The effect of external lateral stabilization on the control of mediolateral stability in walking and running

It is still unclear how humans control mediolateral (ML) stability in walking and even more so for running. Here, foot placement adjustment as a main mechanism of active control of mediolateral stability was compared between walking and running. Moreover, to verify the role of foot placement as a means of active control of ML stability and associated metabolic costs in both modes of locomotion, this study investigated the effect of external lateral stabilization on foot placement control. Ten young adults participated in this study. Kinematic data of the trunk (T₆) and feet (heels) as well as breath-by-breath oxygen consumption data were recorded during walking and running on a treadmill in normal and stabilized conditions. Coordination between ML trunk Center of Mass (CoM) state and subsequent ML foot placement, step width, and step width variability were assessed. Two-way repeated measures ANOVAs (either normal or SPM1d) were used to test for effects of walking vs. running and of normal vs. stabilized locomotion. We found a stronger association between ML trunk CoM state and foot placement in walking than in running from 90-100% of the gait cycle and also a higher step width variability in walking, but no significant differences in step width. The association between trunk CoM state and foot placement was significantly decreased by external lateral stabilization in walking and running, and this reduction was stronger in walking than in running from 75-100% of gait cycle. Surprisingly, energy cost significantly increased by external lateral stabilization, which was more pronounced in running than walking. We conclude that ML foot placement is coordinated to the CoM kinematic state to stabilize both walking and running. This coordination is more tight in walking than in running and appears not to contribute substantially to the energy costs of either mode of locomotion.