short-term plyometric jump training improves repeated ...

SHORT-TERM PLYOMETRIC JUMP TRAINING IMPROVES REPEATED-SPRINT ABILITY IN PREPUBERAL MALE SOCCER PLAYERS YASSINE NEGRA,1 HELMI CHAABENE,2,3 JAIME FERNANDEZ-FERNANDEZ,4 SENDA SAMMOUD,1 RAJA BOUGUEZZI,1 OLAF PRIESKE,2 AND URS GRANACHER2 1

Research Unit (UR17JS01), “Sport Performance, Health & Society,” Higher Institute of Sports and Physical Education of Ksar Said, Tunis, Tunisia; 2Division of Training and Movement Sciences, Research Focus Cognition Sciences, University of Potsdam, Potsdam, Germany; 3High Institute of Sports and Physical Education, Kef, University of Jendouba, Jendouba, Tunisia; and 4Department of Physical Education and Sports, Faculty of Physical Activity and Sports Sciences, University of Leon, Leon, Spain Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKbH4TTImqenVNY7U4bpjOf5lu8UNJfDtf1/f1lKOmA6B4I32CKiPm7t on 06/15/2018

ABSTRACT Negra, Y, Chaabene, H, Fernandez-Fernandez, J, Sammoud, S, Bouguezzi, R, Prieske, O, and Granacher, U. Short-term plyometric jump training improves repeated-sprint ability in prepuberal male soccer players. J Strength Cond Res XX(X): 000–000, 2018—This study examined the effects of a shortterm (i.e., 8 weeks) combined horizontal and vertical plyometric jump training (PJT) program in combination with regular soccer-specific training as compared with soccer-specific training only on jump and change of direction (CoD) performances, speed, and repeated-sprint ability (RSA) in prepuberal male soccer players. Twenty-four players were recruited and randomly assigned to either a PJT group (PJTG; n = 13; 12.7 6 0.2 years) or an active control group (CONG; n = 11; 12.7 6 0.2 years). The outcome measures included tests for the assessment of jump performance (drop jump from 20- to 40cm height [DJ20 and DJ40] and 3-hop test [THT]), speed (20m sprint), CoD (T-test), and RSA (20-m repeated shuttle sprint). Data were analyzed using magnitude-based inferences. Within-group analyses revealed large performance improvements in the T-test (d = 21.2), DJ20 (d = 3.7), DJ40 (d = 3.6), THT (d = 0.6), and the RSAtotal (d = 21.6) in the PJTG. Between-group analyses showed greater performance improvements in the T-test (d = 22.9), 20-m sprint time (d = 22.0), DJ20 (d = 2.4), DJ40 (d = 2.0), THT (d = 1.9), RSAbest (d = 21.9), and the RSAtotal (d = 21.9) in the PJTG compared with CONG. Eight weeks of an in-season PJT in addition to regular soccer-specific training induced larger increases in measures of physical fitness in prepuberal male soccer players

Address correspondence to Dr. Yassine Negra, [email protected]. 00(00)/1–9 Journal of Strength and Conditioning Research 2018 National Strength and Conditioning Association

compared with regular soccer-specific training only. More specifically, PJT was effective in improving RSA performance.

KEY WORDS youth, stretch-shortening cycle, athletic performance, soccer

INTRODUCTION

H

igh-intensity actions such as sprinting, jumping, change of direction (CoD), and kicking represent major performance determinants not only in elite but also in youth soccer players (6,32). According to Lloyd et al. (20), the terms youth and young athletes represent both children (up to the approximate age of 11 years in girls and 13 years in boys) and adolescents (typically including girls aged 12–18 years and boys aged 14–18 years). In this context, scientific data demonstrated that elite soccer players are characterized by high levels of muscle strength, power, and their derivatives (i.e., acceleration, sprinting, jumping, and CoD) (9,12,25). Therefore, the development of physical fitness in young soccer players is crucial to achieve sporting success in their future career. Despite its low contribution to the overall running distance covered in youth’s soccer matches, high-speed sprinting actions primarily contribute to critical and decisive situations in competition (i.e., gaining ball possession, scoring, or conceding goals) (28–30). Vaeyens et al. (42) showed that young elite soccer players are faster, more agile, and more powerful than their subelite peers. Likewise, Le Gall et al. (19) revealed that future international and professional soccer players showed higher speed (10- and 20-m sprint time) and power (maximal anaerobic power and vertical jumping) performances compared with players targeting the amateur level only. In this context, Bradley et al. (2) found that one of the main performance determinants in soccer is the ability to repeat highspeed actions (e.g., repeated-sprint ability [RSA]). Previous research demonstrated a significant relationship between the VOLUME 00 | NUMBER 00 | MONTH 2018 |

