Soccer researchers have examined the physical, physiological, and technical requirements of a range of small-sided games and training methods. In modern soccer, the physiological and physical demands are essential to optimal performance at all levels (adults, youth, and juniors). These demands include high-intensity movements (i.e., sprinting, jumping, cutting, changing of directions, or ball-shooting), moderate-intensity (jogging), and low-intensity (walking). These demands are influenced by different factors, these are: players position, skill level, style of play, and tactical strategies used by the team (25).
Careful inspection of a match reveals that during a typical game, a 2- to 4-second sprint occurs every 90 seconds (3,38); high-speed sprinting only contributes approximately 3% to the total distance covered in children's games (5), and the most crucial moments of the game, such as winning ball possession, scoring, or conceding goals depend on the ability of the athlete to perform these high-speed movements (36). It is generally accepted that high-intensity actions such as sprinting or vertical jumping are integral elements for success in soccer and therefore need to be trained as part of a periodized youth training program (23).
The sport of soccer is requiring athletes to become more athletic, as indicated by short-term muscle power becoming more crucial in many game situations. Jumping ability, acceleration, and sprinting make important contributions to the performance potential of soccer players (23). Some 96% of sprints are shorter than 30 m, and 49% are only over a distance of 10 m (46). Thus, the performance over distances of 10 m or less, and the velocity attained during the first step are considered to be key indicators of player potential (8,9). Because of the advantage of having greater speed, acceleration, and power, a great deal of research has focused on the development of sprint, vertical jump, and agility performance using a myriad of training methods, including speed training, sprint drills, sprinting against resistances, weight training, combined resistance and speed training, and plyometric training (PT) (13,14,39,40,42). Recently, numerous studies have focused on determining which training intervention maximizes improvements in sprint and jumping performance in both adult and young soccer players (9,10,34). The majority of the scientific studies looking at training interventions designed to improve these attributes have proposed in-season short-term training (e.g., 4–8 weeks) programs that use several weekly training interventions (9,10,15,34).
In young soccer players, PT does provide such training stimuli and has been shown to improve explosive actions in pubertal and prepubertal (8,15,19,28,34,50) populations. For example, Meylan and Malatesta (34) observed that 8 weeks of low-intensity PT implemented in conjunction with the soccer-specific training program of adolescent soccer players resulted in significantly improved 10-m sprint times and jump heights. Similarly, Wong et al. (50), after 12 weeks of combined strength and PT that was performed twice a week in addition to soccer training, observed greater increases in vertical jump, ball-shooting speed, and 10-m and 30-m sprint times performance compared with soccer training alone. All these training strategies suggest that the inclusion of a more intense and non–soccer-specific training stimulus, which is integrated into the normal soccer training program, has the potential to induce improvements in jumping and sprinting performances that are greater than maturation or soccer-specific training performance gains. The focus of this article is on the effects of PT and sprint training on physical and technical skill performance.
In previous studies investigating the effect of PT on explosive actions of prepubertal and pubertal children (8,15,19,28,34,50), PT was always focused on improving vertical jump and speed performance. Nevertheless, to the author's knowledge, no study has examined the effects of a short-term plyometric program (9 weeks) supplemented with sprint training as part of regular in-season training in pubertal soccer players. In light of the aforementioned considerations, the aim of this study was to examine the effects of an in-season, short-term plyometric program supplemented with sprint training program on specific explosive actions (i.e., change of directions, ball-shooting speed, and technical drills) among early pubertal soccer players. It was hypothesized that the combination of soccer technical drills and the proposed specific plyometric and sprint training program during a 9-week period would enhance players' explosive actions (i.e., sprinting, change of direction, jumping, and ball-shooting speed) to a greater extent than soccer training alone.
Experimental Approach to the Problem
This study examined the effects of a PT program mixed with acceleration (10 m) and technical skills within regular soccer practice on their capacity of jump, sprint, endurance, and technical skills in young soccer players. The training program was added to the normal soccer training during 18 sessions (9 weeks), and it was mainly characterised by short-term (low-volume) and moderate-intensity sessions (low-impact training program). To determine training effects, the following tests were selected: (a) 10-m sprint time (seconds); (b) 10-m agility test (seconds) with and without ball (starting from the right and left sides); (c) countermovement jump (CMJ) (centimeter) and Abalakov vertical jump (centimeter); (d) Yo-Yo intermittent endurance (IE) test (meter); and (e) ball-shooting speed (km·h−1). All tests were executed before and after the 9-week training period. The initial tests were completed in 2 days (Tuesday and Thursday) as part of a regular testing program. After the initial measurements, subjects were randomly assigned to 2 groups: Control group (CG) (n = 13) (only performed the soccer training program), and combined group (CombG) (n = 13) (soccer training program + plyometric + sprint + dribbling + shooting training). All training sessions were supervised. Every subject in the experimental groups performed the exercises at 6:15 PM. (before the soccer training). All participants attended 2 practices per week lasting for 40 minutes. The subjects were instructed to avoid any strenuous physical activity during the duration of the experiment and to maintain their dietary habits for the whole duration of the study.
