The on-off ground asymmetry during running on sand

During running, the mechanical energy of the centre of mass of the body (COM) fluctuates throughout the step like a spring-mass system bouncing vertically on the ground. This system oscillates around an equilibrium point at which the vertical acceleration of the COM (av) is nil (Cavagna, 2009). The step period can be divided into two parts (Figure 1): the first part (tce) during which av > 0 (lower part of the oscillation), and the second part (tae) during which av≤ 0 (upper part of the oscillation). At all speeds, tce corresponds to one-half period of the oscillation of the bouncing system, whereas tae corresponds to the second half only if the aerial phase is nil. Indeed, if the body leaves the ground, the elastic model is not anymore valid since av = −1 g. Figure 1. Typical traces of one subject. Top: Vertical acceleration (av), velocity (Vv) and displacement (Sv) of the COM as a function of time, while running in the sand at 2.5 m s−1. Bottom: av as a function of Sv