Coordination of the upper and lower limbs for vestibular control of balance

Vestibular stimulation can evoke responses in the arm when it is used for balance. Here we determine how these responses are affected by grip context, and how they are coordinated with the rest of the body. Galvanic vestibular stimulation (GVS) was used to evoke balance responses under three conditions of manual contact with an earth-fixed object: no contact (NC), light grip (< 1N) (LG), and firm grip (FG). As grip progressed along this continuum, we observed an increase in GVS-evoked hand force, with a simultaneous reduction in ground reaction force (GRF) through the feet. During LG, hand force was secondary to the GVS-evoked body sway response, indicating that the arm performed a mostly passive role. In contrast, during FG the arm became actively involved in driving body sway, as revealed by an early force impulse in the opposite direction to that seen in LG. We then examined how the direction of this active hand vector was coordinated with the lower limbs. Consistent with previous findings on sway anisotropy, FG skewed the direction of the GVS-evoked GRF vector towards the axis of baseline postural instability. However, this was effectively cancelled by the hand force vector, such that the whole-body sway response remained aligned with the inter-aural axis, maintaining the craniocentric principle. These results show that a minimum level of grip is necessary before the upper limb plays an active role in vestibular-evoked balance responses. Furthermore, they demonstrate that upper and lower-limb forces are coordinated to produce an appropriate whole-body sway response.