Plyometric-jump training with versus without unstable load to improve physical fitness in trained young volleyball players: a randomized controlled trial

Plyometric-jump training with versus without unstable load to improve physical fitness in trained young volleyball players: a randomized controlled trial | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Plyometric-jump training with versus without unstable load to improve physical fitness in trained young volleyball players: a randomized controlled trial Parsa Soltani, Rodrigo Ramirez-Campillo, Mohammad Fashi, Khosrow Ebrahim, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7474863/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 18 You are reading this latest preprint version Abstract Background: Plyometric-jump training has been recommended as an effective resistance training modality for enhancing vertical jump and lower limb muscle power in young athletes. While unstable surfaces are popular means to address progression and specificity, an alternative approach for applying instability during jumping is the inclusion of unstable loads. The purpose of this study was to compare the effects of unstable load plyometric-jump training (UL-PJT) to plyometric-jump training without unstable load (S-PJT) on physical fitness (i.e. jump performance, muscle strength, balance) of young volleyball players. Methods: Thirty-seven trained male volleyball players aged 16.8±1.1 years were randomly assigned to UL-PJT, S-PJT, and active controls (CON). For 4 weeks (3 sessions/week), UL-PJT group performed jump exercises with external unstable loads (~10% body weight), while S-PJT comprised jump exercises without additional loads. Testing included assessment of jump-and-reach (J&R) height, drop jump (DJ) height, maximal isokinetic knee flexion/extension strength (e.g., peak isokinetic torque [PIT], time to PIT [TTP]), static balance (i.e., overall, anterior-posterior, and medial-lateral stability index). Results: Large group × time interaction effects (all p≤0.048; 0.88≤effect size (ES)≤1.76) were noted for J&R/DJ height, knee extension TTP, and static balance. Post-hoc analyses revealed that overall stability index and J&R height improved after both UL-PJT and S-PJT. Moreover, both training programs induced specific improvements in anterior-posterior (S-PJT) and medial-lateral stability indices (UL-PJT). However, only UL-PJT improved knee extensor TTP and DJ height. Conclusions: The present study demonstrated that 4 weeks of UL-PJT and S-PJT were effective for enhancing J&R and balance performance (i.e., overall stability index) in trained young volleyball players. However, it can be recommended that coaches and practitioners use UL-PJT over S-PJT if the goal is to improve DJ and/or isokinetic knee extensor strength measures. Trial registration: Iranian Registry of Clinical Trials IRCT20250905067129N1 (2025-09-15, retrospectively registered). Instability jump isokinetic strength power balance center of pressure stretch-shortening cycle Figures Figure 1 Figure 2 Figure 3 Figure 4 BACKGROUND Volleyball is a team sport characterized by intermittent high-intensity activities including acceleration, deceleration, jumping, spiking, and blocking interspersed with relatively long periods of recovery ( 1 ). In particular, jumping appears to be a key activity in volleyball players. For instance, it has been reported that elite male volleyball players perform 250–300 attacking and/or blocking jumps during a 5-set match ( 2 ). Depending on the athlete’s position, the total number of jumps performed throughout a season ranged between ~ 13,200 (setters) and ~ 41,400 jumps (middle-blockers) in highly trained male volleyball players ( 3 ). Moreover, young and adult athletes with higher performance levels revealed better jump performance measures compared to athletes with lower performance levels ( 4 , 5 ). Therefore, it appears plausible to consider jump-based exercise programs as an essential training modality in volleyball. Indeed, plyometric-jump training (PJT) has been recommended as an effective resistance training modality for enhancing lower limb muscle power and vertical jump performance ( 6 ). Many plyometric exercises rely on the mechanical properties of the stretch-shortening cycle (SSC) with preactivated muscles experiencing an eccentric phase immediately followed by a concentric phase, utilizing the elastic energy stored during the eccentric (i.e., stretching) phase. This eccentric-concentric coupling produces a more powerful contraction compared with concentric actions alone ( 7 ). Recently, meta-analyses confirmed that traditional PJT (i.e., jumping without external load on stable surfaces [S-PJT]) is safe and effective to improve physical fitness (e.g., jump performance, linear speed) in volleyball players, irrespective of age, sex, and expertise level ( 8 , 9 ). Of note, programming parameters such as load/intensity (e.g., additional loads) ( 10 ), instability/training surface ( 11 ), frequency ( 12 ), direction (e.g., horizontal, vertical) ( 13 ), and/or sequencing (e.g., S-PJT before or after the regular sport-specific training) ( 14 ) can modulate PJT effects. For instance, the eccentric-concentric coupling of dynamic muscle actions during SSC tasks results in the stimulation of a stretch reflex which potentiates performance during the propulsive phase of jumping ( 15 ). These reflex responses depend on the velocity of the stretch and the magnitude of the stretching load ( 15 ). Indeed, to increase the (stretching) load during S-PJT, pre-pubertal soccer players aged 13 years used weighted vests (8% participant’s body weight) during eight weeks of intervention, and improved jump performance, linear speed, change-of-direction speed, and kicking-distance compared to their peers that completed S-PJT ( 10 ). In terms of instability, previous studies applied unstable surface conditions during PJT in young athletes to adhere to the principle of training specificity ( 16 , 17 ). In fact, it was recommended to include instability in resistance training programs to mimic the specific demands of the sport (e.g., jumping and hitting/blocking the ball in volleyball) ( 18 ). Particularly in youth, instability could provide important stimuli for optimal performance and injury prevention as balance and coordination are not fully developed in this age group ( 19 ). However, study findings on the effects of PJT on unstable surfaces vs. S-PJT on physical fitness measures in young athletes revealed inconsistent findings. For instance, in trained pre-pubertal male soccer players aged 12–13 years, 8 weeks of PJT on unstable surfaces (i.e., balance pads) and S-PJT induced similar improvements in jumping (i.e., CMJ, standing long jump), linear and change-of-direction speed, and proactive balance (i.e., Y-balance test). Static balance (i.e., stork balance test), however, was improved after unstable PJT only ( 17 ). Furthermore, eight weeks of PJT improved DJ performance, linear as well as change-of-direction speed, and static balance (i.e., single-leg stance) in trained adolescent male soccer players (mean age: 15 years), irrespective of surface condition. However, gains in CMJ height were larger following S-PJT compared with unstable-surface PJT ( 16 ). A potential explanation for the inconsistent findings in the literature may be the fact that strength/power performance output on unstable surfaces is reduced ( 18 , 19 ). For instance, DJs on unstable compared to stable surfaces resulted in lower jump height and longer time for braking/eccentric phase in physically active adults aged 23 years ( 20 ). An alternative approach for applying instability during PJT is the inclusion of unstable loads (e.g., flexible barbells, weights suspended by elastic bands, aqua bags, water tubes/balloons) in a “top-down strategy” instead of using unstable surfaces (i.e., “bottom-up strategy”) ( 21 , 22 ). With unstable loads, instability is applied at punctum mobile during exercise and not at punctum fixum at the base of support. Ditroilo et al. ( 21 ) reported larger trunk muscle activity and center of pressure displacements when using a water tube during isometric half squats compared with a stable load (i.e., regular barbell) in trained soccer players (mean age: 22 years). Recreationally trained males (mean age: 23 years) improved balance (i.e., single-leg drop landing), trunk muscular endurance, and linear speed after unstable load resistance training compared with stable load and/or variable load resistance training ( 22 ). However, the scarce volume of published studies precludes a robust recommendation for the use of unstable loads during PJT (UL-PJT) to improve components of physical fitness, particularly in young volleyball players. Therefore, this study aimed at comparing the effects of UL-PJT to S-PJT on physical fitness (i.e., jump performance, muscle strength, balance) in young volleyball players. With reference to relevant studies ( 16 , 17 , 22 ), we hypothesized greater jump, strength, and balance improvements after UL-PJT compared with S-PJT. METHODS Participants With reference to the literature ( 22 ), and using G*Power software (version 3.1.9.7, University Düsseldorf, Germany), an a priori analysis showed that a sample size of 30 participants will be sufficient to find significant (p < 0.05) and small-sized interaction effects (effect size [ES] = 0.20) for jump height (statistical power = 80%). To account for potential dropouts, 39 trained male young volleyball players were finally included in the study. At the time of the study onset, the players had ≥ 2 years competing in Tehran volleyball school matches or Tehran League matches, with a sport-specific training volume of ≥ 6 hours per week. Players were paired for jump performance and randomly assigned to two experimental groups (i.e., UL-PJT, S-PJT) and one control group (CON) by one of the authors (PS) drawing lots. Two participants (one in the UL-PJT and one in CON group) withdrew due to personal reasons not related to training, resulting in a final sample size of 37 players (mean age: 16.8 ± 1.1 years) included in the analysis (Fig. 1 ). The anthropometric characteristics of the participants are presented in Table 1 . Timing of peak height velocity (PHV) was estimated from anthropometric outcomes by Moore’s formula ( 23 ). Maturity offset was quantified as the difference between chronological age and estimated timing of PHV. All players were classified as post-PHV ( 24 ). A history of acute or chronic musculoskeletal injury/disorders of the ankle, knees, or lower back that could potentially have affected the outcomes of the study were defined as exclusion criteria. Prior to the study, players and parents/legal guardians were informed about the research methodology as well as potential risks and benefits associated with the study. Written informed consent was obtained from all players involved and/or their parents/legal guardians. The study adheres to the CONSORT guidelines (see supplementary file). Table 1 Trained young volleyball players’ age and anthropometric characteristics. UL-PJT (n = 12) S-PJT (n = 13) Control (n = 12) Age (years) 16.9 ± 1.3 16.8 ± 1.1 16.8 ± 0.9 Body mass (kg) 67.8 ± 7.8 69.7 ± 14.2 70.8 ± 8.3 Body height (cm) 179.2 ± 7.3 181.2 ± 7.9 178.5 ± 5.7 Skeletal muscle mass (kg) 33.0 ± 3.6 32.7 ± 4.8 33.4 ± 3.1 Percentage body fat (%) 12.9 ± 3.6 14.9 ± 5.5 15.5 ± 6.07 Maturity offset (years from PHV) 2.96 ± 1.07 2.99 ± 1.08 2.80 ± 0.66 Data presented as means ± standard deviation; PHV = peak height velocity, S-PJT = stable plyometric-jump training without additional loads, UL-PJT = unstable loaded plyometric-jump training. *** Include Fig. about here *** *** Include Table 1 about here *** Experimental procedure To investigate the effect of UL-PJT vs. S-PJT on measures of physical fitness (i.e., jump performance, muscle strength, balance) in trained young volleyball players, a randomized controlled trial design with parallel groups was used. All measurements were recorded by the same experienced sports scientists (not blinded to group allocation). Body mass, body height, skeletal muscle mass, and percentage body fat of the players were measured (only as descriptive baseline characteristics) using the bioimpedance analysis device Inbody 770 (Seoul, Korea). After a familiarization session in the Sports Sciences Laboratory of Shahid Beheshti