Characterizing the vestibular control of balance in the intrinsic foot muscles.

Background: To maintain standing balance, vestibular cues are processed and integrated with other sensorimotor signals to produce appropriate motor adjustments. Whole-body vestibular-driven postural responses are context-dependent and transformed based upon head and foot posture. Previous reports indicate the importance of intrinsic foot muscles during standing, but it is unclear how vestibular-driven responses of these muscles are modulated by alterations in stability and head posture.

Research Question: The purpose was to investigate the effect of altered mediolateral stability on the modulation of intrinsic foot muscle postural adjustments when driven by vestibular perturbations in anteroposterior and mediolateral directions.

Methods: For experiment 1 (n = 17) and 2 (n = 12), time-domain, vestibular-evoked balance responses to continuous, stochastic electrical vestibular stimulation (EVS) during various foot (narrow, wide, and unipedal) and head yaw postures were assessed for the abductor hallucis (AH), abductor digiti minimi (ADM), medial gastrocnemius (MG), and soleus, as well as ground reaction forces.

Results: With increased mediolateral stance width, AH, ADM, MG, soleus, and whole-body vestibular-evoked balance responses decreased. The AH vestibular-evoked balance response increased for unipedal compared to narrow bipedal stance. The AH vestibular-evoked balance response exhibited an opposite polarity than the ADM when the head was positioned anatomically, indicating that these intrinsic foot muscles function antagonistically towards the summation of whole-body postural adjustments.

Significance: Our findings demonstrate that whole-body vestibular-evoked balance responses were adjusted in response to altered mediolateral stability and head posture, in part, via modification of intrinsic foot muscle activity.