Copyright ª 2018 National Strength and Conditioning Association

1

Plyometric Jump Training in Youth Soccer overall distance covered during the match and the mean sprint time during RSA (37). In this regard, it has previously been shown that RSA outcomes effectively discriminate between professional and amateur soccer players (15). For that reason, the optimal training design that may improve RSA performance in young talented soccer players should be of great interest to coaches and strength and conditioning professionals. Buchheit et al. (3) studied the occurrence and nature of relative RSA in highly trained soccer players according to age, playing position, and playing time. They demonstrated that younger players were exposed to a higher number of repeated-sprint situations compared with their older peers (U13 . U14 . U16). Furthermore, younger compared with older players performed more and longer sprints per sequence (3). Of note, Ratel et al. (38) reported that children are better at compensating and regenerating from repeated high-intensity efforts compared with adults. Specifically, the greater oxidative capacity, the faster phosphocreatine resynthesis, the better acid-base regulation, the faster readjustment of initial cardiorespiratory parameters, and the higher removal of metabolic byproducts in children compared with adults could explain their faster recovery after high-intensity exercise (38). Plyometric jump training (PJT) is well recognized in the scientific literature as a safe and effective training regime to improve a myriad of high-intensity actions in young soccer players such as CoD, sprint, and jump performances (1,28,31,35,36). Although there is preliminary evidence on the effects of PJT in prepuberal male soccer players on components of physical fitness (e.g., muscle power, speed, agility) (1,24,25,30,31), further empirical research is still needed to strengthen and emphasize the effect of PJT on prepuberal male soccer players’ measures of physical fitness. Specifically, despite the high RSA demands in youth soccer (4), PJT effects on RSA are unresolved, particularly in prepuberal soccer players. To the best of the authors’ knowledge, the effects of PJT on RSA and agility have recently been studied in young adolescent soccer players aged 15.7 6 0.2 years (12). The 8-week PJT program consisted of 2 sessions per week and included hurdle and drop jump exercises. The main outcomes of this study demonstrated no significant pre-to-post improvements in all RSA variables (i.e., RSA best time [RSAbest], RSA total time [RSAtotal], and RSA fatigue index [RSAFI]) (effect size [ES] = 0.06, 0.15, and 0.39 for RSAbest, RSAtotal, and RSAFI, respectively). Similarly, agility performance did not improve after training (ES = 0.49). Conversely, significant training-induced enhancements were found in sprint time and repeated CoD performance (ES = 0.7 and 0.94 for sprint time and repeated CoD test, respectively). Finally, the authors suggested that the lack of significant performance improvements in RSA and agility performance may be attributed to the high physical fitness level of the participants before the start of the study or the reduced training load stimulus (i.e., low intensity and

2

the

volume [2 sessions per week with 28–70 ground contacts per session]) of the applied PJT program. In a sample of elite male adolescent soccer players aged 14.5 6 0.5 years, Buchheit et al. (4) compared the effects of 8 weeks of strength (i.e., combined plyometric and short sprint exercises) and repeated shuttle sprint training on RSA. The main findings of this study showed significant gains in RSA outcomes (RSAbest and RSA mean time [RSAmean]) after both training interventions with greater effects of shuttle sprint training compared with strength training. These authors (4) claimed that the improvement in the outcome of high-intensity actions (i.e., speed, jumping, and CoD performance) may have contributed to RSA performance enhancement. Taken together, there is limited longitudinal research that has studied the effects of PJT on measures of physical fitness in prepuberal male soccer players. In addition, in view of the significance of RSA in youth soccer competition (5), the effects of PJT on RSA in prepuberal soccer players still need to be explored. Thus, the aim of this study was to examine the effects of an 8-week PJT program on CoD, speed, jump performance, and RSA in prepuberal male soccer players. We tested the hypothesis that the replacement of some soccer drills with plyometric exercises would enhance prepuberal soccer players’ measures of physical fitness to a greater extent compared with conventional soccer training alone. Specifically, by reference to previous research (3,4), we expected that PJT could improve RSA performance in prepuberal soccer players.

METHODS Experimental Approach to the Problem

A 2-group repeated-measures experimental design was applied to assess whether 8 weeks of biweekly in-season PJT improves selected measures of physical fitness in male prepuberal soccer players relative to their peers who continued to follow their regular in-season soccer-specific training regime. A team of soccer players (U13) was randomly divided into a PJT group (PJTG) and an active control group (CONG). Overall, the PJTG conducted 5 training sessions per week, including 2 PJT sessions that were integrated into the regular soccer-specific training routine of the soccer team. The CONG continued their regular soccer-specific training with 5 sessions per week. Two weeks before baseline testing, 2 familiarization sessions were performed to get participants acquainted with the applied tests. Pre-training and post-training, tests were conducted for the assessment of CoD (i.e., T-test), speed (i.e., 20-m sprint), jumping performance (i.e., drop jump from a height of 20 cm [DJ20] and 40 cm [DJ40], and 3-hop test [THT]), and RSA (i.e., 20-m repeated shuttle sprint). All tests were scheduled at least 48 hours after the last training session or match, at the same time of day (7:30–9:30 AM), and under the same environmental conditions (29–338 C, no wind).

TM

Journal of Strength and Conditioning Research


the

TM

Journal of Strength and Conditioning Research Subjects

Healthy, elite young soccer players (N = 32) from the same regional soccer team were included in this study. Of note, both groups followed the same soccer training program under the supervision of the same coaches. Participants were randomly assigned to a PJTG (mean 6 SD, n = 16; age = 12.7 6 0.2 years; maturity offset = 21.3 6 0.6) or an active CONG (mean 6 SD, n = 16; age = 12.7 6 0.2 years; maturity offset = 21.7 6 0.7). All participants had a background of 5.0 6 1.3 years of systematic soccer training involving 3–5 training sessions per week throughout the soccer season. For personal reasons, 5 participants from the CONG group left the youth soccer training center. Three other participants from the PJTG discontinued the intervention due to changes in their study schedule. This makes a total of 24 prepuberal soccer players who successfully completed the training program (PJTG; n = 13 and CONG; n = 11). Anthropometric characteristics of both groups are detailed in Table 1. Players who missed more than 20% of the total training sessions and/or more than 2 consecutive sessions were excluded from the study. During the period 6 months before the start of the study and over the course of the study, all participants were free of injuries. Maturation status was determined at the beginning and after 8 weeks of training according to the predicted age at peak height velocity (APHV) (21). Maturity offset (expressed in years) was defined as time before or after peak height velocity. All participants and their legal representatives were properly informed about all testing and training procedures, as well as possible benefits and harms related to the study. All procedures were approved by the local institutional review committee of the Higher Institute of Sport and Physical Education, Ksar Said, Tunisia. Verbal and written informed consent was obtained from legal representatives and children after explanation of the experimental protocol and its potential benefits and harms.