University (Tehran, Iran), each player was verbally encouraged to give maximum effort during the tests. Pre-test and post-test measurements were taken 48 hours after the last training session /competition. Testing sessions included a 10–15 min general warm-up of running and dynamic stretching, followed by a specific warm-up of ten submaximal jumps. Before testing, three minutes of recovery were provided after the warm-up ( 25 ). All participants were required i) to have ≥ 8 hours of quality sleep the night before the testing day, ii) to consume a meal high in carbohydrates, iii) to maintain proper hydration prior to the measurements ( 26 ), and iv) to avoid consumption of ergogenic supplements such as caffeine before the test. All tests were completed in one day in a standardized order (i.e., assessment of balance performance prior to jumping, and muscle strength). Assessment of jump performance Vertical jump performance was evaluated using the jump-and-reach (J&R) and DJ tests. For J&R, standing reach height was initially assessed during upright erect standing with both feet in lateral position close to a wall, the dominant arm fully extended in upright direction and the fingers touching a wall-mounted touch-sensitive sensor connected to a digital display (Sargent jump device, Danesh Salar Iranian, Iran). Subsequently, participants performed a countermovement (i.e., knee and hip flexion) with arm swing immediately followed by a rapid and powerful vertical jump. The participants’ task was to touch the wall-mounted scale during flight time at the highest position with their middle finger. Drop jumps were performed with the participants stepping off a 40 cm box, landing on firm floor, and immediately jumping as high as possible. Participants were instructed to touch the wall-mounted scale during the flight at the highest position. Jump height was defined as the difference between standing reach height and jumping reach height. Good-to-excellent test-retest reliability with intraclass correlation coefficients (ICC) of 0.80 to 0.90 and coefficients of variation of 5.9% to 8.6% were reported for Vertec-like jump assessment ( 27 ). For J&R and DJ, players performed three trials with maximal effort (3 min recovery between trials), and the jump heights were averaged for later analysis. Assessment of muscle strength Isokinetic concentric muscle strength of knee flexors and extensors of the dominant leg were assessed (Biodex Multi-Joint System − 4 Pro; Biodex Medical System Inc., Shirley, NY, USA). Leg dominance was determined according to the lateral preference inventory ( 28 ). System calibration was checked each time before testing. Individual adjustment of the equipment involved participants sitting with hip angle at 80° (0° angle corresponding to full hip extension) and straps attached to the equipment to firmly fix the upper body and the hip ( 29 ). The shank of the dominant leg was attached to the lever arm of the dynamometer to control for movement velocity and to record the torque applied by the knee extensors and flexors. The range of motion at the knee joint was 10–100° with 0° corresponding to full knee extension and started from 90° knee flexion. Isokinetic testing included three sets of five maximal unilateral concentric knee extensions-flexion movement at 60°/s. Participants rest was 180 s between trials. Recorded data allowed the calculation of peak isokinetic torque (PIT, i.e., maximal value of the torque-time curve), time to PIT (TTP, i.e., time needed to reach PIT from onset of torque), and average power (i.e., total mechanical work divided by time). Onset of torque was defined as the time point at which torque development exceeded 2.5% of PIT ( 29 ). Isokinetic testing of knee flexor and extensor strength demonstrated excellent test-retest reliability (ICC of 0.98 for knee extension and 0.97 for knee flexion) ( 30 ). For later analysis, the means of PIT, TTP, and average power across the three trials were used. Assessment of balance performance Bipedal static balance was assessed using the Biodex Balance System SD (Biodex Medical System Inc., Shirley, NY, USA). After careful instructions from the test supervisor, participants were asked to stand as quiet and stable as possible during the test. Participants completed three attempts of 20 s, with 10 s rest interval between attempts. According to an initial pilot trial (n = 4), the balance test was performed with a tilt angle level 5–8 (level 1 included maximal tilt angles, i.e. highest instability level). Visual feedback about the participant’s center of gravity was consistently provided by means of the system screen. For later analysis, the overall, anterior-posterior, and medial-lateral stability indices of the system were averaged across the three trials. The Biodex Balance System has demonstrated excellent reliability for overall stability index (ICC = 0.94), anterior-posterior stability index (ICC = 0.95), and 0.93 for medial-lateral stability index (ICC = 0.93) in athletic populations ( 31 ). Plyometric-jump training The study was conducted during the competition period, involving three 90 min weekly sessions of volleyball training, focused on development of individual and team skills. While the CON group kept the regular volleyball training, the intervention groups replaced some technical volleyball training drills with the intervention exercises for 4 weeks ( 32 , 33 ). Players did not participate in any other form of training other than volleyball training ( 34 ). Before all training sessions, all groups included a raise, activate, mobilize and potentiate (RAMP) warm-up, involving running, dynamic stretching, and submaximal jumping ( 34 ). Sessions were supervised by a coach-to-athlete ratio of 1:2 and a 48-hour recovery interval was provided between all training sessions across groups. During the first session of the intervention period, participants in the intervention groups were familiarized with the training methods and protocol. Plyometric-jump training included 12 total sessions (3 per week), each of 15–25 min on a firm gym floor. Participants tracked exercise sessions with an exercise-log, including total duration and session ratings of perceived exertion, the later measured with the modified Borg 10-point scale ( 35 ). The UL-PJT group performed jumps with a barbell and two elastic balloons containing water equal to 10% of the participant’s body weight ( 25 ). The S-PJT group performed the same volume of jumps without any additional loads. A work-to-rest ratio of 1:7 was considered for training sets, and 2 min of rest interval between exercises ( 25 ). Progression during the 4 weeks of PJT was achieved by increasing the total number of jumps per session (i.e., from 48 to 104) and/or hurdle heights (i.e., from 35 to 50 cm) (Table 2 ). Table 2 Description of the 4-weeks plyometric-jump training programs using unstable loads (UL-PJT) or stable conditions without additional loads (S-PJT). Group Exercises Week 1 Week 2 Week 3 Week 4 UL-PJT Hurdle CMJ 3×10 (35 cm)* 4×12 (40 cm) 4×14 (45 cm) 4×16 (50 cm) CMJ 3×6 3×8 4×8 4×10 S-PJT Hurdle CMJ 3×10 (35 cm) 4×12 (40 cm) 4×14 (45 cm) 4×16 (50 cm) CMJ 3×6 3×8 4×8 4×10 CMJ = countermovement jump; S-PJT = stable plyometric-jump training without additional loads; UL-PJT = unstable loaded plyometric-jump training; *: number of sets per exercise × number of jumps per set (hurdle height). *** Include Table 2 about here *** Statistical analysis Data were presented as group mean values ± standard deviations. Normality of the data distribution was assessed using the Shapiro-Wilk test. Variables that did not meet normality assumption (i.e., pre-test overall stability index in UL-PJT, post-test knee extension PIT in CON, pre-test knee flexion PIT in S-PJT) were normalized using a two-step inverse normal transformation procedure ( 36 ). Levene’s test of homogeneity of variances indicated that the variances were equal across groups for all dependent variables (p > 0.05). A 3 (group: UL-PJT, S-PJT, CON) × 2 (time: pre-test, post-test) mixed analysis of variance (ANOVA) with time as the repeated-measures factor was performed to evaluate the main effects of group and time, as well as the group×time interaction effects. In case of group×time interactions, group-specific post-hoc tests (i.e., paired sample t-tests) were performed to identify statistically significant (i.e., p < 0.05) pre- to post-test differences in each group separately. Effect sizes (ES) for mixed ANOVA and paired sample t-tests were calculated and reported as Cohen’s d . Effect sizes were classified as small (ES: 0.20 to 0.49), medium (ES: 0.50 to 0.79), and large (ES ≥ 0.80) ( 37 ). Analyses were performed using Statistical Package for Social Sciences (SPSS; version 27.0). RESULTS All participants (n = 37) received treatment as allocated, with 100% attendance rate, and without reporting training or test-related injury. Pre-test and post-test values for the experimental and control groups are summarized in Table 3 . No between-groups differences were found before interventions, except that CON demonstrated better anterior-posterior and overall stability compared to S-PJT ( p ≤ 0.012, 1.29 ≤ ES ≤ 1.43). Table 3 Performance changes over four weeks of unstable loaded plyometric-jump training (UL-PJT) and stable plyometric-jump training without additional loads (S-PJT) compared with a control (CON) group on measures of physical fitness in trained young volleyball players. Variables UL-PJT (n = 12) S-PJT (n = 13) CON (n = 12) Mixed ANOVA p -value (ES) Pre-test Post-test (Δ%) * Pre-test Post-test (Δ%) Pre-test Post-test Time Group Time × group Jump performance J&R height (cm) 52.4 ± 3.7 55.4 ± 4.2 (5.7) 46.8 ± 7.5 49.5 ± 7.0 (5.7) 49.3 ± 5.3 49.0 ± 5.3 (0.6) < 0.001 (2.21) 0.040 (0.91) < 0.001 (1.76) DJ height (cm) 50.9 ± 3.4 54.1 ± 4.8 (6.2) 45.8 ± 5.7 48.3 ± 6.8 (5.4) 48.3 ± 5.7 48.0 ± 5.6 (0.6) < 0.001 (1.29) 0.037 (0.92) 0.015 (1.06) Muscle strength Knee extension PIT (Nm) 216.8 ± 31.6 219.8 ± 32.4 (1.3) 213.5 ± 50.4 211.2 ± 28.5 (1.0) 206.1 ± 27.0 197.6 ± 13.5 (4.1) 0.601 (0.17) 0.384 (0.48) 0.647 (0.32) Knee flexion PIT (Nm) 108.7 ± 10.9 117.3 ± 19.6 (7.9) 103.5 ± 25.5 106.4 ± 16.5 (2.8) 108.5 ± 22.5 114.1 ± 20.8 (5.1) 0.074 (0.63) 0.488 (0.41) 0.756 (0.25) Knee extension TTP (ms) 590.5 ± 105.2 440.1 ± 96.7 (25.4) 491.7 ± 118.3 445.6 ± 136.7 (9.3) 536.6 ± 115.5 625.8 ± 138.3 (16.6) 0.101 (0.57) 0.030 (0.95) < 0.001 (1.56) Knee flexion TTP (ms) 453.3 ± 57.6 400.5 ± 98.0 (11.6) 509.2 ± 105.9 399.9 ± 106.2 (21.4) 528.6 ± 99.9 401.0 ± 88.0 (24.1) < 0.001 (1.84) 0.468 (0.42) 0.228 (0.60) Knee extension power (W) 147.0 ± 21.8 159.0 ± 22.3 (8.1) 138.3 ± 30.7 151.3 ± 22.9 (9.3) 146.6 ± 16.7 146.3 ± 11.0 (0.2) 0.003 (1.10) 0.583 (0.35) 0.077 (0.80) Knee flexion power (W) 78.4 ± 10.3 81.5 ± 15.5 (3.9) 68.6 ± 17.2 77.7 ± 13.7 (13.2) 79.4 ± 16.8 85.6 ± 16.3 (7.8) 0.004 (1.05) 0.235 (0.59) 0.465 (0.42) Balance performance Overall balance (AU) 1.15 ± 0.39 0.79 ± 0.36 (31.3) 1.35 ± 0.37 0.99 ± 0.38 (26.6) 0.91 ± 0.32 0.95 ± 0.31 (4.3) 0.004 (1.07) 0.078 (0.80) 0.045 (0.89) AP balance (AU) 0.86 ± 0.23 0.60 ± 0.33 (30.2) 0.97 ± 0.27 0.73 ± 0.31 (24.7) 0.60 ± 0.24 0.68 ± 0.28 (13.3) 0.025 (0.80) 0.065 (0.83) 0.048 (0.88) ML balance (AU) 0.62 ± 0.26 0.38 ± 0.16 (38.7) 0.74 ± 0.26 0.55 ± 0.29 (25.6) 0.50 ± 0.16 0.60 ± 0.26 (20.0) 0.026 (0.79) 0.183 (0.64) 0.013 (1.07) Data presented as means ± standard deviation; AP: anterior-posterior; AU = arbitrary units; ES = effect size (Cohen’s d ); DJ = drop jump; J&R = jump-and-reach; ML: medial-lateral; PIT = peak isokinetic torque; S-PJT = stable plyometric-jump training without additional loads;; TTP = time to peak isokinetic torque; UL-PJT = unstable loaded plyometric-jump training; * = values in parenthesis denote relative within-group pre-post test changes. *** Include Table 3 about here *** Jump performance The J&R and DJ height showed large-sized group×time interaction effects ( p ≤ 0.015, 1.06 ≤ ES ≤ 1.76). In contrast to CON, the UL-PJT and S-PJT improved J&R height ( p < 0.001, = 1.53 ≤ ES ≤ 1.54; Fig. 2 ). Only UL-PJT improved DJ height ( p = 0.001, ES = 1.27). *** Include Fig. 2 about here *** Muscle strength Group×time interaction effect was found for knee extension TTP ( p < 0.001, ES = 1.56). Only UL-PJT improved (i.e., decreased time) knee extension TTP ( p < 0.001, ES = 1.28; Fig. 3 ). Further, main effects of time were noted for