a single type of factors leading to success, nor there is a model that could fit all countries or be applied to all sports (19). The issue is that top soccer teams seek talented individuals to develop elite players. Consequently, there is a permanent need to identify and nurture future elite performers to obtain results in high-level soccer. In the search for an answer, this study demonstrated that soccer training simultaneously with 12 weeks of combined plyometric and resistance training leads to enhancement of explosive strength and related parameters, and also improvement in endurance performance, and reduces the occurrence of injuries in young elite soccer players.
The traditional view of talent has been linked to the idea of ability or intelligence as genetically inherited and measurable through specific tests (28). Contemporary views of talent are domain-specific, so they present multiple areas of ability. The research investigating the potential long-term benefits of youth resistance training is needed, and due consideration must be given to the identification of youth with inadequate muscle strength and the promotion of sustainable programs that can prevent injuries of adverse health outcomes later in life (32).
To our knowledge, this is the first study to investigate the effect of combined plyometric resistance training on anthropometry and fitness components, and also number and type of injuries in highly trained young soccer players.
With regard to anthropometry characteristics, height and body mass increased significantly during the 12-week training, EG and CG. Physical activity is generally considered to be necessary for adequate growth and development (18). Recent literature has shown positive growth effects as long as proper nutrition and age-specific physical activity guidelines were met. Training may actually be an effective stimulus for growth and bone mineralization in adolescent (68). Previous studies reported that young soccer players were taller (40) and skeletally more mature compared with their chronological age counterparts (58). These results are probably associated with the level of training intensity (62) or physical activities performed by these adolescents (71).
V[Combining Dot Above]O2max estimated by the YYIRTL1 was the criterion of endurance in this study. Our results showed a significant increase of the estimated V[Combining Dot Above]O2max and the total distance covered during the YYIRTL1 (the average value of the total distance between T0 and T2 was, respectively, 1,015.4 ± 372.0 m and 1,907.7 ± 443.7 m for TG and 993.8 ± 347.7 m and 1,576.9 ± 320.6 m for CG). In CG, the enhancement was inherent in soccer players as observed recently by Hammami et al. (44). Hence, the amount increase of V[Combining Dot Above]O2max was significantly higher in EG. Strength training seems to improve the capacity of the oxygen transport and utilization and thus improve the aerobic capacity in our young EG (69). Recently, Grieco et al. (43) found that strength plyometric training is benefic to improve running economy in young trained.
The results reported by several studies concerning the effect of strength training on aerobic capacity are conflicting. Hickson (45) observed no significant changes in V[Combining Dot Above]O2max after heavy resistance strength training. The adaptation to exercise is related to the type of training stimulus. For heavy resistance, low-repetition strength exercise, such as weight training, results in increases in strength and in muscle cell hypertrophy with little or no increases in the maximum oxygen uptake (46).
Similarly, Hickson et al. (46) have observed unchanged V[Combining Dot Above]O2max over 10 weeks of maximal strength training. However, Gorostiaga et al. (41) observed a significant increase of endurance capacity after resistance training. These divergences in the literature data are associated with differences in training programs, the subject's initial level, and the methods of estimating endurance (41).
Over 12 weeks of training, all our soccer players (EG and CG) improved significantly (p ≤ 0.05) their sprint runs (10 and 20 m) and jumping height (5JT, SJ, and CMJ), which support the results observed recently by Hammami et al. (43) in elite adolescent soccer players. This suggests that improvements in explosive power production are linked primarily with a significant increase in the neural activation. Hence, soccer is a high-intensity game that places considerable demands on the neuromuscular system (8). The neuromuscular system produces maximal leg power output. This is important in soccer because it develops maximal strength and muscular power with high contraction velocities while executing some techniques, such as jumping, tackling, kicking heading, and sprinting, although data for young soccer players in terms of strength, jumps, and sprints are sparsely debated.