| www.nsca.com

fitness tests were performed in a fixed order over 3 consecutive days (Figure 1). The warm-up program for all tests included 5 minutes of submaximal running with CoD exercises, 10 minutes of submaximal plyometrics {2 jump exercises of 20 vertical (i.e., countermovement jump [CMJ]) and 10 horizontal jumps (i.e., 2-footed ankles hop-forward)}, dynamic stretching exercises, and 5 minutes of a sprint-specific warm-up. All tests were separated by a 5–10 minutes break in between to ensure full phosphocreatine restoration (15,41). Each player participated in a familiarization trial and 2 test trials for each test. Another rest period of 3 minutes was provided between trials. The best of the 2 test trials was used for further statistical analyses. Change of Direction. The T-test was administered using the protocol as previously outlined by Negra et al. (32). The time needed to complete the test was used as performance outcome and it was assessed with an electronic timing system (Microgate SARL, Bolzano, Italy). The intraclass correlation coefficient (ICC) for test-retest trials was 0.92 (90% confidence limit [CL]: 0.81–0.96). Speed. Twenty-meter sprint time was assessed using an electronic timing system (Microgate). Participants started in a standing start position 0.3-m before the first infrared photoelectric gate, which was placed 0.75-m above the ground to ensure that it captured trunk movement and avoided false signals through limb motion. In total, 2 singlebeam photoelectric gates were used and placed at the start line and the finish line. The ICC for test-retest reliability was 0.97 (90% CL: 0.94–0.99). The 20-m speed performance distance has been chosen based on previous studies showing that this is the most frequent linear speed distance that occurs during youth soccer competition (34).

Jump Performance. Jump performance was assessed using the DJ with 20 and 40 cm drop height. Jump height was assessed using a photoelectric system (Optojump; Microgate). Participants were General Testing Procedure. Testing was integrated into the instructed to place their hands on their hips and step off the weekly soccer-specific training schedule of participants. Physical platform with the leading leg straight to avoid any initial upward propulsion ensuring a drop height of 20 and 40 cm. TABLE 1. Age and anthropometric data of the included subjects.*† They were also instructed to CONG (n = 11) PJTG (n = 13) jump with short ground contact and as forcefully as possible. Pre-test Post-test Pre-test Post-test Moreover, participants were inAge (y) 12.7 6 0.2 12.9 6 0.2 12.7 6 0.2 12.8 6 0.2 structed to leave the platform Height (cm) 158.6 6 4.5 158.7 6 4.5 152.0 6 6 152.2 6 6 with knees and ankles fully Body mass (kg) 43.7 6 5.7 44.7 6 5.5 39.9 6 5.8 40.1 6 6 extended and to land in a simiMaturity offset 21.3 6 0.6 21.2 6 0.6 21.8 6 0.7 21.7 6 0.7 larly extended position to ensure Predicted APHV 14.0 6 0.7 14.1 6 0.7 14.4 6 0.8 14.5 6 0.8 validity of the test. The ICCs for *PJTG = plyometric jump training group; CONG = control group; APHV = age at peak test-retest reliability were 0.93 height velocity. (90% CL: 0.85–0.97) and 0.92 †Data are presented as mean and SD. (90% CL: 0.83–0.97) for the DJ20 and DJ40, respectively. Procedures

VOLUME 00 | NUMBER 00 | MONTH 2018 |


3

Plyometric Jump Training in Youth Soccer

Figure 1. Schematic representation of the experimental design. CoD = change of direction test; DJ20 = drop jump from 20 cm; DJ40 = drop jump from 40 cm; THT = 3-hop test; RSA = repeated-sprint ability; PJTG = plyometric jump training group; CONG = control group.

Besides the assessment of DJs, THTs were conducted. Athletes started from an upright standing position and performed 3 consecutive maximal hops forward on the dominant leg to reach the maximal horizontal distance. During the test, double-arm swing was allowed (13). The covered distance was measured to the nearest 1 cm using a measuring tape. The ICC for test-retest reliability was 0.95 (90% CL: 0.90–0.98). Repeated-Sprint Ability. The RSA test was conducted using a photocell system (Microgate). Immediately after a warm-up, participants completed a preliminary single shuttle sprint test. The first trial provided the criterion score for the actual shuttle sprint test (37). Participants rested for 5 minutes before starting the actual test. During the first sprint, participants had to achieve at least 97.5% of their criterion score. If 97.5% of the criterion score was not achieved, the subject rested for another 5 minutes and then restarted the test (37). We used such an approach to examine whether participants adopted a coping strategy. Of note, all participants attained their criterion score during the first sprint. Athletes performed six 20-m shuttle sprints with 1808 turns, separated by 25 seconds of passive recovery (33). Three seconds before the commencement of each sprint, players were asked to adopt the ready position until the next start signal. From the starting line, they sprinted for 20 m, touched the second line with one foot, and returned to the starting line as quickly as possible. After 25 seconds of passive recovery, the maneuver was repeated. Participants were instructed to complete all sprints as fast as possible. The best time in a single trial (RSAbest), the total time (RSAtotal), and the fatigue index {RSAFI: FI = 100 3 (total time/ [best time 3 6]) 2 100} were determined. The reliability of this test was examined elsewhere (17). Soccer Training Protocol. The active CONG participated in a regular soccer-specific training program over the 8-week intervention period with 5 training sessions per week lasting between 80 and 90 minutes. The PJTG participated in 3 soccer-specific training sessions per week that were similar to those of the CONG and 2 PJT sessions that substituted 2 soccer-specific training sessions, so that the overall exposition time to training was identical between the 2 groups. In general, soccer training