This study involved a group of 26 young soccer players (all of them were players from the Real Betis Balompié Academy) between the ages of 14 and 15 years (Table 1). None of the subjects had any background in regular strength and power training or competitive sports that involved any kind of strength or power exercises during the treatment. Exclusion criteria included subjects with potential medical problems or a history of ankle, knee, or back pathology in the 3 months before the study or subjects with medical or orthopaedic problems who compromised their participation or performance in this study or any lower extremity reconstructive surgery in the past 2 years or unresolved musculoskeletal disorders. All subjects and their parents or legal guardians were carefully informed about the experiment procedures and about the potential risk and benefits associated with participation in the study and signed an informed consent document before any of the tests were performed. The study was conducted in accordance with the Declaration of Helsinki II, and it was approved by the ethics committee of the responsible department. This study was performed between March 2012 and May 2012.
The players were carefully familiarized with the test procedures of voluntary force and power production during several submaximal actions a few days before the measurements were taken, and the tests were also previously done for control training purposes. All tests to determine the vertical jump, sprint, agility, endurance, and technical skills capacity were carried out each 9 weeks (pretest and post-test), after 9 weeks of treatment. The performance tests were completed in 2 days. On day 1, the following tests were completed: measurement of height, body mass, CMJ test (centimeter), Abalakov test (centimeter), and Yo-Yo IE test (meter). On day 2, the 10-m sprint time (seconds), agility test (seconds), and ball-shooting speed test (km·h−1) were completed. Before the tests and after completing the anthropometric measurements, subjects carried out a standardized warm-up consisting of 5 minutes' submaximal running at 9 km·h−1 followed by light stretching. Subjects then performed 4 minutes of specific soccer warm-up (changes of directions, sprints, jumps, and heading including pass the ball and dribbling) and 4 minutes of stretching (first static and then dynamic). Additionally, sufficient rest was allowed between all tests to limit the effects of fatigue in subsequent tests.
Before the physical tests, body height, body weight (Seca 222; Seca-balance, New York, NY, USA), and body fat percentage of the subjects were determined. The fat percentage was calculated by means of measurements of skinfold thickness using a Harpenden skinfold calliper (ASSIST Creative Resources, Ltd., London, United Kingdom). The 7-site Jackson-Pollock formula (45), validated for use with athletes, was used in this study to estimate body density. Body fat percentage was then calculated using the appropriate formula recommended by American College of Sports Medicine (2), based on age and ethnicity. Two experienced testers assessed the anthropometric measurements throughout the entire study. The intraclass correlation coefficient (ICC) was 0.98.
Vertical Jump Tests
A CMJ and Abalakov vertical jump test were used to maximize stretch-shortening cycle activity and to assess explosive strength of the lower extremity muscles. Both tests were performed using an electronic contact platform (Ergo Jump Plus Bosco System, Muscle Lab. V7. 18; Ergotest Technology, Langesund, Norway). During the CMJ, the subject was instructed to rest his hands on his hips, but during the Abalakov test, the use of hand was free while performing a downward movement followed by a maximal effort vertical jump. All subjects were instructed to land in an upright position and to bend the knees after landing. The 2 extreme values of the 5 trials, with a pause of 10 seconds between jumps, were eliminated (the best and the worst), and the average of the 3 central values was used for the subsequent statistical analysis for each type of vertical jump. The ICC was 0.93 for CMJ and 0.91 for Abalakov vertical jump measurements indicating high reliability.
Ten-Meter Sprint Time
Sprint times were recorded for 2 distances: 0–5 m and 0–10 m. The 10-m sprint test was conducted outdoors on the soccer pitch (artificial grass). For all sprint tests, the subjects started the test using a standardized crouch start and commenced sprinting with a random sonorous sound. Infrared beams were positioned at the sprint distance to be measured with a photoelectric cell (Muscle Lab. V7.18; Ergotest Technology). Subjects were given 2 practice trials performed at half speed after a thorough warm-up to familiarize them with the timing device. Two trials were completed, and the best performance trial was used for the subsequent statistical analysis. Three minutes of rest were permitted between 10-m trials. The ICC was 0.96.
