Population responses of medial gastrocnemius motor units to electrical vestibular stimulation

Electrical vestibular stimulation (EVS) can be used to manipulate the vestibular input to the medial gastrocnemius muscle (MG). The aims of this study were: 1) to identify the lowest noisy EVS amplitude that results in modulation of MG activity; 2) to identify whether a motor unit response to EVS is influenced by its baseline firing characteristics. Eight participants stood quietly with their head rotated to the left while receiving EVS at different intensities (0-5mA). MG activation was measured using high-density surface electromyography. Coherence between EVS and populations of MG motor units was observed when stimulation intensity was ≥ 0.5mA, indicating that modulation of MG firing rate can be achieved with low-amplitude noisy EVS. Motor units with lower firing rate and higher coefficient of variation during quiet standing showed larger responses to EVS, suggesting that these motor units may contribute more than others to MG responses to vestibular perturbations.