Mechanical demand shapes anticipatory kinematic adjustments differently in drop and jump landings: Evidence from a...

Landing from a jump presents distinct challenges that require accurate prediction of both ground contact timing and the characteristics of the forthcoming impact forces to generate anticipatory kinematic adjustments to efficiently dissipate energy. This study quantified these adjustments under simulated microgravity to characterize their temporal and magnitude features. Nine participants performed countermovement jumps (CMJ) and drop-landings (DL) under simulated microgravity during parabolic flights, using a subject loading system to apply downward force in weightlessness (0g). Ground reaction forces and sagittal-plane kinematics were recorded to compute hip, knee and ankle joint angles, angular velocities, and joint flexion onsets. Loading rate, peak vertical ground reactions forces and extra work were computed to quantify the effect of anticipatory adjustments. A linear mixed-effects model examined the effect of energy to be dissipated at touchdown (E_TD), task (CMJ or DL) and their interaction on onsets of joint flexion, joint angles and angular velocities at touchdown (TD). Joint flexion onsets occurred later relative to TD with increased E_TD, to reach smaller joint flexion angles at TD. When expressed relative to the instant of downward fall initiation, latency of joint flexion also varied with E_TD, indicating that adjustments do not rely on fixed timing reference. Joint angular velocities at touchdown increased with E_TD in CMJ, suggesting a combined modulation of onset timing and angular acceleration to achieve the required velocity at TD. During DL, angular velocities at TD were lower, leading to reduced extra work and greater loading rates. Overall, these findings highlighted the critical role of anticipatory kinematic adjustments and the importance of fall initiation in energy dissipation during landing.