Recently, it has been found that performances on all 3 tasks were closely correlated. Greater squat strength was associated with higher jump height and also faster 5-m and 20-m sprint times. Interestingly, absolute squat strength was more closely linked to 5 m and jump performances, whereas relative strength (1RM divided by body mass) was more closely linked to 20-m sprint time. The investigators conclude that leg strength is closely associated with both sprint and jump performances in well-trained players. Stronger athletes tend to jump higher and spring faster. They also mention earlier studies that show positive effects of strength training on performance (16).
The increase of sprint running performance was significantly greater (p ≤ 0.05) in EG compared with CG after the strength training. The improvement in sprint performance occurred concomitantly with greater increases in 5JT and SJ performances in EG compared with CG during the training period. Our results agreed with several studies (30,84). However, Thomas et al. (77) observed vertical jump height improvements after 6 weeks of plyometric training, whereas no change in sprint performance was observed in soccer players from a professional soccer academy group (10–17 older). This result can, at least in part, be explained by the higher body height in EG. Indeed, Wong et al. (83) observed that body height was significantly correlated (p ≤ 0.05) with sprint times in young soccer players. The improvement in sprint performance occurred concomitantly with greater increases in 5JT and SJ performances in EG compared with CG during the training period. The focus of strength training has turned to neural components adaptations (12). It is established that high resistance training shows faster mobility of the nerve activity (53). Hence, trained athletes are able to recruit motor units faster, which results in a faster recruitment of the muscle fibers. This mechanism produces maximal strength in shorter time. These data indicate that strength training is effective for enhancement of short distance sprint and jumping performance in young soccer players.
However, measuring agility is substantial fitness component in soccer. Indeed, acceleration, deceleration, and direction changes occur frequently in soccer game. Therefore, T-test agility was applied in this investigation. Both EG and CG improved their T-test running time after the program training. The improvement rate was better in EG compared with CG between T1 and T2. Hence, combined plyometric and resistance training program enhanced explosive muscle power and flexibility. Our results are consistent with those found by Váczi et al. (78), after 6 weeks of in-season plyometric training, and Meylan and Malatesta (62), in children soccer players after 8 weeks of plyometric training. Enhancement in agility performance can be attributed to neural adaptations, in particular to improve intermuscular coordination.
Maximum strength can be improved through neuromuscular stabilization training. Neuromuscular stabilization training improves intramuscular coordination and intermuscular coordination. Intramuscular coordination is the ability of the neuromuscular system to allow optimum levels of motor unit recruitment and motor unit synchronization, allowing high levels of force production, stabilization, and force reduction. Intermuscular coordination is the ability of the neuromuscular system to allow agonists, antagonists, stabilizers, and neutralizers to work synergistically in an integrated, multiplanar environment. This leads to decreased Golgi tendon organ inhibition, decreased antagonistic inhibition, increased dynamic joint stabilization, and increased neuromuscular efficiency (60). These neurophysiological changes together may improve the ability to store and release elastic energy during the stretch-shortening cycle. Specifically, on landing after a depth jump, an increased level of preactivation enables the muscle sarcomeres to maintain their length, whereas the tendons keep elongating and store elastic energy (54).
In this 12-week study of young soccer players, a total of 17 injuries were recorded. Most injuries were located at the lower limb. Previous studies showed similar results (37). Most soccer injuries were caused by trauma; the proportion of overuse injuries accounts for between 9 and 34% of all injuries (1,39). Soccer injuries affect predominately the ankle and knee and also the muscles of the thigh and calf (26,57). This was expected because the nature of the game and the high demands placed on the lower limb. Soccer players are often exposed to demanding training and competition schedules, which may include repeated high-intensity exercise sessions performed on consecutive days, multiple times per week (52).
Each training and game place high physical demands on players as they experience repeated moderate and rapid accelerations and decelerations, explosive jumps, and muscle damage from eccentric loading or contact trauma (37). Muscle strains were the most common type of injury occurred in CG (46.2%), followed by ankle sprain (23.1%). In previous studies examining injuries in young soccer players, ankle injuries were, consistently, the most common type of injury (50).
However, in this investigation, injury rate was higher in CG (13 injuries) than in EG (4 injuries). This finding seemed to complement earlier study, which reported that muscularly weak boys were more susceptible to injury compared with peers of the same chronological age (6).