knee flexion TTP, as well as in knee extension and flexion power output ( p ≤ 0.004, 1.05 ≤ ES ≤ 1.84), irrespective of group. *** Include Fig. 3 about here *** Balance performance Group-time interaction effects were noted for overall, anterior-posterior, and medial-lateral stability ( p ≤ 0.048, 0.88 ≤ ES ≤ 1.07). Post-hoc tests showed that UL-PJT and S-PJT ( p ≤ 0.022, 0.73 ≤ ES ≤ 0.79) improved (i.e. reduced values) overall stability (Fig. 4 ). Further, S-PJT improved anterior-posterior stability index ( p = 0.035, ES = 0.65). Moreover, only UL-PJT improved medial–lateral stability ( p = 0.020, ES = 0.78). *** Include Fig. 4 about here *** DISCUSSION The main findings of this randomized controlled trial were that i) both UL-PJT and S-PJT improved J&R height, ii) DJ height and knee extensor TTP were improved following UL-PJT only, and iii) UL-PJT improved the medial-lateral component (i.e., stability index), whereas the S-PJT improved the anterior-posterior component during the static balance test. In general, volleyball players perform a considerable number of jumps during regular training sessions and competitions ( 3 ), which may reduce their responsiveness to additional jump-related training stimuli ( 38 ). However, the improvement in J&R height after UL-PJT and S-PJT confirmed the effectiveness of PJT interventions adapted to the needs of trained young volleyball players. Indeed, Fig. 2 indicated that virtually all individuals improved J&R height after the 4-weeks intervention period. This is well in-line with previous interventions of similar duration in young and adult team sport athletes ( 32 , 33 ). For instance, Ricart-Luna et al. ( 33 ) examined the effects of a 4-weeks mesocycle (3 sessions/week) of loaded PJT versus weightlifting training in highly-trained male and female basketball players aged 12 to 16 years and found that both groups similarly improved CMJ height and horizontal broad jump distance. Further, highly-trained female volleyball players (mean age: 20 years) performed 4 weeks of combined S-PJT and loaded PJT during the pre-competitive period and improved CMJ height compared with an active control group ( 32 ). Considering that greater jump heights are commonly achieved by higher-level players ( 4 , 5 ), improved vertical jump performance may offer a key potential advantage to developing volleyball players. Interestingly and in contrast to the J&R test, DJ height (i.e., maximal jump height achieved after dropping off a 40-cm box) improved only following UL-PJT. Although not measured in our study, it can be speculated that the DJ test required greater ground reaction forces and leg muscle stiffness during the landing phase on the stable ground when compared with the J&R test, requiring considerable (eccentric) leg muscle strength ( 39 )(e.g., assessed as landing rate of force development). Including additional and unstable loads during PJT may have facilitated the ability to rapidly develop forces in the lower limbs. Partly in support of this notion, only UL-PJT improved TTP (i.e., decreased time to reach PIT) of the knee extensors, key muscles acting during the landing phase of DJ ( 40 – 42 ). Future studies would be required to confirm the potential effect of UL-PJT on kinetic, kinematic, and/or neuromuscular adaptations related to jump performance. Notably, while knee extension TTP improved after UL-PJT, other isokinetic measures (i.e., PIT, average power) in the knee extensors and flexors were not significantly affected by the interventions. The task specificity of jumping exercises (e.g., multiarticular, bilateral, dynamic, high-velocity) and isokinetic testing (e.g., monoarticular, unilateral, isokinetic, 60°/s) may have reduced the transfer effects of UL-PJT and S-PJT to muscle strength adaptations in the present study ( 43 ). Nonetheless, the improvement in TTP after UL-PJT is a novel finding, partially supported by previous reports ( 44 , 45 ). For instance, male physical education students (mean age: 23 years) improved knee extension TTP (at 180°/s) after 6 weeks (3–4 sessions/week) of S-PJT, whole-body vibration training, and combined S-PJT and whole-body vibration training, with greater gains after the combined training program ( 44 ). In trained male under-17 soccer players, knee extension TTP (at 60°/s) was improved after 6 weeks (2 sessions/week) of PJT, particularly after loaded PJT (+ 10% body mass) compared with S-PJT ( 45 ). Future studies need to directly determine if unstable loads may provide superior stimuli during PJT when compared with stable loads. In terms of balance performance, both UL-PJT and S-PJT improved overall balance, although only UL-PJT improved the medial–lateral component, while S-PJT improved the anterior-posterior component. The superior effects of PJT on balance performance have been reported previously ( 46 ), including PJT interventions with and without instability conditions ( 11 ). Jump training exercises may increase the activation of stabilizing muscles, particularly in the trunk and lower limbs, leading to improved postural control ( 20 ), and the present findings suggest specific adaptations according to jumping conditions (i.e., UL-PJT improved medial–lateral balance; S-PJT improved anterior-posterior balance). Moreover, jump exercises may exert a favorable overload on proprioceptive feedback and sensorimotor integration, allowing for more precise control of body position. Further, jump drills may reinforce vestibular and visual control strategies, which are crucial for maintaining stability under dynamic conditions. Therefore, both UL-PJT and S-PJT seems similarly effective to enhance overall balance performance, although future studies may be required to i) determine if their combination may potentiate both medial–lateral and anterior-posterior stability, ii) determine the physiological-biomechanical mechanisms underlying the specific balance improvements after each mode of jump training. Potential limitations According to the training integration principle, multicomponent interventions should be emphasized to optimize athletic performance (e.g. jump, strength, balance), reduce risk of injuries, and for holistic development of athletes, particularly youth athletes ( 47 ). Indeed, the UL-PJT was ecologically combined with volleyball players’ multidimensional training routine. However, UL-PJT also combined both load and instability, whereas the effects of each component (load or instability) were not examined independently. Therefore, future studies would be needed to determine whether the benefits observed in the current study were due to the combined effect or whether instability itself contributed additional effects compared to loaded training in stable surface. CONCLUSIONS Volleyball players improved maximal vertical jump height (J&R test) and overall balance after both PJT modes. However, only UL-PJT improved DJ height which may be attributed to group-specific gains in knee extension TTP. Moreover, both training programs induced specific anterior-posterior balance improvement (S-PJT) and medial-lateral balance improvement (UL-PJT). Coaches and practitioners could use UL-PJT over S-PJT if the goal is to improve DJ and/or isokinetic knee extensor strength measures in trained young (i.e., post-PHV) volleyball players. Future studies would be required to confirm if such adaptations can be transferred to competitive actions (e.g., spiking, blocking). Abbreviations ANOVA = analysis of variance CMJ = countermovement jump CON = control group DJ = drop jump ES = effect size J&R = jump-and-reach PHV = peak-height velocity PIT = peak isokinetic torque PJT = plyometric-jump training S-PJT = stable plyometric-jump training without additional loads SSC = stretch-shortening cycle TTP = time to peak isokinetic torque UL-PJT = unstable loaded plyometric-jump training. Declarations Ethics approval and consent to participate The study was conducted in accordance with the latest version of the declaration of Helsinki. It was approved by the Research Ethics Committee of Shahid Beheshti University (IR.SBU.REC.1404.006). Prior to the commencement of the study, written informed consent was obtained from all players involved and/or their parents/legal guardians. Consent for publication All authors read and approved the final manuscript and gave their consent for publication. Availability of data and materials The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request. Competing interests PS, RRC, MF, KE, JLM, and OP declare that they have no competing interests. Funding No source of funding was used during the study. Author contributions PS conceived the study design with guidance from MF and KE, who supervised the project and contributed to the conceptualization. PS collected the data and performed analyses together with OP. PS and OP prepared the first draft of the manuscript. RRC contributed to data interpretation and critically revised the manuscript together with JLM. All authors read and approved the final version of the article. Acknowledgements The authors thank all the athletes who volunteered to participate in the study and those who collaborated with data collection. References Sheppard JM, Gabbett TJ, Stanganelli L-CR. 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Additional Declarations No competing interests reported. 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14:45:38","extension":"html","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":146843,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7474863/v1/8b8c152463114c44d8518c74.html"},{"id":92876938,"identity":"e1e64f84-2309-4b7a-b5dc-5871c211bd27","added_by":"auto","created_at":"2025-10-06 14:53:37","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":99882,"visible":true,"origin":"","legend":"\u003cp\u003eStudy flow chart.\u003c/p\u003e","description":"","filename":"Fig1flowChart300.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7474863/v1/25619e08802dc4a6ad1fe21d.jpg"},{"id":92876274,"identity":"540edb8c-090a-4152-ad0e-f6616ff21c13","added_by":"auto","created_at":"2025-10-06 14:45:37","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":46016,"visible":true,"origin":"","legend":"\u003cp\u003ePre- to post-test changes in jump-and-reach height following unstable loaded plyometric-jump training (UL-PJT) and stable plyometric-jump training without additional loads (S-PJT) compared with a control (CON) group on measures of in trained young volleyball players.\u003c/p\u003e","description":"","filename":"Fig2JR300.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7474863/v1/ffd926dfed95c6c8cc957960.jpg"},{"id":92876940,"identity":"d5363871-145d-486a-8306-9c89f39b5f48","added_by":"auto","created_at":"2025-10-06 14:53:37","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":49483,"visible":true,"origin":"","legend":"\u003cp\u003ePre- to post-test changes in time-to-peak isokinetic torque following unstable loaded plyometric-jump training (UL-PJT) and stable plyometric-jump training without additional loads (S-PJT) compared with a control (CON) group on measures of in trained young volleyball players.\u003c/p\u003e","description":"","filename":"Fig3timeToPeak300.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7474863/v1/f1bcd33c899892736d93f2ad.jpg"},{"id":92876277,"identity":"a635583e-dd4d-437b-ade4-3f7d33401175","added_by":"auto","created_at":"2025-10-06 14:45:37","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":45725,"visible":true,"origin":"","legend":"\u003cp\u003ePre- to post-test changes in overall stability index (i.e., static balance) following unstable loaded plyometric-jump training (UL-PJT) and stable plyometric-jump training without additional loads (S-PJT) compared with a control (CON) group on measures of in trained young volleyball players.\u003c/p\u003e","description":"","filename":"Fig4balanceoverall300.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7474863/v1/a05ec9f593142368c9310393.jpg"},{"id":92878629,"identity":"8fd4c1ed-52f3-4ba1-9020-a719016e7063","added_by":"auto","created_at":"2025-10-06 15:09:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1167499,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7474863/v1/1a76cde9-a640-4945-8f00-aa2e9452945f.