Upright postural control is essential for performing tasks of daily living. Widening stance width increases postural stability and decreases the amplitude of vestibular-evoked whole-body balance responses. The vestibular-evoked whole-body balance response is the summation of all muscle activity contributing to the postural task. Recent reports indicate the importance of the abductor hallucis (AH) and digiti minimi (ADM) during standing balance – similar to other postural muscles – but it is unclear how vestibulomyogenic responses are modulated when mediolateral stability is manipulated. The purpose was to investigate the effects of increased mediolateral stability on vestibular-evoked balance responses in muscles with a primary force generating function in the mediolateral (i.e., toe abductors) and anteroposterior (i.e., plantar flexors) direction. Continuous, random electrical vestibular stimulation (EVS; peak-to-peak amplitude: ±4mA; frequency bandwidth: 0-25Hz) was delivered via carbon-rubber electrodes placed over the mastoid processes. Indwelling electromyography (EMG) of the AH and ADM and surface EMG of the medial gastrocnemius and soleus were sampled. Participants were exposed to four 90-s trials of EVS with vision occluded, head facing forward, and feet together (FT; ~2cm) or apart (FA; ~26cm). Vestibular-evoked balance responses were evaluated using a cross-correlation like analysis (i.e., cumulant density) to determine the relationship between EVS and EMG. The vestibular-evoked balance response decreased by 60-90% from FT to FA in all muscles (p<0.05), with the toe abductors displaying the largest reductions. These findings indicate that the vestibulomyogenic response in muscles preferentially acting in the mediolateral and anteroposterior direction are reduced by increased mediolateral stability.
Funded by NSERC.