Study design
Participants were randomly allocated to the MRHT or FDHT intervention programs. All participating athletes exercised during the intervention period for 8-weeks with two weekly MRHT or FDHT sessions. Similar to other previously published training studies in athletic populations Chaouachi, Granacher [12], a passive control group could not be incorporated as the two experimental groups were national level elite players and there were no comparable players available that would provide similar baseline values. As previous studies have already demonstrated the effectiveness of plyometric and balance training in youth [12], the objective of the present study was to compare the effects of two specific DJ training programs on measures of physical fitness in youth elite volleyball players.
Pre and post training, DJ performance was assessed from 30, 40, and 50-cm DJ boxes. In addition, dynamic balance was assessed using the Y-balance test and the composite score (CS-YBT). Linear sprint speed was assessed over 5, 10 and 20-m and CoD speed using the T-half test.
***TABLE 1 NEAR HERE***
Participants
With reference to the study of Ramirez-Campillo, Alvarez [10] on the effects of different drop heights on DJ performance in youth male soccer players, an a priori power analysis, with a type I error rate of 0.05 and 80% statistical power, was computed. The analysis indicated that 30 participants would be sufficient to observe a significant interaction effect (effect size Cohen’s f = 0.27 for the DJ performance using a box height of 20-cm. Overall, 30 youth male volleyball players from a regional volleyball team were enrolled in this study. The participants performed systematic volleyball training for the last 4 years with 4 to 5 weekly training sessions and a match played during the weekend. Legal guardians and participants provided written informed consent and assent after a thorough explanation of the objectives and the scope of the research project, including the procedures, risks, and benefits of the study. The local ethics committee approved the study (National Centre Medicine and Science of Sports of Tunis [CNMSS-LR09SEP01]) and the procedures were in accordance with the latest version of the Declaration of Helsinki. Written informed consent was obtained from parents/legal representatives of all participants prior to the start of the study.
Procedures
Drop jump performance
To assess DJ performance, participants performed maximal DJs from three drop heights (i.e., 20, 30, 40-cm). DJ performance was measured using an Ergojump ® system (Ergojump, Globus Italia, Codogne, Italy). For the execution of DJs, participants stood in an upright position on the boxes, feet shoulder-wide apart, with the hands placed on the hips. Participants were asked to step off the box with their dominant leg, drop down to land evenly on both feet (no heel contact allowed) and jump-off the ground at maximal-effort to perform a double-leg vertical jump. All participants were instructed to jump as high as possible and to keep ground contact times as short as possible. Two DJ trials were completed for each height with a one minute rest between jumps. The best out of two trials was retained for further analysis.
Dynamic balance
Dynamic balance was evaluated using the CS-YBT [13],16. All test trials were conducted barefoot. Participants stood on the dominant leg, with the most distal aspect of their big toe on the center of the footplate from the YBT kit. The participants were then asked to push the reach-indicator block with the free limb in the anterior, posterior medial, and posterior lateral directions while maintaining their single-limb stance on the central footplate [14]. CS-YBT data collection followed the protocol of Kang, Kim [14]. Participants were not allowed to lift the heel of the stance leg during YBT performance. Maximal reach distances were recorded to the nearest 0.5 cm using the YBT kit.
Linear sprint speed
The performance of a 20-m linear sprint speed test was measured using photocell gates (Brower Timing Systems, Salt Lake City, Utah, USA, accuracy of 0.01 s) placed 0.4 m above the ground. Split sprint times at 5-m, and 20-m were analyzed. Participants started the test with one foot at the starting line in an erect standing position and were instructed to accelerate as fast as possible. A starting signal was not provided in order to avoid the effect of reaction time. The rest period between single sprint trials amounted to ~5-min. The best out of two trials in terms of fastest sprint time was maintained for further analysis.
Change-of-direction speed
We carried out the T-half test as previously outlined by Sassi, Dardouri [15] to assess CoD speed. This test is useful in determining the speed in directional changes (forward sprinting, left and right shuffling, and backpedaling). Each participant performed three trials with a three minute rest after each trial.
Drop jump training programs
During the 8 weeks training intervention, the MRHT or a FDHT groups replaced 20 minutes per session of the technical volleyball training content with DJs, twice a week separated by at least 48-h. A detailed weekly description of the usual volleyball training applied during this period is shown in Table 2. Participants in both intervention groups performed 60 weekly DJ contacts, divided into 3 sets of 10 repetitions of DJs per session, with 15 and 90 seconds of rest between repetitions and sets (28), respectively. Overall training volumes were similar between MRHT and FDHT.
In MRHT, the maximal rebound height was established according to the best rebound height. For this purpose, participants performed DJs from different drop heights (20, 30, 40, 50-cm) and the drop height was used for training where the athletes achieved the largest rebound height. For example, if the player achieved his best rebound height from a drop height of 40-cm, he would use a rebound height of 40-cm during training. The tests to establish a maximal rebound height were repeated on a weekly basis to allow training progression.
Athletes in the FDHT performed DJs over the 8 weeks’ exercise period following a fixed programming scheme from 30-cm drop heights. Participants in the MRHT and FDHT were asked to aim at maximal rebound height and minimal contact time during each jump. Throughout all exercise sessions, the instructor-to-player ratio of 1:1 was maintained. Before the start of MRHT or FDHT, participants completed a standardized warm-up consisting of low-intensity jogging, CoD, DJ, and dynamic lower-limbs stretching.
**** TABLE 2 NEAR HERE ****
Statistical analyses
All data analyses were performed using SPSS 26.0 (SPSS, Inc., Chicago, IL, USA).The level of significance was set a priori at p < 0.05. Data were tested and confirmed for normal distribution using the Shapiro-Wilk test. Subsequently a 2 (groups: MRHT, FDHT) × 2 (time: pre, post) analysis of variance (ANOVA) was computed with repeated measures on time to establish the exercise effects of the different DJ training programs on measures of physical fitness. If group-by-time interactions reached the level of significance, group-specific post-hoc tests (i.e., paired sample t-tests) were computed to identify the comparisons that were statistically significant. Additionally, effect sizes were determined by converting partial eta-squared from the ANOVA output to Cohen’s d [16]. Cohen’s d was classified as small (0.00 ≤ d ≤ 0.49), medium (0.50 ≤ d ≤ 0.79), and large (d ≥ 0.80). Test re-test reliability of the assessed variables was computed using Cronbach’s model of ICCs and standard error of measurements (SEM) according to the method introduced by Hopkins, Marshall [17].