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Plyometric-jump training with versus without unstable load to improve physical fitness in trained young volleyball players: a randomized controlled trial","fulltext":[{"header":"BACKGROUND","content":"\u003cp\u003eVolleyball is a team sport characterized by intermittent high-intensity activities including acceleration, deceleration, jumping, spiking, and blocking interspersed with relatively long periods of recovery (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e). In particular, jumping appears to be a key activity in volleyball players. For instance, it has been reported that elite male volleyball players perform 250\u0026ndash;300 attacking and/or blocking jumps during a 5-set match (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e). Depending on the athlete\u0026rsquo;s position, the total number of jumps performed throughout a season ranged between ~\u0026thinsp;13,200 (setters) and ~\u0026thinsp;41,400 jumps (middle-blockers) in highly trained male volleyball players (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e). Moreover, young and adult athletes with higher performance levels revealed better jump performance measures compared to athletes with lower performance levels (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e). Therefore, it appears plausible to consider jump-based exercise programs as an essential training modality in volleyball.\u003c/p\u003e\u003cp\u003eIndeed, plyometric-jump training (PJT) has been recommended as an effective resistance training modality for enhancing lower limb muscle power and vertical jump performance (\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e). Many plyometric exercises rely on the mechanical properties of the stretch-shortening cycle (SSC) with preactivated muscles experiencing an eccentric phase immediately followed by a concentric phase, utilizing the elastic energy stored during the eccentric (i.e., stretching) phase. This eccentric-concentric coupling produces a more powerful contraction compared with concentric actions alone (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e). Recently, meta-analyses confirmed that traditional PJT (i.e., jumping without external load on stable surfaces [S-PJT]) is safe and effective to improve physical fitness (e.g., jump performance, linear speed) in volleyball players, irrespective of age, sex, and expertise level (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e). Of note, programming parameters such as load/intensity (e.g., additional loads) (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e), instability/training surface (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e), frequency (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e), direction (e.g., horizontal, vertical) (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e), and/or sequencing (e.g., S-PJT before or after the regular sport-specific training) (\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e) can modulate PJT effects. For instance, the eccentric-concentric coupling of dynamic muscle actions during SSC tasks results in the stimulation of a stretch reflex which potentiates performance during the propulsive phase of jumping (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e). These reflex responses depend on the velocity of the stretch and the magnitude of the stretching load (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e). Indeed, to increase the (stretching) load during S-PJT, pre-pubertal soccer players aged 13 years used weighted vests (8% participant\u0026rsquo;s body weight) during eight weeks of intervention, and improved jump performance, linear speed, change-of-direction speed, and kicking-distance compared to their peers that completed S-PJT (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn terms of instability, previous studies applied unstable surface conditions during PJT in young athletes to adhere to the principle of training specificity (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e). In fact, it was recommended to include instability in resistance training programs to mimic the specific demands of the sport (e.g., jumping and hitting/blocking the ball in volleyball) (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e). Particularly in youth, instability could provide important stimuli for optimal performance and injury prevention as balance and coordination are not fully developed in this age group (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e). However, study findings on the effects of PJT on unstable surfaces vs. S-PJT on physical fitness measures in young athletes revealed inconsistent findings. For instance, in trained pre-pubertal male soccer players aged 12\u0026ndash;13 years, 8 weeks of PJT on unstable surfaces (i.e., balance pads) and S-PJT induced similar improvements in jumping (i.e., CMJ, standing long jump), linear and change-of-direction speed, and proactive balance (i.e., Y-balance test). Static balance (i.e., stork balance test), however, was improved after unstable PJT only (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e). Furthermore, eight weeks of PJT improved DJ performance, linear as well as change-of-direction speed, and static balance (i.e., single-leg stance) in trained adolescent male soccer players (mean age: 15 years), irrespective of surface condition. However, gains in CMJ height were larger following S-PJT compared with unstable-surface PJT (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e). A potential explanation for the inconsistent findings in the literature may be the fact that strength/power performance output on unstable surfaces is reduced (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e). For instance, DJs on unstable compared to stable surfaces resulted in lower jump height and longer time for braking/eccentric phase in physically active adults aged 23 years (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAn alternative approach for applying instability during PJT is the inclusion of unstable loads (e.g., flexible barbells, weights suspended by elastic bands, aqua bags, water tubes/balloons) in a \u0026ldquo;top-down strategy\u0026rdquo; instead of using unstable surfaces (i.e., \u0026ldquo;bottom-up strategy\u0026rdquo;) (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e). With unstable loads, instability is applied at \u003cem\u003epunctum mobile\u003c/em\u003e during exercise and not at \u003cem\u003epunctum fixum\u003c/em\u003e at the base of support. Ditroilo et al. (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e) reported larger trunk muscle activity and center of pressure displacements when using a water tube during isometric half squats compared with a stable load (i.e., regular barbell) in trained soccer players (mean age: 22 years). Recreationally trained males (mean age: 23 years) improved balance (i.e., single-leg drop landing), trunk muscular endurance, and linear speed after unstable load resistance training compared with stable load and/or variable load resistance training (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e). However, the scarce volume of published studies precludes a robust recommendation for the use of unstable loads during PJT (UL-PJT) to improve components of physical fitness, particularly in young volleyball players.\u003c/p\u003e\u003cp\u003eTherefore, this study aimed at comparing the effects of UL-PJT to S-PJT on physical fitness (i.e., jump performance, muscle strength, balance) in young volleyball players. With reference to relevant studies (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e), we hypothesized greater jump, strength, and balance improvements after UL-PJT compared with S-PJT.\u003c/p\u003e"},{"header":"METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003eParticipants\u003c/h2\u003e\n \u003cp\u003eWith reference to the literature (\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e), and using G*Power software (version 3.1.9.7, University D\u0026uuml;sseldorf, Germany), an \u003cem\u003ea priori\u003c/em\u003e analysis showed that a sample size of 30 participants will be sufficient to find significant (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) and small-sized interaction effects (effect size [ES]\u0026thinsp;=\u0026thinsp;0.20) for jump height (statistical power\u0026thinsp;=\u0026thinsp;80%). To account for potential dropouts, 39 trained male young volleyball players were finally included in the study. At the time of the study onset, the players had\u0026thinsp;\u0026ge;\u0026thinsp;2 years competing in Tehran volleyball school matches or Tehran League matches, with a sport-specific training volume of \u0026ge;\u0026thinsp;6 hours per week. Players were paired for jump performance and randomly assigned to two experimental groups (i.e., UL-PJT, S-PJT) and one control group (CON) by one of the authors (PS) drawing lots. Two participants (one in the UL-PJT and one in CON group) withdrew due to personal reasons not related to training, resulting in a final sample size of 37 players (mean age: 16.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1 years) included in the analysis (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). The anthropometric characteristics of the participants are presented in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. Timing of peak height velocity (PHV) was estimated from anthropometric outcomes by Moore\u0026rsquo;s formula (\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e). Maturity offset was quantified as the difference between chronological age and estimated timing of PHV. All players were classified as post-PHV (\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e). A history of acute or chronic musculoskeletal injury/disorders of the ankle, knees, or lower back that could potentially have affected the outcomes of the study were defined as exclusion criteria. Prior to the study, players and parents/legal guardians were informed about the research methodology as well as potential risks and benefits associated with the study. Written informed consent was obtained from all players involved and/or their parents/legal guardians. The study adheres to the CONSORT guidelines (see supplementary file).\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eTrained young volleyball players\u0026rsquo; age and anthropometric characteristics.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eUL-PJT (n\u0026thinsp;=\u0026thinsp;12)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eS-PJT (n\u0026thinsp;=\u0026thinsp;13)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eControl (n\u0026thinsp;=\u0026thinsp;12)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAge (years)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e16.9\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e16.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e16.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eBody mass (kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e67.8\u0026thinsp;\u0026plusmn;\u0026thinsp;7.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e69.7\u0026thinsp;\u0026plusmn;\u0026thinsp;14.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e70.8\u0026thinsp;\u0026plusmn;\u0026thinsp;8.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eBody height (cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e179.2\u0026thinsp;\u0026plusmn;\u0026thinsp;7.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e181.2\u0026thinsp;\u0026plusmn;\u0026thinsp;7.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e178.5\u0026thinsp;\u0026plusmn;\u0026thinsp;5.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSkeletal muscle mass (kg)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e33.0\u0026thinsp;\u0026plusmn;\u0026thinsp;3.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e32.7\u0026thinsp;\u0026plusmn;\u0026thinsp;4.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e33.4\u0026thinsp;\u0026plusmn;\u0026thinsp;3.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePercentage body fat (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e12.9\u0026thinsp;\u0026plusmn;\u0026thinsp;3.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e14.9\u0026thinsp;\u0026plusmn;\u0026thinsp;5.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15.5\u0026thinsp;\u0026plusmn;\u0026thinsp;6.07\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eMaturity offset (years from PHV)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.96\u0026thinsp;\u0026plusmn;\u0026thinsp;1.