Predictive control of body mass trajectory in a two-step sequence

During a step, the body enters a state of falling as a foot is lifted from the ground. This has important implications for the control of body trajectory when stepping in different directions. A model has been proposed in which the central nervous system controls body trajectory by a predictive, ballistic “throw” of the body occurring just before the stepping foot leaves the ground. Here we investigate this model and ask how far into the future the body trajectory is predetermined by this initial ballistic throw. We measured body centre-of-mass (CoM) and foot trajectories during two-step sequences involving stepping onto illuminated targets, one for each foot. The targets were varied spatially such that leading foot placement could be dissociated from final CoM position. The results showed that the body throw was altered when stepping in different directions. However, the throw varied only with leading foot placement and not with final CoM position. Thus, provided the leading foot was placed identically, the same throw was used for steps in which body trajectories would later diverge. Furthermore, these trajectories did not diverge while the stepping foot was in the air but occurred after it touched down. The results are consistent with the ballistic stepping model. However, they suggest that the throw is limited in its predictive capacity, being concerned only with where the stepping foot is to be placed rather than final CoM position. This constitutes a one-step-at-a-time strategy that allows the body to be safely “caught” between steps.