Stabilization demands of walking modulate the vestibular contributions to gait

Stable walking relies critically on signals of head motion provided by the vestibular system, which are modulated within each muscle and throughout the gait cycle. It is unclear, however, whether this vestibular contribution also varies according to the stability of the walking task. Here we investigate how vestibular signals influence muscles relevant for gait stability (medial gastrocnemius, gluteus medius and erector spinae) - as well as their net contribution to ground reaction forces - while humans walked normally, with mediolateral stabilization, wide and narrow steps. We estimated the coherence of an electrical vestibular stimulus (EVS) with muscle activity and mediolateral ground reaction forces, together with the local dynamic stability of trunk kinematics. Walking with external stabilization increased local dynamic stability and decreased coherence between EVS and all muscles/forces compared to normal walking. Wide-base walking also decreased vestibulo-motor coherence, though gait stability did not differ. Conversely, narrow-base walking increased local dynamic stability, but produced muscle-specific increases and decreases in coherence that resulted in a net increase in vestibulo-motor coherence with ground reaction forces. Overall, our results show that while vestibular contributions may vary with gait stability, they more critically depend on the stabilization demands (i.e. control effort) needed to maintain stable walking patterns.