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.99\u0026thinsp;\u0026plusmn;\u0026thinsp;1.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.66\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"4\"\u003e\n \u003cp\u003eData presented as means\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation; PHV\u0026thinsp;=\u0026thinsp;peak height velocity, S-PJT\u0026thinsp;=\u0026thinsp;stable plyometric-jump training without additional loads, UL-PJT\u0026thinsp;=\u0026thinsp;unstable loaded plyometric-jump training.\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003e*** Include Fig. \u0026nbsp;about here ***\u003c/p\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003cp\u003e*** Include Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e about here ***\u003c/p\u003e\n \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\n \u003ch2\u003eExperimental procedure\u003c/h2\u003e\n \u003cp\u003eTo investigate the effect of UL-PJT vs. S-PJT on measures of physical fitness (i.e., jump performance, muscle strength, balance) in trained young volleyball players, a randomized controlled trial design with parallel groups was used. All measurements were recorded by the same experienced sports scientists (not blinded to group allocation). Body mass, body height, skeletal muscle mass, and percentage body fat of the players were measured (only as descriptive baseline characteristics) using the bioimpedance analysis device Inbody 770 (Seoul, Korea). After a familiarization session in the Sports Sciences Laboratory of Shahid Beheshti University (Tehran, Iran), each player was verbally encouraged to give maximum effort during the tests. Pre-test and post-test measurements were taken 48 hours after the last training session /competition. Testing sessions included a 10\u0026ndash;15 min general warm-up of running and dynamic stretching, followed by a specific warm-up of ten submaximal jumps. Before testing, three minutes of recovery were provided after the warm-up (\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e). All participants were required i) to have \u0026ge;\u0026thinsp;8 hours of quality sleep the night before the testing day, ii) to consume a meal high in carbohydrates, iii) to maintain proper hydration prior to the measurements (\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e), and iv) to avoid consumption of ergogenic supplements such as caffeine before the test. All tests were completed in one day in a standardized order (i.e., assessment of balance performance prior to jumping, and muscle strength).\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003ch3\u003eAssessment of jump performance\u003c/h3\u003e\n\u003cp\u003eVertical jump performance was evaluated using the jump-and-reach (J\u0026amp;R) and DJ tests. For J\u0026amp;R, standing reach height was initially assessed during upright erect standing with both feet in lateral position close to a wall, the dominant arm fully extended in upright direction and the fingers touching a wall-mounted touch-sensitive sensor connected to a digital display (Sargent jump device, Danesh Salar Iranian, Iran). Subsequently, participants performed a countermovement (i.e., knee and hip flexion) with arm swing immediately followed by a rapid and powerful vertical jump. The participants\u0026rsquo; task was to touch the wall-mounted scale during flight time at the highest position with their middle finger. Drop jumps were performed with the participants stepping off a 40 cm box, landing on firm floor, and immediately jumping as high as possible. Participants were instructed to touch the wall-mounted scale during the flight at the highest position. Jump height was defined as the difference between standing reach height and jumping reach height. Good-to-excellent test-retest reliability with intraclass correlation coefficients (ICC) of 0.80 to 0.90 and coefficients of variation of 5.9% to 8.6% were reported for Vertec-like jump assessment (\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e). For J\u0026amp;R and DJ, players performed three trials with maximal effort (3 min recovery between trials), and the jump heights were averaged for later analysis.\u003c/p\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003eAssessment of muscle strength\u003c/h2\u003e\n \u003cp\u003eIsokinetic concentric muscle strength of knee flexors and extensors of the dominant leg were assessed (Biodex Multi-Joint System \u0026minus;\u0026thinsp;4 Pro; Biodex Medical System Inc., Shirley, NY, USA). Leg dominance was determined according to the lateral preference inventory (\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e). System calibration was checked each time before testing. Individual adjustment of the equipment involved participants sitting with hip angle at 80\u0026deg; (0\u0026deg; angle corresponding to full hip extension) and straps attached to the equipment to firmly fix the upper body and the hip (\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e). The shank of the dominant leg was attached to the lever arm of the dynamometer to control for movement velocity and to record the torque applied by the knee extensors and flexors. The range of motion at the knee joint was 10\u0026ndash;100\u0026deg; with 0\u0026deg; corresponding to full knee extension and started from 90\u0026deg; knee flexion. Isokinetic testing included three sets of five maximal unilateral concentric knee extensions-flexion movement at 60\u0026deg;/s. Participants rest was 180 s between trials. Recorded data allowed the calculation of peak isokinetic torque (PIT, i.e., maximal value of the torque-time curve), time to PIT (TTP, i.e., time needed to reach PIT from onset of torque), and average power (i.e., total mechanical work divided by time). Onset of torque was defined as the time point at which torque development exceeded 2.5% of PIT (\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e). Isokinetic testing of knee flexor and extensor strength demonstrated excellent test-retest reliability (ICC of 0.98 for knee extension and 0.97 for knee flexion) (\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e). For later analysis, the means of PIT, TTP, and average power across the three trials were used.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eAssessment of balance performance\u003c/h3\u003e\n\u003cp\u003eBipedal static balance was assessed using the Biodex Balance System SD (Biodex Medical System Inc., Shirley, NY, USA). After careful instructions from the test supervisor, participants were asked to stand as quiet and stable as possible during the test. Participants completed three attempts of 20 s, with 10 s rest interval between attempts. According to an initial pilot trial (n\u0026thinsp;=\u0026thinsp;4), the balance test was performed with a tilt angle level 5\u0026ndash;8 (level 1 included maximal tilt angles, i.e. highest instability level). Visual feedback about the participant\u0026rsquo;s center of gravity was consistently provided by means of the system screen. For later analysis, the overall, anterior-posterior, and medial-lateral stability indices of the system were averaged across the three trials. The Biodex Balance System has demonstrated excellent reliability for overall stability index (ICC\u0026thinsp;=\u0026thinsp;0.94), anterior-posterior stability index (ICC\u0026thinsp;=\u0026thinsp;0.95), and 0.93 for medial-lateral stability index (ICC\u0026thinsp;=\u0026thinsp;0.93) in athletic populations (\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e).\u003c/p\u003e\n\u003ch3\u003ePlyometric-jump training\u003c/h3\u003e\n\u003cp\u003eThe study was conducted during the competition period, involving three 90 min weekly sessions of volleyball training, focused on development of individual and team skills. While the CON group kept the regular volleyball training, the intervention groups replaced some technical volleyball training drills with the intervention exercises for 4 weeks (\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e). Players did not participate in any other form of training other than volleyball training (\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e). Before all training sessions, all groups included a raise, activate, mobilize and potentiate (RAMP) warm-up, involving running, dynamic stretching, and submaximal jumping (\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e). Sessions were supervised by a coach-to-athlete ratio of 1:2 and a 48-hour recovery interval was provided between all training sessions across groups.\u003c/p\u003e\n\u003cp\u003eDuring the first session of the intervention period, participants in the intervention groups were familiarized with the training methods and protocol. Plyometric-jump training included 12 total sessions (3 per week), each of 15\u0026ndash;25 min on a firm gym floor. Participants tracked exercise sessions with an exercise-log, including total duration and session ratings of perceived exertion, the later measured with the modified Borg 10-point scale (\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e). The UL-PJT group performed jumps with a barbell and two elastic balloons containing water equal to 10% of the participant\u0026rsquo;s body weight (\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e). The S-PJT group performed the same volume of jumps without any additional loads. A work-to-rest ratio of 1:7 was considered for training sets, and 2 min of rest interval between exercises (\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e). Progression during the 4 weeks of PJT was achieved by increasing the total number of jumps per session (i.e., from 48 to 104) and/or hurdle heights (i.e., from 35 to 50 cm) (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n \u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eDescription of the 4-weeks plyometric-jump training programs using unstable loads (UL-PJT) or stable conditions without additional loads (S-PJT).\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eGroup\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eExercises\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWeek 1\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWeek 2\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWeek 3\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWeek 4\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eUL-PJT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHurdle CMJ\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u0026times;10 (35 cm)*\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;12 (40 cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;14 (45 cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;16 (50 cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCMJ\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u0026times;6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u0026times;8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;10\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eS-PJT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHurdle CMJ\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u0026times;10 (35 cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;12 (40 cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;14 (45 cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;16 (50 cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCMJ\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u0026times;6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u0026times;8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u0026times;10\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"6\"\u003e\n \u003cp\u003eCMJ\u0026thinsp;=\u0026thinsp;countermovement jump; S-PJT\u0026thinsp;=\u0026thinsp;stable plyometric-jump training without additional loads; UL-PJT\u0026thinsp;=\u0026thinsp;unstable loaded plyometric-jump training; *: number of sets per exercise \u0026times; number of jumps per set (hurdle height).