Field-to-laboratory biomechanical profiling of reactive strength index in male volleyball drop jumps.

BACKGROUND: Reactive strength index (RSI) is widely used in drop-jump testing, but the score alone does not show how the landing-to-take-off phase is organized. OBJECTIVE: This study examined whether RSI in male volleyball drop jumps can be interpreted more clearly by combining field-accessible performance metrics with time-continuous knee-joint biomechanical waveforms. METHODS: Forty-four male volleyball participants were classified as trained athletes (n = 22) or recreational participants (n = 22). Each completed 30-cm drop jumps using force platforms and three-dimensional motion capture. Field outcomes included jump height, contact time, RSI, and countermovement depth. Laboratory outcomes included dominant-limb knee angle, moment, and power waveforms, with supplementary lower-limb peak joint and work-distribution outcomes. Discrete outcomes were analyzed with mixed models and covariate-adjusted models; waveforms were compared in Python using one-dimensional statistical parametric mapping, with supplementary body mass-normalized kinetic waveform analyses. RESULTS: Compared with recreational participants, trained athletes had shorter contact time (adjusted β = -0.093 s, 95% CI: -0.157 to -0.030), smaller countermovement depth (adjusted β = -7.03 cm, 95% CI: -11.98 to -2.08), and higher RSI (adjusted β = 0.273 m/s, 95% CI: 0.131-0.415). Knee propulsive work was higher before anthropometric adjustment, whereas the adjusted result was statistically inconclusive. Waveform analysis showed less knee flexion across most of contact, greater knee moment during middle-to-late contact, and phase-specific knee-power differences. CONCLUSION: Higher RSI in trained male volleyball athletes was associated with a compact field-level landing-rebound pattern, defined by shorter contact time and smaller countermovement depth, supported by lower-limb discrete outcomes and knee-joint waveform features rather than a single biomechanical feature. Combining field-accessible metrics with lower-limb biomechanical analysis may provide a more interpretable profile of reactive strength in volleyball drop jumps.