Biomechanical and neuromuscular adaptations in joint contributions during loaded countermovement jumps.

This study investigates the relative contributions of the hip, knee, and ankle joints during lower limb extension exercises under varying supramaximal loads in elite volleyball players, utilizing advanced kinematic and dynamic metrics. Thirty-three elite male volleyball athletes from the Chinese National Team participated in this controlled experimental study. The study employed a repeated measures ANOVA to assess differences across loading conditions, with effect sizes calculated using partial eta-squared (η²). Significant shifts in joint contributions were observed as external loads increased, with the knee joint being the primary contributor at lower loads and the hip joint assuming dominance as the load increased. Specifically, the Net Joint Moments (NJMs) at the knee decreased significantly (95%CI, p < 0.001), while those at the hip increased correspondingly (95%CI, p < 0.001), highlighting the biomechanical demands of higher loads. Despite these shifts, the maximum flexion angles remained stable across all conditions. The findings suggest that unweighted countermovement jumps (CMJs) optimize knee muscle strength, while low-load CMJs are more effective for enhancing hip muscle strength and explosive power. These insights are critical for developing training programs that improve performance and reduce injury risk in high-intensity sports.