The incidence of soccer injury has been investigated in several studies; for youth players, the reported incidences range from 0.5 to 13.7 injuries per 1,000 hours exposure (7). In our study, the estimated total rate injuries per 1,000 hours of exposure for CG and EG were, respectively, 0.70 and 2.32. This difference was significantly higher for CG. From the data, it can be concluded that strength program training might be beneficial preventive measures for the soccer players. Our results were comparable to the literature. Twelve studies on injury prevention in youth and soccer players (13–19 years old) were found in the literature, of which six were designed to reduce the overall rate of injury (25,74,75), whereas the others focused on specific injuries (81). In most studies, there was evidence that injury prevention programs were effective (25,50,61,81).
One of the CG has a clavicle fracture when falling, responsable of loss time between 3 and 6 weeks. Faude (33) suggests that one approach might reduce the risk of falling by implementing particular preventive training, focusing on the improvement of coordination, balance, or neuromuscular performance (25,59,61). That is, the combined plyometric and resistance training has proven to be effective strategy for reducing injuries amount in young soccer. Current researches indicate a low risk of injury in young athletes who undertake an appropriate resistance training program. However, there are discrepancies between studies because of differences in methodology relating to injury definition, data collection strategies, and observation periods (33).
Besides, most resistance and plyometric training–related injuries are the result of indecorous exercise technique and inappropriate loads. Hence, Faigenbaum and Myer (31) have a concern regards toward plyometric training. Indeed, it can be safe and effective if it prescribed, individualized, and progressed appropriately over the time. Overall, resistance training is beneficial for increasing bone strength and muscle strength and then reduces the risk for injuries related to muscle imbalance (agonist/antagonist) (48). Resistance training has also been suggested to reduce the risk for musculoskeletal injuries or perhaps reduce the severity of such injury. Although studies reporting the direct effect of resistance training on injury rate reduction are limited, the physiological adaptations seen consequent to resistance training on bone, connective tissue, and muscle do imply enhanced protection against injury for individuals who participate in such a training program. Because bone is living tissue, it has the ability to remodel and adapt to the physical stresses imposed on it. Although bone will respond to many types of training programs, especially those with high strain such as jumping or running, it does seem that resistance training provides the greatest osteogenic (increase in bone mineral density) effect. Resistance training is beneficial for increasing bone strength, and muscular strength also seems to be positively related to bone mineral content and bone strength (17). Although to date there has been little research conducted on the direct effect of resistance training on connective tissue adaptations, what studies there are have reported increases in both the size and strength of ligaments and tendons (66). Finally, resistance training also has an important role in reducing the risk of musculoskeletal injuries related to muscle imbalance, expressed as either an agonist to antagonist ratio (i.e., knee flexors/knee extensors) or as a bilateral comparison (i.e., right and left knee flexors). Correction of the existing imbalance through a resistance training program is important to reducing the individual's risk for muscle injury.
This study is the first to include a strength training intervention for young elite soccer players over one season. This study shows that soccer training simultaneously with 12 weeks of combined plyometric and resistance training leads to enhancement of anthropometric and explosive strength and related parameters, and also improvement in endurance performance, and reduces the injury occurrence in young elite soccer players.
These results suggest that strength training program directly supervised by a strength and conditioning specialists over a period of 12 weeks led to a significant improvement in measured variables, compared with a CG of well-trained soccer players.
Combining this information with physiological and physical data, the improvements in muscle strength and power and improvement in sport performance are common benefits resulting from resistance training programs. In addition, resistance training has also been suggested to reduce the risk for musculoskeletal injuries or at least reduce the severity of such injury. The physiological adaptations seen consequent to resistance training on bone, connective tissue, and muscle do imply enhanced protection against injury for individuals who participate in such a resistance training program.
The authors gratefully acknowledge the soccer players who participated in this study with great enthusiasm. The authors pay a great tribute to Tunisian Football Association for its unfailing collaboration. The authors thank Professor Jens Bangsbo for his correction of the manuscript and suggestions.
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