4

the

included training of fast footwork, technical skills and moves, position games, and tactical games (Table 2). Plyometric Jump Training. Details related to the number of sets, ground contacts per set, and total ground contact time per session and per week are displayed in Table 3. The PJT program was conducted during the second half of the inseason period. Before every PJT session, a standardized 8–12-minute warm-up was completed including lowintensity running, coordination exercises, dynamic movements (i.e., lunges, skips), sprints, and dynamic stretching for the lower-limb muscles. Each session lasted between 80 and 90 minutes with 25–35 minutes of plyometric drills. The remaining training time comprised technical (e.g., free kicks, corners) and tactical (i.e., defensive and offensive) drills. At the beginning of each training week, the first PJT session was performed at least 48 hours after the last soccer match that was scheduled on the weekend. The second PJT session was completed 72 hours after the first session. The PJT protocol was based on a previously published study (30). Notably, the first PJT session of each training week focused on improving the vertical leap, whereas the second PJT session focused on exercising horizontal jumps. During every vertical PJT session, CMJs were conducted and during every horizontal PJT session, 2-footed ankle hop-forward exercises were performed. Both sessions consisted of 5–8 sets and 10–15 repetitions. During vertical and horizontal jumps, participants were instructed to jump as high as possible and as far as

TABLE 2. Athletes’ training characteristics during the 8-week intervention period.*

No. of training sessions No. of matches No. of PJT sessions

CONG (n = 11)

PJTG (n = 13)

38 07 0

38 07 16

*CONG = control group; PJTG = plyometric jump training group.

TM



the

TM


TABLE 3. Plyometric jump training program.* Ground contacts Week Volume sets 3 (reps) Session 1 Session 2 1 2 3 4 5 6 7 8

5–6 sets (10 reps) 5–6 sets (10 reps) 5–6 sets (12 reps) 5–6 sets (12 reps) 5–6 sets (15 reps) 6–7 sets (15 reps) 7 sets (15 reps) 8 sets (15 reps)

50 50 60 60 75 90 100 120

60 60 72 72 90 105 100 120

*reps = repetitions.

possible in a cyclic manner with a minimal ground contact time, respectively. The total number of ground contacts per week was 50 during the first week and this number was progressively increased to 120 after 8 weeks of training to limit stress on the muscle-tendon unit (30). A 90-second rest interval was provided between each set of exercise to allow for sufficient recovery time (6). Statistical Analyses

Results are presented as mean values and SDs. All data were log-transformed for analysis to reduce bias arising from non-

| www.nsca.com

uniformity error and then analyzed for practical significance using magnitude-based inferences (15). For within/betweengroup comparisons, the chances that the differences in performance were better/greater (i.e., greater than the smallest worthwhile change [0.2 multiplied by the betweenparticipants SD, based on the Cohen d principle]), similar, or worse/smaller were calculated. Quantitative chances of beneficial/better or detrimental/poorer effects were assessed qualitatively as follows: ,1%, almost certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25–75%, possibly; 75–95%, likely; 95–99%, very likely; and .99%, almost certainly (15). If the chances of having beneficial/better and detrimental/poorer performances were both .5%, the true difference was rated as unclear. Otherwise, we interpreted that change as the observed chance (15). For more clarity, all improvements are presented as positive changes, so that both positive (i.e., increased DJ height) and negative differences (i.e., reduced time in a sprint test) are in the same direction. All analyses were performed using specific spreadsheets available at www.sportsci.org. Standardized differences (i.e., ES) were also calculated to determine the magnitude of differences between the groups for each variable using the method previously described by Hopkins et al. (15) (i.e., ,0.4 = small; 0.41–0.7 = moderate; and .0.7 = large magnitudes of change). Intraclass correlation coefficients were obtained using a specific spreadsheet available on www. sportsci.org and interpreted as poor (0–0.49), moderate (0.5–0.69), high (0.7–0.89), and very high (.0.9) (27).

TABLE 4. Performance change in the plyometric and control groups before and after 8 weeks of training (mean 6 SD).* Variables

Pre-test

Post-test

PJTG T-test (s) 11.1 6 0.5 10.3 6 0.5 20-m (s) 3.4 6 0.2 3.3 6 0.2 DJ20 (cm) 25.4 6 3.6 27.8 6 4.2 DJ40 (cm) 25.8 6 5.3 28.2 6 5.2 THT (m) 5.3 6 0.5 5.7 6 0.6 RSAbest (s) 7.9 6 0.4 7.7 6 0.4 47.8 6 2.5 RSAtotal (s) 48.8 6 2.2 RSAFI 3.3 6 1.4 2.9 6 1.3 CONG T-test (s) 11.38 6 0.44 11.73 6 0.39 20-m (s) 3.8 6 0.2 3.6 6 0.2 DJ20 (cm) 20.5 6 2.3 19.8 6 2.4 DJ40 (cm) 19.0 6 5.0 19.9 6 3.1 THT (m) 4.6 6 0.4 4.6 6 0.5 RSAbest (s) 8.3 6 0.3 8.4 6 0.3 RSAtotal (s) 51.5 6 1.4 52.2 6 1.6 RSAFI 3.7 6 1.6 3.4 6 1.5