\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003cp\u003e*** Include Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e about here ***\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003eStatistical analysis\u003c/h2\u003e\n \u003cp\u003eData were presented as group mean values\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviations. Normality of the data distribution was assessed using the Shapiro-Wilk test. Variables that did not meet normality assumption (i.e., pre-test overall stability index in UL-PJT, post-test knee extension PIT in CON, pre-test knee flexion PIT in S-PJT) were normalized using a two-step inverse normal transformation procedure (\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e). Levene\u0026rsquo;s test of homogeneity of variances indicated that the variances were equal across groups for all dependent variables (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05). A 3 (group: UL-PJT, S-PJT, CON) \u0026times; 2 (time: pre-test, post-test) mixed analysis of variance (ANOVA) with time as the repeated-measures factor was performed to evaluate the main effects of group and time, as well as the group\u0026times;time interaction effects. In case of group\u0026times;time interactions, group-specific post-hoc tests (i.e., paired sample t-tests) were performed to identify statistically significant (i.e., \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) pre- to post-test differences in each group separately. Effect sizes (ES) for mixed ANOVA and paired sample t-tests were calculated and reported as Cohen\u0026rsquo;s \u003cem\u003ed\u003c/em\u003e. Effect sizes were classified as small (ES: 0.20 to 0.49), medium (ES: 0.50 to 0.79), and large (ES\u0026thinsp;\u0026ge;\u0026thinsp;0.80) (\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e). Analyses were performed using Statistical Package for Social Sciences (SPSS; version 27.0).\u003c/p\u003e\n\u003c/div\u003e"},{"header":"RESULTS","content":"\u003cp\u003eAll participants (n\u0026thinsp;=\u0026thinsp;37) received treatment as allocated, with 100% attendance rate, and without reporting training or test-related injury. Pre-test and post-test values for the experimental and control groups are summarized in Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. No between-groups differences were found before interventions, except that CON demonstrated better anterior-posterior and overall stability compared to S-PJT (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.012, 1.29\u0026thinsp;\u0026le;\u0026thinsp;ES\u0026thinsp;\u0026le;\u0026thinsp;1.43).\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n \u003ctable id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003ePerformance changes over four weeks of unstable loaded plyometric-jump training (UL-PJT) and stable plyometric-jump training without additional loads (S-PJT) compared with a control (CON) group on measures of physical fitness in trained young volleyball players.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eVariables\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eUL-PJT\u003c/p\u003e\n \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;12)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eS-PJT\u003c/p\u003e\n \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;13)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eCON\u003c/p\u003e\n \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;12)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003eMixed ANOVA\u003c/p\u003e\n \u003cp\u003e\u003cem\u003ep\u003c/em\u003e-value (ES)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePre-test\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePost-test\u003c/p\u003e\n \u003cp\u003e(\u0026Delta;%) *\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePre-test\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePost-test (\u0026Delta;%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePre-test\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePost-test\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTime\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eGroup\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTime \u0026times; group\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"10\"\u003e\n \u003cp\u003e\u003cem\u003eJump performance\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eJ\u0026amp;R height (cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e52.4\u0026thinsp;\u0026plusmn;\u0026thinsp;3.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e55.4\u0026thinsp;\u0026plusmn;\u0026thinsp;4.2\u003c/p\u003e\n \u003cp\u003e(5.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e46.8\u0026thinsp;\u0026plusmn;\u0026thinsp;7.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e49.5\u0026thinsp;\u0026plusmn;\u0026thinsp;7.0 (5.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e49.3\u0026thinsp;\u0026plusmn;\u0026thinsp;5.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e49.0\u0026thinsp;\u0026plusmn;\u0026thinsp;5.3 (0.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;0.001 (2.21)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.040 (0.91)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;0.001 (1.76)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eDJ height (cm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e50.9\u0026thinsp;\u0026plusmn;\u0026thinsp;3.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e54.1\u0026thinsp;\u0026plusmn;\u0026thinsp;4.8\u003c/p\u003e\n \u003cp\u003e(6.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e45.8\u0026thinsp;\u0026plusmn;\u0026thinsp;5.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e48.3\u0026thinsp;\u0026plusmn;\u0026thinsp;6.8 (5.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e48.3\u0026thinsp;\u0026plusmn;\u0026thinsp;5.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e48.0\u0026thinsp;\u0026plusmn;\u0026thinsp;5.6 (0.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;0.001 (1.29)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.037 (0.92)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.015 (1.06)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"10\"\u003e\n \u003cp\u003e\u003cem\u003eMuscle strength\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eKnee extension PIT (Nm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e216.8\u0026thinsp;\u0026plusmn;\u0026thinsp;31.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e219.8\u0026thinsp;\u0026plusmn;\u0026thinsp;32.4\u003c/p\u003e\n \u003cp\u003e(1.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e213.5\u0026thinsp;\u0026plusmn;\u0026thinsp;50.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e211.2\u0026thinsp;\u0026plusmn;\u0026thinsp;28.5 (1.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e206.1\u0026thinsp;\u0026plusmn;\u0026thinsp;27.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e197.6\u0026thinsp;\u0026plusmn;\u0026thinsp;13.5 (4.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.601 (0.17)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.384 (0.48)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.647 (0.32)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eKnee flexion PIT (Nm)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e108.7\u0026thinsp;\u0026plusmn;\u0026thinsp;10.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e117.3\u0026thinsp;\u0026plusmn;\u0026thinsp;19.6\u003c/p\u003e\n \u003cp\u003e(7.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e103.5\u0026thinsp;\u0026plusmn;\u0026thinsp;25.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e106.4\u0026thinsp;\u0026plusmn;\u0026thinsp;16.5 (2.8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e108.5\u0026thinsp;\u0026plusmn;\u0026thinsp;22.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e114.1\u0026thinsp;\u0026plusmn;\u0026thinsp;20.8 (5.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.074 (0.63)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.488 (0.41)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.756 (0.25)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eKnee extension TTP (ms)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e590.5\u0026thinsp;\u0026plusmn;\u0026thinsp;105.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e440.1\u0026thinsp;\u0026plusmn;\u0026thinsp;96.7\u003c/p\u003e\n \u003cp\u003e(25.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e491.7\u0026thinsp;\u0026plusmn;\u0026thinsp;118.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e445.6\u0026thinsp;\u0026plusmn;\u0026thinsp;136.7 (9.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e536.6\u0026thinsp;\u0026plusmn;\u0026thinsp;115.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e625.8\u0026thinsp;\u0026plusmn;\u0026thinsp;138.3 (16.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.101 (0.57)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.030 (0.95)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;0.001 (1.56)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eKnee flexion TTP (ms)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e453.3\u0026thinsp;\u0026plusmn;\u0026thinsp;57.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e400.5\u0026thinsp;\u0026plusmn;\u0026thinsp;98.0\u003c/p\u003e\n \u003cp\u003e(11.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e509.2\u0026thinsp;\u0026plusmn;\u0026thinsp;105.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e399.9\u0026thinsp;\u0026plusmn;\u0026thinsp;106.2 (21.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e528.6\u0026thinsp;\u0026plusmn;\u0026thinsp;99.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e401.0\u0026thinsp;\u0026plusmn;\u0026thinsp;88.0 (24.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;0.001 (1.84)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.468 (0.42)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.228 (0.60)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eKnee extension power (W)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e147.0\u0026thinsp;\u0026plusmn;\u0026thinsp;21.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e159.0\u0026thinsp;\u0026plusmn;\u0026thinsp;22.3\u003c/p\u003e\n \u003cp\u003e(8.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e138.3\u0026thinsp;\u0026plusmn;\u0026thinsp;30.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e151.3\u0026thinsp;\u0026plusmn;\u0026thinsp;22.9 (9.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e146.6\u0026thinsp;\u0026plusmn;\u0026thinsp;16.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e146.3\u0026thinsp;\u0026plusmn;\u0026thinsp;11.0 (0.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.003 (1.10)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.583 (0.35)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.077 (0.80)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eKnee flexion power (W)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e78.4\u0026thinsp;\u0026plusmn;\u0026thinsp;10.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e81.5\u0026thinsp;\u0026plusmn;\u0026thinsp;15.5\u003c/p\u003e\n \u003cp\u003e(3.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e68.6\u0026thinsp;\u0026plusmn;\u0026thinsp;17.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e77.7\u0026thinsp;\u0026plusmn;\u0026thinsp;13.7 (13.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e79.4\u0026thinsp;\u0026plusmn;\u0026thinsp;16.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e85.6\u0026thinsp;\u0026plusmn;\u0026thinsp;16.3 (7.8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.004 (1.05)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.235 (0.59)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.465 (0.42)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"10\"\u003e\n \u003cp\u003e\u003cem\u003eBalance performance\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eOverall balance (AU)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.39\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.79\u0026thinsp;\u0026plusmn;\u0026thinsp;0.36\u003c/p\u003e\n \u003cp\u003e(31.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.35\u0026thinsp;\u0026plusmn;\u0026thinsp;0.37\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.99\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38 (26.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.32\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.95\u0026thinsp;\u0026plusmn;\u0026thinsp;0.31 (4.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.004 (1.07)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.078 (0.80)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.045 (0.89)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAP balance (AU)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.86\u0026thinsp;\u0026plusmn;\u0026thinsp;0.23\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.33\u003c/p\u003e\n \u003cp\u003e(30.