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

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).