Skipping as the gait of choice in hypo-gravity : metabolic and biomechanical insights from level and gradient experiments on Earth

Studies from Cavagna and Margaria, and more recently from our research group, demonstrated the dynamical limits of walking and running when moving in low gravity. We also studied \u2018skipping\u2019 (i.e. the placement of two successive feet on the ground followed by the flight), a gait displayed by kids, rarely by adults (but frequently by lemurs), which has the biomechanical potential to be used as the gait of choice in those conditions, as witnessed by astronauts of Apollo missions who spontaneously adopted it to move faster on the Lunar surface. In preparation to simulated and \u2018real\u2019 hypo-gravity experiments on this gait, we present here the extension of previous knowledge about skipping on Earth, focusing on the gradients most commonly found on the Moon. We used metabography to assess metabolic cost of transport and 3D motion analysis to elucidate mechanical aspects of that locomotion type. The results confirm at all gradients the higher (average) ground reaction force during the contact phase, with respect to running at the same speed, which would allow to confidently face the Lunar and Martian surface where the dust and regoliths affect, in addition to a lower gravity, the locomotion dynamics. Skipping is confirmed to imply a higher metabolic cost than \u2018normal\u2019 locomotion for humans, a problem that would be mitigated by the lower body weight in hypo-gravity conditions