Standardized difference (90% CL) Chances

Qualitative assessment

21.2 20.2 3.7 3.6 0.6 20.2 21.6 20.6

(21.6 to 20.7) (20.4 to 20.1) (1.7 to 5.7) (0.9 to 6.3) (0.4 to 0.9) (20.4 to 20.1) (22.6 to 20.6) (21.6 to 0.4)

100/0/0 63/37/0 100/0/0 98/1/1 99/1/0 57/43/0 99/1/0 74/16/10

Almost certainly positive Possibly positive Almost certainly positive Almost certainly positive Almost certainly positive Possibly positive Almost certainly positive Unclear

0.5 20.2 20.8 1.2 0.0 0.2 0.9 20.4

(0.2 to 0.7) (20.3 to 0.0) (22.0 to 0.3) (22.2 to 4.6) (20.3 to 0.3) (0.0 to 0.4) (20.1 to 1.9) (21.6 to 0.9)

0/8/92 44/56/0 7/11/82 69/7/24 17/72/11 0/57/42 4/8/88 58/19/23

Likely negative Possibly positive Unclear Unclear Unclear Possibly negative Likely negative Unclear

*CL = confidence limit; PJTG = plyometric jump training group; DJ = drop jump; THT = 3-hop test; RSA = repeated-sprint ability; FI = fatigue index; CONG = control group.



5

Plyometric Jump Training in Youth Soccer

Figure 2. Comparison of changes in performance tests between groups. Standardized differences (effect size [ES]). CONG = control group; PJTG = plyometric jump training group; DJ40 = drop jump from 40 cm; DJ20 = drop jump from 20 cm; THT = 3-hop test; RSA = repeated-sprint ability; FI = fatigue index.

RESULTS All participants of the PJTG and the CONG received treatments as allocated. All participants completed the training program and none reported any training or test-related injuries. There were no statistically significant baseline differences between groups (all p . 0.05) with regards to chronological age, height, body mass, maturity offset, APHV, and soccer experience (i.e., training background) (Table 1). Within-group analyses are shown in Table 4. Large performance improvements in T-test, DJ20, DJ40, THT, and RSAtotal were found for the PJTG compared with pre-test. For 20-m sprint and RSAbest, PJTG showed small performance improvements from pre-test to post-test. For the CONG, small performance improvements were shown in the 20-m sprint time test compared with the pre-test. However, T-test, RSAbest, and RSAtotal were substantially altered from pre-test to post-test in the CONG. Between-group analyses are illustrated in Figure 2. Results from magnitude-based inferences showed greater performance improvements in all tests in favor of the PJTG except for RSAFI. Particularly, greater improvements were found in PJTG compared with CONG T-test (212.0% [90% CL: 214.5 to 29.4], 100/0/0 with chances for greater/similar/ smaller values, respectively), the 20-m sprint time test (29.6% [90% CL: 212.7 to 26.5], 100/0/0), the DJ20 test (39.5% [90% CL: 26.9–53.3], 100/0/0), DJ40 test (41% [90% CL: 25.2–58.8], 100/0/0), THT test (24.3% [90% CL: 15.1– 34.3], 100/0/0), RSAbest (28.1% [90% CL: 210.9 to 25.3], 100/0/0), and RSAtotal (28.5% [90% CL: 211.3 to 25.7], 100/0/0). For the RSAFI, the chances for greater/similar/

6

the

smaller performance between groups were rated unclear (213.9% [90% CL: 238.7 to 21], 60/29/11).

DISCUSSION To the best of the authors’ knowledge, this is the first study that examined the effects of PJT program on RSA scores in prepuberal male soccer athletes in addition to measures of CoD, speed, and jump performance. Our results demonstrated that 8 weeks of PJT in combination with regular soccer training induced greater performance improvements in all measures of physical fitness (standardized differences ranging from 1.9 to 2.9) except for the RSAFI compared with CONG. For CONG, results did not show any performance enhancements in all measures of physical fitness except for the 20-m sprint test that showed a small increase from pretest to post-test with a possibly positive chance. The results of this study showed greater CoD performance improvement after training in the PJTG (Δ7%) compared with the CONG (Δ3%). This is consistent with the findings of Negra et al. (28) that revealed a significant improvement in the Illinois CoD test after 8 (Δ2%, ES = 1.3) and 12 (Δ3.3%, ES = 1.8) weeks of PJT in prepuberal male soccer players. Likewise, Negra et al. (30) noted a significant improvement in the Illinois CoD test after 8 weeks of PJT conducted on a stable surface (D3%, ES = 1.5) and unstable surface (D3%, ES = 1.46) in prepuberal male soccer players. So¨hnlein et al. (40) revealed a significant improvement in hurdle agility run test (A¨-6.1%; ES = 0.6) after 16 weeks of PJT in early- to mid-pubertal elite soccer players. Similarly, Meylan and Malatesta (24) reported that 8 weeks of PJT resulted in a significant improvement in agility