Ten-Meter Agility Test
The agility test was performed on the field, with soccer shoes, and consisted of four 60-degree changes of direction over 10 m (34). 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 because they sprinted and changed direction. This test was selected because it required acceleration, deceleration, and balance control, which are facets of agility (44). Its relative simplicity minimized the role of learning effects. Four trials were completed starting from each side (2 dribbling with the ball and 2 without ball), and the best performance trial was used for the subsequent statistical analysis. Three minutes of rest was permitted between trials. The ICC was 0.93 from the right side and 0.92 from the left side. For the agility dribbling with the ball, the ICC was 0.82 from the right side and 0.83 from the left side.
Ball-Shooting Speed Test
Ballistic strength production during a soccer shoot was evaluated in a soccer field. For the shoot, the subjects were instructed to use their preferred technique to shoot a soccer ball as fast as possible through a standard goal. Shoot tests were undertaken after a 10-minute standardized warm-up and using a standard soccer ball (mass, 440 g; circumference, 0.69 m). To simulate a typical soccer action, the players were allowed to prepare the shoot and they were instructed to shoot with maximal velocity toward the center of the goal from the penalty line (11 m far from the net). The coaches supervised this test closely to ensure that the required techniques were followed. Each subject continued until 10 correct consecutive shoots had been recorded (5 with each leg). Five to ten seconds of rest was allowed between the shoots. The ball-shooting speed was measured using Stalker Sports Radar (Texas, USA). The radar device was positioned on a tripod behind the shooter. The 2 extreme values of the trials were eliminated (the best and the worst), and the mean of the central values was used for the subsequent statistical analysis. The ICC was 0.90 with the right leg and 0.89 with the left leg.
Yo-Yo Intermittent Endurance Run: Level One
Yo-Yo IE test was performed according to the procedures suggested by Castagna et al. (6) and Krustup et al. (29). Because soccer includes high-intensity intermittent bouts of exercise, which stresses the anaerobic glycolysis metabolic pathway, the Yo-Yo IE is considered to closely match movement patterns seen in a soccer match. It was used to estimate the V[Combining Dot Above]O2max and to measure the IE capacity of younger soccer players. In this study, the Yo-Yo IE test started in stage 10, at 12.52 km·h−1, in standard 20-m intervals. The total distance covered in the shuttles was recorded for analysis, but the distance covered during the rest interval was excluded (6). The ICC was 0.93.
Soccer training took place 4 days per week (Monday, Wednesday, Thursday, and Friday). The experimental group (CombG) supplemented the soccer training with a proposed plyometric-sprint training program. During this time, CG underwent regular soccer training (40 minutes) and no additional sprint and technique were performed. This program consisted of exercises performed at maximal voluntary intensity using player's body weight (or body weight plus light resistances). The proposed plyometric-sprint training program took place 2 days per week (Monday and Thursday) during 9 weeks of treatment. Each session lasted 40 minutes and consisted of the following components: 10 minutes of standard warm-up (5 minutes' submaximal running at 6 km·h−1 and several displacements, stretching exercises for 5 minutes, and 2 submaximal exercises of jump), 25 minutes of plyometric-speed and technical drills work, and 5 minutes of stretching exercises. The plyometric-sprint exercises consisted of 1/2 squat with jump, skipping, stride length, sidelong jumps of 30-cm hurdle, vertical jumps in 30-cm hurdle, and second support of triple jump; 10-m sprint + technical dribbling + shoot (the same shoot of test) were added to all exercises in each repetition in CombG. The resting period between each series was 1 minute. The CG played an “extra” 40 minutes of soccer to equate total training and carried out the same testing protocols as the other groups. The training was performed on the artificial grass (the same as competition), with the subjects using appropriated soccer-equipped boots and clothes. All the training sessions were fully supervised by a certified strength and conditioning specialist to ensure that the proper technique was performed. All the subjects were carefully instructed before the treatment and received a practical demonstration and performed familiarization trials with the exercises. The treatment was performed before the start of the regular soccer training session. The subjects were instructed to avoid any strenuous physical activity, in addition to the programmed training intervention, for the duration of the experiment. Additionally, the subjects were encouraged to maintain their normal hydration levels, sleep, and dietary habits for the duration of the study. The training program followed by the experimental groups is outlined in Table 2.