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.97\u0026thinsp;\u0026plusmn;\u0026thinsp;0.27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.73\u0026thinsp;\u0026plusmn;\u0026thinsp;0.31 (24.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.24\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.68\u0026thinsp;\u0026plusmn;\u0026thinsp;0.28 (13.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.025 (0.80)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.065 (0.83)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.048 (0.88)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eML balance (AU)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.62\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.38\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e\n \u003cp\u003e(38.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.74\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.55\u0026thinsp;\u0026plusmn;\u0026thinsp;0.29 (25.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26 (20.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.026 (0.79)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.183 (0.64)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.013 (1.07)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"10\"\u003e\n \u003cp\u003eData presented as means\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation; AP: anterior-posterior; AU\u0026thinsp;=\u0026thinsp;arbitrary units; ES\u0026thinsp;=\u0026thinsp;effect size (Cohen\u0026rsquo;s \u003cem\u003ed\u003c/em\u003e); DJ\u0026thinsp;=\u0026thinsp;drop jump; J\u0026amp;R\u0026thinsp;=\u0026thinsp;jump-and-reach; ML: medial-lateral; PIT\u0026thinsp;=\u0026thinsp;peak isokinetic torque; S-PJT\u0026thinsp;=\u0026thinsp;stable plyometric-jump training without additional loads;; TTP\u0026thinsp;=\u0026thinsp;time to peak isokinetic torque; UL-PJT\u0026thinsp;=\u0026thinsp;unstable loaded plyometric-jump training; * = values in parenthesis denote relative within-group pre-post test changes.\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n \u003cp\u003e*** Include Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e about here ***\u003c/p\u003e\n \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\n \u003ch2\u003eJump performance\u003c/h2\u003e\n \u003cp\u003eThe J\u0026amp;R and DJ height showed large-sized group\u0026times;time interaction effects (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.015, 1.06\u0026thinsp;\u0026le;\u0026thinsp;ES\u0026thinsp;\u0026le;\u0026thinsp;1.76). In contrast to CON, the UL-PJT and S-PJT improved J\u0026amp;R height (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, = 1.53\u0026thinsp;\u0026le;\u0026thinsp;ES\u0026thinsp;\u0026le;\u0026thinsp;1.54; Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). Only UL-PJT improved DJ height (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.001, ES\u0026thinsp;=\u0026thinsp;1.27).\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\n \u003cp\u003e*** Include Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e about here ***\u003c/p\u003e\n \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e\n \u003ch2\u003eMuscle strength\u003c/h2\u003e\n \u003cp\u003eGroup\u0026times;time interaction effect was found for knee extension TTP (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, ES\u0026thinsp;=\u0026thinsp;1.56). Only UL-PJT improved (i.e., decreased time) knee extension TTP (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, ES\u0026thinsp;=\u0026thinsp;1.28; Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). Further, main effects of time were noted for knee flexion TTP, as well as in knee extension and flexion power output (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.004, 1.05\u0026thinsp;\u0026le;\u0026thinsp;ES\u0026thinsp;\u0026le;\u0026thinsp;1.84), irrespective of group.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\n \u003cp\u003e*** Include Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e about here ***\u003c/p\u003e\n \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e\n \u003ch2\u003eBalance performance\u003c/h2\u003e\n \u003cp\u003eGroup-time interaction effects were noted for overall, anterior-posterior, and medial-lateral stability (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.048, 0.88\u0026thinsp;\u0026le;\u0026thinsp;ES\u0026thinsp;\u0026le;\u0026thinsp;1.07). Post-hoc tests showed that UL-PJT and S-PJT (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;0.022, 0.73\u0026thinsp;\u0026le;\u0026thinsp;ES\u0026thinsp;\u0026le;\u0026thinsp;0.79) improved (i.e. reduced values) overall stability (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e). Further, S-PJT improved anterior-posterior stability index (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.035, ES\u0026thinsp;=\u0026thinsp;0.65). Moreover, only UL-PJT improved medial\u0026ndash;lateral stability (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.020, ES\u0026thinsp;=\u0026thinsp;0.78).\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\n \u003cp\u003e*** Include Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e about here ***\u003c/p\u003e\n\u003c/div\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eThe main findings of this randomized controlled trial were that i) both UL-PJT and S-PJT improved J\u0026amp;R height, ii) DJ height and knee extensor TTP were improved following UL-PJT only, and iii) UL-PJT improved the medial-lateral component (i.e., stability index), whereas the S-PJT improved the anterior-posterior component during the static balance test.\u003c/p\u003e\u003cp\u003eIn general, volleyball players perform a considerable number of jumps during regular training sessions and competitions (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e), which may reduce their responsiveness to additional jump-related training stimuli (\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e). However, the improvement in J\u0026amp;R height after UL-PJT and S-PJT confirmed the effectiveness of PJT interventions adapted to the needs of trained young volleyball players. Indeed, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e indicated that virtually all individuals improved J\u0026amp;R height after the 4-weeks intervention period. This is well in-line with previous interventions of similar duration in young and adult team sport athletes (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e). For instance, Ricart-Luna et al. (\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e) examined the effects of a 4-weeks mesocycle (3 sessions/week) of loaded PJT versus weightlifting training in highly-trained male and female basketball players aged 12 to 16 years and found that both groups similarly improved CMJ height and horizontal broad jump distance. Further, highly-trained female volleyball players (mean age: 20 years) performed 4 weeks of combined S-PJT and loaded PJT during the pre-competitive period and improved CMJ height compared with an active control group (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e). Considering that greater jump heights are commonly achieved by higher-level players (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e), improved vertical jump performance may offer a key potential advantage to developing volleyball players.\u003c/p\u003e\u003cp\u003eInterestingly and in contrast to the J\u0026amp;R test, DJ height (i.e., maximal jump height achieved after dropping off a 40-cm box) improved only following UL-PJT. Although not measured in our study, it can be speculated that the DJ test required greater ground reaction forces and leg muscle stiffness during the landing phase on the stable ground when compared with the J\u0026amp;R test, requiring considerable (eccentric) leg muscle strength (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e)(e.g., assessed as landing rate of force development). Including additional and unstable loads during PJT may have facilitated the ability to rapidly develop forces in the lower limbs. Partly in support of this notion, only UL-PJT improved TTP (i.e., decreased time to reach PIT) of the knee extensors, key muscles acting during the landing phase of DJ (\u003cspan additionalcitationids=\"CR41\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e). Future studies would be required to confirm the potential effect of UL-PJT on kinetic, kinematic, and/or neuromuscular adaptations related to jump performance.\u003c/p\u003e\u003cp\u003eNotably, while knee extension TTP improved after UL-PJT, other isokinetic measures (i.e., PIT, average power) in the knee extensors and flexors were not significantly affected by the interventions. The task specificity of jumping exercises (e.g., multiarticular, bilateral, dynamic, high-velocity) and isokinetic testing (e.g., monoarticular, unilateral, isokinetic, 60\u0026deg;/s) may have reduced the transfer effects of UL-PJT and S-PJT to muscle strength adaptations in the present study (\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e). Nonetheless, the improvement in TTP after UL-PJT is a novel finding, partially supported by previous reports (\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e). For instance, male physical education students (mean age: 23 years) improved knee extension TTP (at 180\u0026deg;/s) after 6 weeks (3\u0026ndash;4 sessions/week) of S-PJT, whole-body vibration training, and combined S-PJT and whole-body vibration training, with greater gains after the combined training program (\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e). In trained male under-17 soccer players, knee extension TTP (at 60\u0026deg;/s) was improved after 6 weeks (2 sessions/week) of PJT, particularly after loaded PJT (+\u0026thinsp;10% body mass) compared with S-PJT (\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e). Future studies need to directly determine if unstable loads may provide superior stimuli during PJT when compared with stable loads.\u003c/p\u003e\u003cp\u003eIn terms of balance performance, both UL-PJT and S-PJT improved overall balance, although only UL-PJT improved the medial\u0026ndash;lateral component, while S-PJT improved the anterior-posterior component. The superior effects of PJT on balance performance have been reported previously (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e), including PJT interventions with and without instability conditions (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e). Jump training exercises may increase the activation of stabilizing muscles, particularly in the trunk and lower limbs, leading to improved postural control (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e), and the present findings suggest specific adaptations according to jumping conditions (i.e., UL-PJT improved medial\u0026ndash;lateral balance; S-PJT improved anterior-posterior balance). Moreover, jump exercises may exert a favorable \u003cem\u003eoverload\u003c/em\u003e on proprioceptive feedback and sensorimotor integration, allowing for more precise control of body position. Further, jump drills may reinforce vestibular and visual control strategies, which are crucial for maintaining stability under dynamic conditions. Therefore, both UL-PJT and S-PJT seems similarly effective to enhance overall balance performance, although future studies may be required to i) determine if their combination may potentiate both medial\u0026ndash;lateral and anterior-posterior stability, ii) determine the physiological-biomechanical mechanisms underlying the specific balance improvements after each mode of jump training.\u003c/p\u003e\u003cdiv id=\"Sec22\" class=\"Section2\"\u003e\u003ch2\u003ePotential limitations\u003c/h2\u003e\u003cp\u003eAccording to the training integration principle, multicomponent interventions should be emphasized to optimize athletic performance (e.g. jump, strength, balance), reduce risk of injuries, and for holistic development of athletes, particularly youth athletes (\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e). Indeed, the UL-PJT was ecologically combined with volleyball players\u0026rsquo; multidimensional training routine. However, UL-PJT also combined both load and instability, whereas the effects of each component (load or instability) were not examined independently. Therefore, future studies would be needed to determine whether the benefits observed in the current study were due to the combined effect or whether instability itself contributed additional effects compared to loaded training in stable surface.