TM



the

TM

Journal of Strength and Conditioning Research performance (Δ-9.6%, ES = 2.8) in young male soccer players aged 13.3 6 0.6 years. Granacher et al. (10) observed a significant performance improvement in CoD performance (Δ2.9%, ES = 1.2) after 8 weeks of PJT in subelite adolescent male soccer players. It has previously been shown that PJT improves the rate of force development, a preeminent factor to optimize CoD performance (26,39). Similarly, Sheppard and Young (39), revealed that PJT can improve the eccentric strength of thigh muscles, a prevalent component in CoD during the deceleration phase of impulsive movement (7). Recently, Chaabene et al. (8) claimed that eccentric strength constitutes a prominent factor to decelerate and stabilize the body during CoD tasks. Moreover, it has been suggested that PJT can decrease ground reaction times through the increase in muscular force output and movement efficiency, thereby positively affecting COD performance (10). Given that we did not conduct any kinetic/kinematic and/or physiological tests, we would have to speculate on the underlying mechanisms responsible for the observed improvements. Further research is needed to clarify this issue. Our findings indicated that PJTG showed a larger performance improvement (Δ4%) in the speed test compared with CONG (Δ3%). These findings are in line with those recently established by Chaabene and Negra (6) who demonstrated a significant increase in speed performance (20-m) after 8 weeks of both high-volume (20-m [D2%, ES = 0.8]) and low-volume (20-m [D3%, ES = 0.8]) PJT program in prepuberal male soccer players. Similar results were obtained by Michailidis et al. (25) who revealed a significant improvement in speed performance (20-m) after 6 (Δ2.2%) and 12 (Δ3.5%) weeks of PJT in prepuberal male soccer players. The observed increases in speed performance after PJT are most likely caused by increases in neuromuscular activation of the trained muscles (11). More specifically, increases in the number and/or firing frequencies of active motor units and/or changes in the recruitment pattern of the motor units, primarily of fast-twitch muscle fibers, might account for the observed results (11). In addition, improvement in speed performance may also result from increases in maximal muscle strength and power, allowing the athletes to explode from the start faster or have longer stride lengths (9). In addition, by reason of including horizontal stimulus in our PJT program, this may increase the chances of gaining adaptations due to the importance of horizontal force production and application in speed performance (9). Of note, our results revealed a significant improvement in 20-m sprint performance in the CONG. This improvement is in line with previously published findings of Michailidis et al. (25). These authors suggested that soccer training alone might contribute to speed development, probably because of the frequent performance of short sprints incorporated in most soccerspecific training sessions. Our findings demonstrated higher jump performance improvements for the PJTG (i.e., DJ20 [Δ9.3%], DJ40 [Δ9%], and THT [Δ7.7%]) compared with CONG (i.e.,

| www.nsca.com

DJ20 [Δ-3%], DJ40 [Δ4%], and THT [Δ0.4%]) after training. These findings corroborate previous data (25). For instance, Chaabene and Negra (6) studied the effect of an 8-week high-volume and low-volume PJT program and revealed significant improvements in squat jump [SJ] and CMJ height performance after both programs (SJ: Δ16% and 17%; and CMJ: Δ14% and 14%, after high and low PJT program, respectively) after training in prepuberal male soccer players. The same authors showed a significant increase in horizontal jump performance (standing long jump [SLJ] Δ7.12% and 6.80 after high and low plyometric training, respectively) after training. Similarly, Michailidis et al. (25) observed significant improvements in SJ and CMJ height after 6 (SJ: Δ14.3% and CMJ: Δ18.5%) and 12 weeks (SJ: Δ23.3% and CMJ: Δ27.6%) of PJT in prepuberal male soccer players. With respect to horizontal jump performance, the same authors reported significant improvements after 6 (SLJ: Δ2.6% and multiple 5-bound hopping [MB5]: Δ14.6%) and 12 weeks (SLJ: Δ4.2% and MB5: Δ22.9%) of training. In the same context, Ramı´rez-Campillo et al. (36) reported significant increases in CMJ height after 8 weeks of PJT (Δ4.3%, ES = 0.2) in young male soccer players aged 13.2 6 1.8 years. In a previous published review, Johnson et al. (18) revealed that a well-structured PJT intervention significantly enhances jumping performance in youth children with large ESs ranged from 1.4 to 2.2. In agreement with these outcomes, our findings illustrate that prepuberal male soccer players can largely improve their vertical and horizontal jump performance by means of a PJT in substitution to their standard soccer training program. Therefore, the current PJT program seems to be effective if the goal is to improve vertical and horizontal jump performance of prepuberal male soccer players. Given the lack of circulating anabolic hormones (e.g., testosterone) in prepuberal athletes that most likely causes muscle hypertrophy, it might be speculated that the marked improvements in both jumping tests could mainly be caused by neural adaptations after PJT (22). To the best of the authors’ knowledge, this is the first study that examined the effects of PJT on RSA performance in prepuberal male soccer players. Our results revealed that RSAbest and RSAtotal showed greater performance enhancements after training in the PJTG [Δ1.8% and 1.2% for RSAbest and RSAtotal, respectively] compared with the CONG [Δ-1.6 and 1.3% for RSAbest and RSAtotal, respectively]. However, the chances of having greater/similar/smaller performance improvements between PJTG and CONG in RSAIF were rated as unclear. This improvement in RSA performance was likely related to the fact that the change in explosive performance after a PJT program may contribute to CoD improvement during RSA test with CoD (4) or running economy (23). As previously stated, PJTG revealed a meaningful improvement in jumping test performance, which may transfer into improved running economy and enhance aerobic performance (23). However, a recent study conducted by Hammami et al. (12) revealed no significant differences in VOLUME 00 | NUMBER 00 | MONTH 2018 |


7

Plyometric Jump Training in Youth Soccer RSA parameters after 8 weeks of PJT in adolescent soccer players. The disparity between the results of the current study and that of Hammami et al. (12) seems to be due to differences in the applied methods (prepuberal vs. adolescent soccer players, period of the season [pre-season vs. inseason], players’ training background [trained vs. highly trained], total soccer training load per week [5 sessions vs. 7 sessions], and training volume [1,184 ground contacts vs. 722 ground contacts at the end of the training program]). The improvement in RSA performance during the current study may be due to the significant enhancement in the outcome of high-intensity actions (i.e., speed, jumping performance, and CoD) after training according to previous findings (4). Particularly, the observed improvements were most likely a result of enhancements in explosive power through improvements in motor unit synchronization, stretch-shortening cycle efficiency, or musculotendinous stiffness (14). We acknowledge that training load should be matched between experimental groups to avoid this important training parameter negatively affects study findings. However, given that both experimental groups exercised at the same soccer facility with the same coaches in charge, we are confident that training load was similar between the 2 groups.