Descriptive statistics (mean ± SD) for the different variables were calculated. The ICC was used to determine the reliability of the measurements. The distribution of each variable was examined with the Kolmogorov–Smirnov normality test. The training-related effects and the differences between the groups were assessed using a multiple analysis of variance with the contrast F of Snedecor. When a significant F-value was achieved, Bonferroni post hoc procedures were performed to locate the pairwise differences between the mean values. Holm's correction was used to control type 1 and 2 errors. The effect sizes (ESs) were calculated. Statistical significance was accepted at an α level of p ≤ 0.05.
No significant differences in the anthropometric variables measured (body weight, height, and % body fat) were observed in the pretest between the experimental and CGs. After 9 weeks of training, no significant changes were observed in any of the physical characteristics analyzed.
Statistically significant increases (p ≤ 0.05) occurred in the experimental group in CMJ (centimeter) (CombG [3 cm, 9.4%, ES = 0.9]) and in the Abalakov vertical jump (CombG [5.4 cm, 15.5%, ES = 1.3]). Significant differences (p ≤ 0.01) were observed after training in the magnitude of the increase between the experimental group and CG (Table 3).
During 9 weeks of training, statistically significant decreases (p ≤ 0.05) occurred in 5-m and 10-m sprint time in CombG (−0.07 seconds, 8.6%, ES = 0.7), (−0.09 seconds, 4.8%, ES = 0.9, respectively). Significant differences (p ≤ 0.01) were observed after training in the magnitude of the decrease between CombG and CG group (Table 3).
Statistically significant decreases (p ≤ 0.05) occurred in the experimental group in Agility test starting from the right side [CombG (−0.33 seconds, 7.9%, ES = 1.1)] and from the left side (CombG [−0.25 seconds, 5.8%, ES = 0.8]). Significant differences (p ≤ 0.01) were observed after training in the magnitude of the increase between the experimental group and CG (Table 3).
Agility Test With Ball
Statistically significant decreases (p ≤ 0.05) occurred in the experimental group in Agility test starting from the right side (CombG [−0.38 seconds, 7.3%, ES = 0.9]) and from the left side (CombG [−0.36 seconds, 6.5%, ES = 1.2]). Significant differences (p ≤ 0.01) were observed after training in the magnitude of the increase between the experimental group and CG (Table 3).
Ball-shooting speed (km·h−1) significantly increased (p ≤ 0.05) in the experimental group with the right leg (CombG [7.2 km·h−1, 9.1%, ES = 0.8]) and with the left leg (CombG [7.6 km·h−1, 10.1%, ES = 0.7]). Significant differences (p ≤ 0.01) were observed after training in the magnitude of the increase between CombG and CG (Table 3).
Yo-Yo Intermittent Endurance Test
No significant increases (p ≤ 0.05) were observed in the Yo-Yo IE test in the experimental group and CG. No significant differences (p ≤ 0.05) were observed after training in the magnitude of the increase between the groups (Table 3).
A novel approach in this study was to examine the effect of 9 weeks of a combined plyometric and sprint training program in pubertal (14–15 years) soccer players in an attempt to maximize physical and technical skill performance (i.e., sprinting, jumping, agility, ball-shooting speed ability, and IE). Our results substantiated our hypothesis in that the combination of soccer technical drills and specific plyometric and sprint training with additional training time in-season improves jumping and sprinting performance in a group of adolescent soccer players. Furthermore, agility and ball-shooting speed performance were significantly enhanced in the experimental group after the plyometric and sprint training program. These results tend to support most of the previous published studies performed examining these types of training interventions with young soccer players (15,28,34,50). Although previous studies selected players younger or older than those selected in this study and measured adaptations in shorter or longer training interventions, generally the training strategies used confirmed that the inclusion of a specific plyometric and sprint training stimulus into the normal soccer training program produces larger jump, sprint, agility, and ball-shooting speed performances than natural growth and specific soccer practice generates. Moreover, 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.
