Contribution of vestibular afference to estimation of centre of mass state for stabilization of walking differs between anteroposterior and mediolateral directions

Walking without falling requires correcting the deviations of the centre of mass (CoM) trajectory relative to the base of support. This process is partially under feedback control. We investigated whether vestibular afference contributes to estimating CoM state to stabilize walking. We disturbed the vestibular afference by electrical vestibular stimulation (EVS), which typically leads to responses coherent with the stimulus and directed opposite to movement direction encoded by the induced vestibular signal. With the head facing forward, EVS induces mediolateral perturbations, which shift into the anteroposterior direction when the head is turned sideward in standing. Thirteen participants walked on a treadmill for 8 min at 78 steps/min and 2.8 km/h in four conditions, defined by the presence of EVS and head orientations (facing forward or leftward). A linear regression between the CoM state, described as the extrapolated CoM, and the delayed ground reaction force (GRF) was fitted, which was used to identify a 'feedback model'. Negative correlations were found in all conditions, indicating that delayed compensatory actions are modulated to correct the deviation of CoM state. EVS significantly increased the magnitude of the GRF not predicted by the XCoM, i.e. the residual error of the feedback model. Contrary to our hypothesis, the increase in the anteroposterior direction was significantly smaller when walking with head facing leftward than when facing forward. Our findings indicate that vestibular afference contributes to estimating CoM state for walking stabilization in the mediolateral direction, but they do not support such a contribution for the anteroposterior direction.