PRACTICAL APPLICATIONS Findings of this study demonstrated that a short-term (i.e., 8 weeks), low-volume (i.e., 60–120 foot contacts per session) PJT program conducted twice per week in replacement of soccer-specific drills induced significant performance improvements in measures of physical fitness (i.e., CoD, speed, and jump performance) in prepuberal male soccer players. Short-term PJT program was, particularly, effective in generating significant RSA performance enhancements (i.e., RSAbest and RSAtotal), whereas regular soccer sport-specific training did not. These findings suggest that regular soccerspecific training should be combined with PJT to improve measures of physical fitness in prepuberal male soccer players. Furthermore, the currently applied PJT program comprised between 27 and 38% of the total soccer training volume, which is why it can easily be integrated twice a week before the regular technical-tactical soccer-specific training. As this study addressed the effects of PJT on physical fitness in a prepuberal cohort, future studies should examine maturity- and sex-related effects of PJT on measures of physical fitness.

ACKNOWLEDGMENTS Y. Negra and H. Chaabene have contributed equally.

REFERENCES 1. Bouguezzi, R, Chaabene, H, Negra, Y, Ramirez-Campillo, R, Jlalia, Z, Mkaouer, B, et al. Effects of equal volume different plyometric training frequency on athletic performance of prepuberal male soccer players. J Strength Cond Res, 2018. In press.

8

the

2. Bradley, PS, Mascio, MD, Peart, D, Olsen, P, and Sheldon, B. Highintensity activity profiles of elite soccer players at different performance levels. J Strength Cond Res 24: 2343–2351, 2009. 3. Buchheit, M, Mendez-villanueva, A, Simpson, BM, and Bourdon, PC. Repeated-sprint sequences during youth soccer matches. Int J Sport Med 31: 709–716, 2010. 4. Buchheit, M, Mendez-Villanueva, A, Delhomel, G, Brughelli, M, and Ahmaidi, S. Improving repeated sprint ability in young elite soccer players: Repeated shuttle sprints vs. explosive strength training. J Strength Cond Res 24: 2715–2722, 2010. 5. Castagna, C, Francini, L, Krustrup, P, Fernandez-da-Silva, J, Povoas, SCA, Bernardini, A, et al. Reliability characteristics and applicability of a repeated sprint ability test in male young soccer players. J Strength Cond Res 32:1538–1544, 2017. In press. 6. Chaabene, H and Negra, Y. The effect of plyometric training volume in prepubertal male soccer players’ athletic performance. Int J Sports Physiol Perform 12: 1205–1211, 2017. 7. Chaabene, H. Change of direction tasks: Does the eccentric muscle contraction really matter? Scientific Pages Sports Med 1: 1–2, 2017. 8. Chaabene, H, Prieske, O, Negra, Y, and Granacher, U. Change of direction speed: Toward a strength training approach with accentuated eccentric muscle actions. Sports Med, 2018. In press. 9. Christou, M, Smilios, I, Sotiropoulos, K, Volaklis, K, Pilianidis, T, and Tokmakidis, SP. Effects of resistance training on the physical capacities of adolescent soccer players. J Strength Cond Res 20: 783– 791, 2006. 10. Granacher, U, Prieske, O, Majewski, M, Busch, D, and Muehlbauer, T. The role of instability with plyometric training in sub-elite adolescent soccer players. Int J Sports Med 36: 386–394, 2015. 11. Hakkinen, K and Komi, PV. Effect of explosive type strength training on electromyographic and force production characteristics of leg extensor muscles during concentric and various stretch-shortening cycle exercises. Scand J Sport Sci 7: 65–76, 1985. 12. Hammami, M, Negra, Y, Aouadi, R, Sheppard, RJ, and Chelly, MS. Effects on an in season plyometric training program on repeated change of direction and sprint performance in the junior soccer players. J Strength Cond Res 30: 3312–3320, 2016. 13. Hamilton, RT, Shultz, SJ, Schmitz, RJ, and Perrin, DH. Triple-hop distance as a valid predictor of lower limb strength and power. J Athl Train 43: 144–151, 2008. 14. Harris, N, Cronin, J, and Keogh, J. Contraction force specificity and its relationship to functional performance. J Sport Sci 25: 201–212, 2007. 15. Hopkins, WG, Marshall, SW, Batterham, AM, and Hanin, J. Progressive statistics for studies in sports medicine and exercise science. Med Sci Sport Exerc 41: 3–13, 2009. 16. Hultman, E, Bergstrom, J, and Anderson, NM. Breakdown and resynthesis of phosphorylcreatine and adenosine triphosphate in connection with muscular work in man. Scand J Clin Invest 19: 56– 66, 1967. 17. Impellizzeri, FM, Rampinini, E, Castagna, C, Bishop, D, Ferrari, BD, Tibaudi, A, et al. Validity of a repeated-sprint test for football. Int J Sports Med 29: 899–905, 2008. 18. Johnson, BA, Salzberg, CL, and Stevenson, DA. A systematic review: Plyometric training programs for young children. J Strength Cond Res 25: 2623–2633, 2011. 19. le Gall, F, Carling, C, Williams, M, and Reilly, T. Anthropometric and fitness characteristics of international, professional and amateur male graduate soccer players from an elite youth academy. J Sci Med Sport 13: 90–95, 2010. 20. Lloyd, RS, Cronin, JB, Faigenbaum, AD, Haff, GG, Howard, R, Kraemer, WJ, et al. National Strength and Conditioning Association position statement on long-term athletic development. J Strength Cond Res 30: 1491–1509, 2016.