Several studies have shown the effectiveness of PT in improving vertical jump (8,15,33,41,50). In this study, significant CMJ (3 cm, 9.4%) and Abalakov vertical jump (5.4 cm, 15.5%) improvements were observed between baseline and post-test in the experimental group. The improvements concur with those of previous studies (8,34,50), showing that a combined program of different modalities of strength training and power-oriented strength training and plyometrics can significantly increase vertical jump performance. Thereby, the improvement observed in the jump ability in this study seems reasonable, and seems to result from the change in the level of neuromuscular activation (neural factors) and motor coordination, in response to specific PT (15). However, previous studies have also shown no improvement in the vertical jump after combined strength training when slow or normal contraction speed was used in the training (20,35,47). The discrepancy between the results in this study and those from previous studies might be attributed to several reasons: differences in the length of the training programme and by the higher training loads and volumes used in the studies; the specificity of the training and the athletic ability; the speed of movement rather than the resistance or load was more important and positively affected the jump performance of young soccer players; the players were very young and not specialists in plyometric and strength training in contrast to the greater training experience and initial training status of players in previous research; the differences in soccer players' training history (i.e., with or without systematic strength and power-oriented training); and the competitive level or the procedures used to measure vertical jumping performance may explain these discrepancies. In addition, the current results for the CMJ and Abalakov vertical jump after the combined plyometric, sprint, shooting, and agility training program also seem to be greater than simple training methods composed of resistance training or PT followed by children despite a greater training load.
Various studies have suggested that the strength training can improve the sprint ability of young soccer players (9,20,27). Other researches have focused on the development of sprint performance using speed training or sprinting against resistances (14). Meanwhile, a common trend in training programs indicates that a combination of methods is more effective for enhancing performance rather than stand-alone approaches (1). Additionally, positive results in sprint ability have been obtained when strength training was combined with PT (13,28). However, to our knowledge, no studies have compared the effectiveness of combined plyometric and sprint training on maximal sprint capacity in pubertal soccer players. The present research suggests significant improvement in 10-m sprint time after 9 weeks of the plyometric and sprint intervention in the CombG (ES = 0.9). This finding suggested that a combined training program provides the most powerful stimulus in improving the various parameters of sprint ability. We hypothesized here that combined training was superior to 1 training mode alone. One plausible explanation could be related to the effect of a positive number of exercises to ensure a sufficient performance of participants' neuromuscular and metabolic systems. Thus, it can be argued that greater improvement have been achieved by combining the number of exercises.
The basic movement patterns in soccer also require high levels of agility (17,37). Because an earlier study (31) demonstrated that agility and acceleration are independent qualities in soccer, it was necessary to assess them with specific testing. Previous studies in early and pubertal soccer players have used different agility tests distances from 40 to 50 m (26,36). However, the test of 10-m agility (with and without ball) seemed to be the most relevant to assess the specific quality of agility in soccer because of the high frequency of short high-intensity sprints during a game and the specificity of dribbling. Previous studies have demonstrated the efficiency of a plyometric program to improve specific agility actions of young soccer players (34,48), and Besier et al. (4) have recommended the inclusion of plyometrics in soccer training to familiarize players with unanticipated changes in direction. The percentage of change in the experimental group in performance after a training period in this study (5.8–7.9%) is in accordance with previous findings on agility ability in young soccer players. As is habitual with similar research, (1,18,49) we hypothesized here that combined training is superior to 1 training mode alone. However, the differences, although favorable to the group that trained with the combination of exercises, were lower than expected in agility ability with and without dribbling with the ball. This significant change in agility time performance in the experimental group (with and without ball) demonstrated that a combined plyometric and sprint 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 specific plyometric drills selected contained many powerful lateral movements and change of directions, which had an impact on the capacity to improve the agility ability faster.
Several studies found that specific strength training improved kicking performance in male soccer players (12,16,32,47). Other studies showed that a combined strength and power training (50) or lower-limb PT (43) could meaningfully increase kicking speed performance. Results of this study are in agreement with this statement because it was shown that the combined plyometric and sprint training program caused significant increases (ES = 0.7–0.8) in ball-shooting speed between pretraining and post-training values for the experimental group, whereas there were no differences for CG. Factors thought to influence ball-shooting speed could include the specificity of the training and the athletic ability (CombG); lower-body and trunk muscle strength that is directly responsible for increasing the speed of the foot (30); or that the linear velocity of the foot and ankle and angular velocity of all joints improves shooting performance (32). In addition, improvements in ball-shooting speed could be caused by the increased transfer of energy from proximal to distal segments, which may have contributed to a higher ball speed value after the combined plyometric and speed training intervention program (43). In addition, given the importance of the relationship between speed and accuracy in the kicking performance, it is important to highlight the lack of soccer shooting accuracy measurement as a limitation of this study.