\u003c/p\u003e\u003c/div\u003e"},{"header":"CONCLUSIONS","content":"\u003cp\u003eVolleyball players improved maximal vertical jump height (J\u0026amp;R test) and overall balance after both PJT modes. However, only UL-PJT improved DJ height which may be attributed to group-specific gains in knee extension TTP. Moreover, both training programs induced specific anterior-posterior balance improvement (S-PJT) and medial-lateral balance improvement (UL-PJT). Coaches and practitioners could use UL-PJT over S-PJT if the goal is to improve DJ and/or isokinetic knee extensor strength measures in trained young (i.e., post-PHV) volleyball players. Future studies would be required to confirm if such adaptations can be transferred to competitive actions (e.g., spiking, blocking).\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eANOVA = analysis of variance\u003c/p\u003e\n\u003cp\u003eCMJ = countermovement jump\u003c/p\u003e\n\u003cp\u003eCON = control group\u003c/p\u003e\n\u003cp\u003eDJ =\u0026nbsp;drop jump\u003c/p\u003e\n\u003cp\u003eES = effect size\u003c/p\u003e\n\u003cp\u003eJ\u0026amp;R = jump-and-reach\u003c/p\u003e\n\u003cp\u003ePHV = peak-height velocity\u003c/p\u003e\n\u003cp\u003ePIT = peak isokinetic torque\u003c/p\u003e\n\u003cp\u003ePJT = plyometric-jump training\u003c/p\u003e\n\u003cp\u003eS-PJT = stable plyometric-jump training without additional loads\u003c/p\u003e\n\u003cp\u003eSSC = stretch-shortening cycle\u003c/p\u003e\n\u003cp\u003eTTP =\u0026nbsp;time to peak isokinetic torque\u003c/p\u003e\n\u003cp\u003eUL-PJT = unstable loaded plyometric-jump training.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study was conducted in accordance with the latest version of the declaration of Helsinki. It was approved by the Research Ethics Committee of Shahid Beheshti University (IR.SBU.REC.1404.006). Prior to the commencement of the study, written informed consent was obtained from all players involved and/or their parents/legal guardians.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors read and approved the final manuscript and gave their consent for publication.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePS, RRC, MF, KE, JLM, and OP declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo source of funding was used during the study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePS conceived the study design with guidance from MF and KE, who supervised the project and contributed to the conceptualization. PS collected the data and performed analyses together with OP. PS and OP prepared the first draft of the manuscript. RRC contributed to data interpretation and critically revised the manuscript together with JLM. All authors read and approved the final version of the article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors thank all the athletes who volunteered to participate in the study and those who collaborated with data collection.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eSheppard JM, Gabbett TJ, Stanganelli L-CR. An analysis of playing positions in elite men\u0026apos;s volleyball: considerations for competition demands and physiologic characteristics. The Journal of Strength \u0026amp; Conditioning Research. 2009;23(6):1858-66.\u003c/li\u003e\n\u003cli\u003eMartinez DB. Consideration for power and capacity in volleyball vertical jump performance. 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Effects of surface instability on neuromuscular performance during drop jumps and landings. European Journal of Applied Physiology. 2013;113(12):2943-51.\u003c/li\u003e\n\u003cli\u003eDitroilo M, O\u0026rsquo;Sullivan R, Harnan B, Crossey A, Gillmor B, Dardis W, Grainger A. Water-filled training tubes increase core muscle activation and somatosensory control of balance during squat. Journal of sports sciences. 2018;36(17):2002-8.\u003c/li\u003e\n\u003cli\u003eMason BR, Lea R, McKune AJ, Pyne DB, Ball NB. Comparison of stable load, unstable load and variable resistance training interventions. International Journal of Sports Science \u0026amp; Coaching. 2025:17479541251333955.\u003c/li\u003e\n\u003cli\u003eMoore SA, McKay HA, Macdonald H, Nettlefold L, Baxter-Jones A, Cameron N, Brasher P. Enhancing a somatic maturity prediction model. Med Sci Sports Exerc. 2015;47(8):1755-64.\u003c/li\u003e\n\u003cli\u003eRumpf MC, Cronin JB, Mohamad IN, Mohamad S, Oliver J, Hughes M. 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The Journal of Strength \u0026amp; Conditioning Research. 2020;34(12):3395-402.\u003c/li\u003e\n\u003cli\u003eTorres-Banduc M, Ramirez-Campillo R, Andrade DC, Calleja-Gonz\u0026aacute;lez J, Nikolaidis PT, McMahon JJ, Comfort P. Kinematic and neuromuscular measures of intensity during drop jumps in female volleyball players. Frontiers in Psychology. 2021;12:724070.\u003c/li\u003e\n\u003cli\u003eSaeterbakken AH, Stien N, Paulsen G, Behm DG, Andersen V, Solstad TEJ, Prieske O. Task specificity of dynamic resistance training and its transferability to non-trained isometric muscle strength: a systematic review with meta-analysis. Sports Medicine. 2025:1-26.\u003c/li\u003e\n\u003cli\u003eRostamkhany H, Nikbakht H, Sadeqi H. Plyometric training effect on lower limb biomechanical parameters. Sleep and Hypnosis. 2018;20(3):166-73.\u003c/li\u003e\n\u003cli\u003eNiknam A, Koushkie Jahromi M, Hemmatinafar M, Dehghani AR, Oviedo GR. Plyometric training with additional load improves jumping performance and isokinetic strength parameters of knee extensors and flexors in young male soccer players. Journal of Sports Sciences. 2024;42(21):1986-2004.\u003c/li\u003e\n\u003cli\u003eRamachandran AK, Singh U, Ramirez-Campillo R, Clemente FM, Afonso J, Granacher U. Effects of plyometric jump training on balance performance in healthy participants: a systematic review with meta-analysis. Frontiers in physiology. 2021;12:730945.\u003c/li\u003e\n\u003cli\u003eMyer GD, Faigenbaum AD, Chu DA, Falkel J, Ford KR, Best TM, Hewett TE. Integrative training for children and adolescents: techniques and practices for reducing sports-related injuries and enhancing athletic performance. The Physician and sportsmedicine. 2011;39(1):74-84.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bmc-sports-science-medicine-and-rehabilitation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ssmr","sideBox":"Learn more about [BMC Sports Science, Medicine and Rehabilitation](http://bmcsportsscimedrehabil.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/ssmr/default.aspx","title":"BMC Sports Science, Medicine and Rehabilitation","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Instability, jump, isokinetic strength, power, balance, center of pressure, stretch-shortening cycle","lastPublishedDoi":"10.21203/rs.3.rs-7474863/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7474863/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground: \u003c/strong\u003ePlyometric-jump training has been recommended as an effective resistance training modality for enhancing vertical jump and lower limb muscle power in young athletes. While unstable surfaces are popular means to address progression and specificity, an alternative approach for applying instability during jumping is the inclusion of unstable loads. The purpose of this study was to compare the effects of unstable load plyometric-jump training (UL-PJT) to plyometric-jump training without unstable load (S-PJT) on physical fitness (i.e. jump performance, muscle strength, balance) of young volleyball players.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMethods: \u003c/strong\u003eThirty-seven trained male volleyball players aged 16.8±1.1 years were randomly assigned to UL-PJT, S-PJT, and active controls (CON). For 4 weeks (3 sessions/week), UL-PJT group performed jump exercises with external unstable loads (~10% body weight), while S-PJT comprised jump exercises without additional loads. Testing included assessment of jump-and-reach (J\u0026amp;R) height, drop jump (DJ) height, maximal isokinetic knee flexion/extension strength (e.g., peak isokinetic torque [PIT], time to PIT [TTP]), static balance (i.e., overall, anterior-posterior, and medial-lateral stability index).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults:\u003c/strong\u003e Large group × time interaction effects (all p≤0.048; 0.88≤effect size (ES)≤1.76) were noted for J\u0026amp;R/DJ height, knee extension TTP, and static balance. Post-hoc analyses revealed that overall stability index and J\u0026amp;R height improved after both UL-PJT and S-PJT. Moreover, both training programs induced specific improvements in anterior-posterior (S-PJT) and medial-lateral stability indices (UL-PJT). However, only UL-PJT improved knee extensor TTP and DJ height.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusions: \u003c/strong\u003eThe present study demonstrated that 4 weeks of UL-PJT and S-PJT were effective for enhancing J\u0026amp;R and balance performance (i.e., overall stability index) in trained young volleyball players. However, it can be recommended that coaches and practitioners use UL-PJT over S-PJT if the goal is to improve DJ and/or isokinetic knee extensor strength measures.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTrial registration: \u003c/strong\u003eIranian Registry of Clinical Trials IRCT20250905067129N1 (2025-09-15, retrospectively registered).\u003c/p\u003e","manuscriptTitle":"Plyometric-jump training with versus without unstable load to improve physical fitness in trained young volleyball players: a randomized controlled trial","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-06 14:45:32","doi":"10.21203/rs.3.rs-7474863/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-16T12:11:07+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-30T19:26:53+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-28T13:59:44+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-27T22:13:24+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"287573173363990888565269435849364272882","date":"2025-11-27T03:52:41+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-22T09:13:20+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"108278141364453167800083066628263384844","date":"2025-11-20T08:25:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"175075882907538798499031305535740358196","date":"2025-11-19T17:12:07+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"256423239105944085236913744130946043805","date":"2025-11-19T13:04:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"262009181784111186608108247514799886116","date":"2025-11-19T11:31:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"100494158807932488882567486039986308470","date":"2025-11-13T06:38:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"273760545173877915869650475346561303997","date":"2025-11-10T10:55:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"273867863438719892318319778470580728969","date":"2025-09-26T09:36:01+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-24T07:43:46+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-09-21T06:30:17+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-09-19T13:07:32+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-09-15T21:32:08+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Sports Science, Medicine and Rehabilitation","date":"2025-09-15T21:28:48+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bmc-sports-science-medicine-and-rehabilitation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ssmr","sideBox":"Learn more about [BMC Sports Science, Medicine and Rehabilitation](http://bmcsportsscimedrehabil.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/ssmr/default.aspx","title":"BMC Sports Science, Medicine and Rehabilitation","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"9e6cebd0-3bac-43ae-b1ea-d02f40dc56dc","owner":[],"postedDate":"October 6th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-02-18T12:53:40+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-06 14:45:32","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7474863","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7474863","identity":"rs-7474863","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}