TM



the

TM

Journal of Strength and Conditioning Research 21. Malina, RM and Koziel, SM. Validation of maturity offset in a longitudinal sample of polish boys. J Sports Sci 32: 424–437, 2014. 22. Markovic, G and Mikulic, P. Neuro-musculoskeletal and performance adaptations to lower-extremity plyometric training. Sports Med 40: 859–895, 2010. 23. Marta, C, Marinho, DA, Barbosa, TM, Izquierdo, M, and Marques, MC. Effects of concurrent training on explosive strength and VO2max in prepubescent children. Int J Sports Med 34: 888–896, 2013. 24. Meylan, C and 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. 25. Michailidis, Y, Fatouros, IG, Primpa, E, Michailidis, C, Avloniti, A, Chatzinikolaou, A, et al. Plyometrics’ trainability in preadolescent soccer athletes. J Strength Cond Res 27: 38–49, 2013. 26. Miller, MG, Herniman, TJ, Ricard, MD, Cheatham, CC, and Michael, TJ. The effects of a 6-week plyometric training program on agility. J Sports Sci Med 5: 459–465, 2006. 27. Munro, BH. Correlation coefficients: measuring the association of two variables. In: Statistical methods for health care research. Philadelphia, PA: Lippincott Williams & Wilkins, 2005. pp. 465–475. 28. Negra, Y, Chaabene, H, Stoeggl, T, Hammami, M, Chelly, MS, and Hachana, Y. Effectiveness and time course adaptation of resistance training vs. plyometric training in pre-pubertal soccer players. J Sport Health Sci, 2016. In press. 29. Negra, Y, Chaabene, H, Hammami, M, Hachana, Y, and Granacher, U. Effects of high-velocity resistance training on athletic performance in prepuberal male soccer athletes. J Strength Cond Res 30: 3290–3297, 2016.

| www.nsca.com

32. Negra, Y, Chaabene, H, Hammami, M, Amara, S, Sammoud, S, Mkaouer, B, et al. Agility in young athletes: Is it a different ability from speed and power. J Strength Cond Res 31: 727–735, 2017. 33. Padulo, J, Tabben, M, Ardigo`, LP, Ionel, M, Popa, C, Gevat, C, et al. Repeated sprint ability related to recovery time in young soccer players. Res Sports Med 23: 412–423, 2015. 34. Paul, DJ and Nassis, GP. Physical fitness testing in youth soccer: Issues and considerations regarding reliability, validity and sensitivity. Pediatr Exerc Sci 27: 301–313, 2015. 35. Prieske, O, Muehlbauer, T, Borde, R, Gube, M, Bruhn, S, Behm, DG, et al. Neuromuscular and athletic performance following core strength training in elite youth soccer: Role of instability. Scand J Med Sci Sports 26: 48–56, 2016. 36. Ramı´rez-Campillo, R, Meylan, C, Alvarez, C, Henriquez-Olguin, C, Martinez, C, Canas-Jamett, R, et al. Effects of in-season low-volume high-intensity plyometric training on explosive actions and endurance of young soccer players. J Strength Cond Res 28: 1335–1342, 2014. 37. Rampinini, E, Bishop, D, Marcora, S, Ferrari Bravo, D, Sassi, R, and Impellizzeri, FM. Validity of simple field tests as indicators of matchrelated physical performance in top-level professional soccer players. Int J Sports Med 28: 228–235, 2007. 38. Ratel, S, Duche, P, and Williams, CA. Muscle fatigue during highintensity exercise in children. Sports Med 36: 1031–1065, 2006. 39. Sheppard, JM and Young, WB. Agility literature review: Classification, training and testing. J Sport Sci 24: 919–932, 2006. 40. So¨hnlein, Q, Muller, E, and Stoggl, TL. The effect of 16-week plyometric training on explosive actions in early to mid-puberty elite soccer players. J Strength Cond Res 28: 2105–2114, 2014.

30. Negra, Y, Chaabene, H, Sammoud, S, Bouguezzi, R, Abbes, MA, Hachana, Y, et al. Effects of plyometric training on physical fitness in prepuberal soccer athletes. Int J Sports Med 38: 370–377, 2017.

41. Tomlin, DL and Wenger, HA. The relationship between aerobic fitness and recovery from high intensity intermittent exercise. Sports Med 31: 1–11, 2001.

31. Negra, Y, Chaabene, H, Sammoud, S, Bouguezzi, R, Mkaouer, B, Hachana, Y, et al. Effects of plyometric training on components of physical fitness in prepuberal male soccer athletes: The role of surface instability. J Strength Cond Res 31: 3295–3304, 2017.

42. Vaeyens, R, Malina, RM, Janssens, M, Van Renterghem, B, Bourgois, J, Vrijens, J, et al. A multidisciplinary selection model for youth soccer: The Ghent youth soccer project. Br J Sports Med 40: 928– 934, 2006.



9