An interesting finding in this study was that the experimental group did not demonstrate a significant increase in IE (ES = 0.3) after combined plyometric and sprint training. The differences, although favorable to the group that trained with the combination of exercises, were lower than expected in IE performance. This disagrees with the results previously described that strength and high-intensity training, in the form of dynamic exercises (i.e., squat, weighted CMJ, drop jump, and sprints), has been reported to enhance an individual's ability to rapidly develop endurance, obtain significant gains in the V[Combining Dot Above]O2max, and allow for greater improvement in IE (7,21,24,50). However, the results obtained are compatible with the results of some previous studies (11,22), which suggested an incompatibility of strength training and enhance IE. A possible explanation for no increase in the results of the experimental group could be the specificity of training, and the limited number of specific exercises to ensure a sufficient improvement of player endurance and cardiovascular system or because the workout was not integrated properly. Thus, it can be argued that greater improvement could have been achieved by increasing the volume and density of the treatment. Another factor that could possibly contribute to the different outcomes between previous investigations with respect to the associations between IE performances is the training and athlete's background.
In summary, this data clearly demonstrated that adding combined plyometric and sprint training in previously moderately trained pubertal soccer players seems to be a good stimulus for improving jumping, sprinting and agility ability, and ball-shooting performance. Moreover, for young soccer players who do not have previous experience with periodized plyometric and sprint training, a general adaptation phase must be scheduled to ensure proper movement technique and safety. As a result, the applicability of combined plyometric and sprint training together with regular soccer training could be performed during the season with no concomitant interference on endurance performance.
The practical implication of this research would be that pubertal soccer players can enhance jumping, sprinting and agility ability, and ball-shooting speed performance by undertaking a 9-week in-season program of combined plyometric and sprint training involving exercises for lower body (squat jump, loaded and unloaded jump, bounding, hurdling, and sprint exercises). The performance improvements shown in this study are of great interest for soccer coaches and are directly applicable to pubertal soccer players, because the performance of this sport relies greatly on the specific on-field vertical jump, maximal sprint, and agility abilities that were enhanced by the high-intensity oriented training regimen. Moreover, for young soccer players who do not have previous experience with plyometric and sprint training, a general adaptation phase is scheduled to ensure proper movement technique and safety. Coaches should consider progressive increases in the load and ensure that exercises are performed on soft landing surfaces, reducing the probability of player injury. The outcomes may help coaches and sport scientists formulate better guidelines and recommendations for athlete assessment and selection, training prescription and monitoring, and preparation for competition. Such improvements can be beneficial to winning challenges and could be transferred into game-play performance.
The authors have no professional relationships with companies or manufacturers that might benefit from the results of this study. There is no financial support for this project. No funds were received for this study from National Institutes of Health, Welcome Trust, University or others. The results of this study do not constitute endorsement of any product by the authors or the National Strength and Conditioning Association.
1. Adams K, O'Shea J, O'Shea K, Climstein M. The effects of six weeks of squat, plyometric and squat-plyometric training on power development. J Appl Sports Sci Res 6: 36–41, 1992.
2. American College of Sports Medicine (ACSM). ACSM’s Guidelines for Exercise Testing and Prescription (7th ed.). Philadelphia, PA: Lippincott Williams &Wilkins, 2006.
3. Bangsbo J, Norregaard L, Thorso F. Activity profile of competition soccer. Can J Sport Sci 16: 110–116, 1991.
4. Besier TF, Lloyd DG, Ackland TR, Cochrane JL. Anticipatory effects on knee joint loading during running and cutting maneuvers. Med Sci Sports Exerc 33: 1176–1181, 2001.
5. Castagna C, D'Ottavio S, Abt G. Activity profile of young soccer players during actual match play. J Strength Cond Res 17: 775–780, 2003.
6. Castagna C, Impellizzeri FM, Chamari K, Carlomagno D, Rampinini E. Aerobic fitness and yo-yo continuous and intermittent tests performances in soccer player: A correlation study. J Strength Cond Res 20: 320–325, 2006.
7. Chamari K, Hachana Y, Ahmed YB, Galy O, Sghaïer F, Chatard JC, Hue O, Wisloff U. Field and laboratory testing in young elite soccer players. Br J Sports Med 38: 191–196, 2004.
8. Chelly MS, Cherif N, BenAmar M, Hermassi S, Fathloun M, Bouhlel E, Tabka Z, Shephard R. Relationships of peak leg power, 1-RM half back squat and leg muscle volume to 5-m sprint performance of junior soccer players. J Strength Cond Res 24: 266–271, 2010.
