Timing of human rhythmic jumping with vertical floor vibrations tends towards mechanically efficient solutions

Jumping on vibrating platforms is described not only by the frequency of jumping (FoJ) but also by the timing of key events in a cycle of jumping relative to vibrations. This study aimed to capture timing and efficiency-related adaptations during jumping on vertically vibrating platforms. Whole body kinematic and kinetic data were collected as ten participants jumped on a sinusoidally vibrating platform of 2.0, 2.4 and 2.8 Hz at 2 m/s². FoJ matched platform frequency, and audio cues were provided to time the jump landing at four positions relative to the platform position: reference position on its way down, lowest position, reference position on its way up, and highest position. FoJ, jump timing, impact factor, contact ratio, mechanical work and leg stiffness were calculated for each jump cycle. Results confirmed that the impact factor, contact ratio, mechanical work, and leg stiffness are timing-dependent. Results also showed that despite being cued, participants adjusted their timing to take off from a higher platform position during its downward motion and land at a lower position during the upward motion while maintaining FoJ. Importantly, participants tended towards efficiency by employing jump timings related to lower energy input, appropriate contact ratio and lower forces. This study provides evidence of jump timing behaviour relative to platform motion being dominated by the efficiency of jumping. Practically, it may be crucial to consider this aspect when estimating human-induced loads on lively assembly structures.