9. Chelly MS, Fathloun M, Cherif N, Ben Amar M, Tabka Z, Van Praagh E. Effects of a back squat training program on leg power, jump- and sprint performances in junior soccer players. J Strength Cond Res 23: 2241–2249, 2009.
10. Christou M, Smilios I, Sotiropoulos K, Volaklis K, Pilianidis T, Tokmakidis SP. Effects of resistance training on the physical capacities of adolescent soccer players. J Strength Cond Res 20: 783–791, 2006.
11. Chromiac JA, Mulvaney DR. A review: The effects of combined strength and endurance training on strength development. J Appl Sport Sci Res 4: 55–60, 1990.
12. De Proft E, Cabri J, Dufour W, Clarys JP. Strength training and kick performance in soccer players. In: Science and Football. Proceedings of the 1st World Congress of Science and Football. Reilly T., Lees A., Davids K., Murphy W.J., eds. London, United Kingdom: E & FN SPON, 1988. pp: 108–113.
13. Delecluse C, Van Coppenolle H, Willems E, Van Leemputte M, Diels R, Goris M. Influence of high-resistance and high- velocity training on sprint performance. Med Sci Sports Exerc 27: 1203–1209, 1995.
14. Delecluse C. Influence of strength training on sprint running performance. Current findings and implications for training. Sports Med 24: 147–156, 1997.
15. Diallo O, Dore E, Duche P, Van Praagh E. Effects of plyometric training followed by a reduced training programme on physical performance in prepubescent soccer players. J Sports Med Phys Fitness 41: 342–348, 2001.
16. Dutta P, Subramanium S. Effect of six weeks isokinetic strength training combined with skill training on football kicking perfor- mance. In: Science and Football IV. Proceedings of the 4th World Congress of Science and Football. Spinks W., Reilly T., Murphy A., eds. New York, NY: Routledge, 2002. pp: 333–340.
17. Ellis L, Gastin P, Lawrence S, Savage B, Buckeridge A, Stapff A, Tumilty D, Quinn A, Woolford S, Young W. Protocols for the physiological assessment of team sports players. In: Physiological Tests for Elite Athletes. Gore C.J., ed. Champaign, IL: Human Kinetics, 2000. pp: 128–144.
18. Fatouros IG, Jamurtas AZ, Leontsini D, Taxildaris K, Aggelousis N, Kostopoulos N, Buckenmeyer P. Evaluation of plyometric exercise training, weight training, and their combination on vertical jumping performance and leg strength. J Strength Cond Res 14: 470–476, 2000.
19. Ferrete C, Requena B, Suarez-Arrones L, Saez de Villarreal E. Effect of strength and high-intensity training on jumping, sprinting, and intermittent endurance performance in prepubertal soccer players. J Strength Cond Res 28: 413–422, 2014.
20. Gorostiaga EM, Izquierdo M, Ruesta M, Iribarren J, Gonzalez- Badillo JJ, Ibanez J. Strength training effects on physical performance and serum hormones in young soccer players. Eur J Appl Physiol 91: 698–707, 2004.
21. Helgerud J, Engen LC, Wisloff U, Hoff J. Aerobic endurance training improves soccer performance. Med Sci Sports Exerc 33: 1925–1931, 2001.
22. Hennessy LC, Watson AWS. The interference effects of training for strength and endurance simultaneously. J Strength Cond Res 8: 12–19, 1994.
23. Hoff J, Helgerud J. Endurance and strength training for soccer players: Physiological considerations. Sports Med 34: 165–180, 2004.
24. Hoff J, Helgerud J. Maximal strength training enhances running economy and aerobic endurance performance. In: Football (Soccer): New Developments Physical Training Research. Hoff J., Helgerud J., eds. Trondheim, Norway: Norwegian University of Science and Technology, 2002. pp: 39–55.
25. Implellizzeri FM, Marcora SM, Castagna C, Reilly T, Sassi A, Iaia FM, Rampinini E. Physiological and performance effects of generic versus specific aerobic training in soccer players. Int J Sports Med 27: 483–492, 2006.
26. Ingle L, Sleap M, Tolfrey K. The effect of a complex training and detraining programme on selected strength and power variables in early pubertal boys. J Sports Sci 24: 987–997, 2006.
27. Jullien H, Bisch C, Largouet N, Manouvrier C, Carling CJ, Amiard V. Does a short period of lower limb strength training improve performance in field-bases tests of running and agility
in young professional soccer players? J Strength Cond Res 22: 404–411, 2008.
28. Kotzamanidis C, Chatzopoulos D, Michailidis C, Papaiakovou G, Patikas D. The effect of a combined high-intensity strength and speed training program on the running and jumping ability of soccer players. J Strength Cond Res 19: 369–375, 2005.
29. Krustrup P, Mohr M, Amstrup T, Rysgaard T, Johansen J, Steensberg A, Pedersen PK, Bangsbo J. The yo-yo intermittent recovery test: Physiological response, reliability, and validity. Med Sci Sports Exerc 35: 697–705, 2003.
30. Lees A, Nolan L. The biomechanics of soccer: A review. J Sports Sci 16: 211–234, 1998.
31. Little T, Williams AG. Specificity of acceleration
, maximum speed, and agility
in professional soccer players. J Strength Cond Res 19: 76–78, 2005.
32. Manolopoulos E, Papadopoulos C, Kellis E. Effects of combined strength and kick coordination training on soccer kick biomechanics in amateur players. Scand J Med Sci Sports 16: 102–110, 2006.
33. Markovic G. Does plyometric training improve vertical jump
height? A meta-analytical review. Br J Sports Med 41: 349–355, 2007.
34. Meylan C, Malatesta D. Effects of in-season plyometric training within soccer practice on explosive actions of young players. J Strength Cond Res 23: 2605–2613, 2009.
35. Mujika I, Santisteban J, Castagna C. In-season effect of short-term sprint and power training programs on elite junior soccer players. J Strength Cond Res 23: 2581–2587, 2009.
36. Reilly T, Bangsbo J, Franks A. Anthropometric and physiological predispositions for elite soccer. J Sports Sci 18: 669–683, 2000.
37. Reilly T, Doran D. Fitness assessment. In: Science and Soccer(2nd ed.). Reilly T., Williams M.A., eds: Routledge, 2003. pp. 21–46.
38. Reilly T, Thomas V. A motion analysis of work-rate in different positional roles in professional football match-play. J Hum Mov Stud 2: 87–97, 1976.
39. Rimmer E, Sleivert G. Effects of plyometric intervention program on sprint performance. J Strength Cond Res 14: 295–301, 2000.
40. Saez de Villarreal E, Gonzalez-Badillo JJ, Izquierdo M. Low and moderate plyometric training frequency produce greater jumping and sprinting gains compared with high frequency. J Strength Cond Res 22: 715–725, 2008.
41. Saez de Villarreal E, Kellis E, Kraemer WJ, Izquierdo M. Determining variables of plyometric training for improving vertical jump
height performance: A meta-analysis. J Strength Cond Res 23: 495–506, 2009.
42. Saez de Villarreal E, Requena B, Izquierdo M, Gonzalez-Badillo JJ. Enhancing sprint and strength performance after combined vs maximal power, heavy-resistance and plyometric training alone. J Sci Med Sport 16: 146–150, 2013.
43. Sedano Campo S, Vaeyens R, Philippaerts RM, Redondo JC, De Benito AM, Cuadrado C. Effects of lower-limb plyometric training on body composition, explosive strength, and kicking speed in female soccer players. J Strength Cond Res 23: 1714–1722, 2009.
44. Sheppard JM, Young WB. Agility
literature review: Classifications, training and testing. J Sports Sci 24: 919–932, 2006.
45. Sinning WE, Dolny DG, Little KD, Cunningham LN, Racaniello A, Siconolfi SF, Sholes JL. Validity of “generalize” equations for body composition analysis in male athletes. Med Sci Sports Exerc 17: 124–130, 1985.
46. Stolen T, Chamari K, Castagna C, Wisloff U. Physiology of soccer: An update. Sports Med 35: 501–536, 2005.
47. Taiana F, Grehaigne JF, Cometti G. The influence of maximal strength training of lower limbs of soccer players on their physical and kick performances. J Sports Sci 10: 170, 1992.
48. Thomas K, French D, Hayes PR. The effect of two plyometric training techniques on muscular power and agility
in youth soccer players. J Strength Cond Res 23: 332–335, 2009.
49. Wilson GF, Murphy AJ, Giorgi A. Weight and PT: Effects on eccentric and concentric force production. Can J Appl Physiol 21: 301–315, 1996.
50. Wong PL, Chamari K, Wisloff U. Effects of 12-week on-field combined strength and power training on physical performance among U-14 young soccer players. J Strength Cond Res 24: 644–652, 2010.