ADAPTIVE NEUROMUSCULAR CONTROL STRATEGIES OF THE SOLEUS AND MEDIAL GASTROCNEMIUS DURING UNSTABLE STANDING

Humans adapt standing postural control strategies in response to unstable surfaces. This adaptation is primarily mediated by the medial gastrocnemius (MG) and soleus (SOL), the principal muscles involved in postural control; however, the neural mechanisms underlying their adaptive control remain poorly understood. This study aimed to elucidate changes in the neural control of the SOL and MG during unstable standing. Twenty males stood on stable and unstable surface. Electroencephalogram from the sensorimotor cortex and high-density surface electromyogram (HDsEMG) from the bilateral MG and SOL were recorded, and the HDsEMG was decomposed into motor unit activity using a blind source separation algorithm. Using motor unit spike trains, intramuscular, intermuscular, and corticomuscular coherence (CMC) were calculated for each muscle. Intermuscular coherence was calculated between the left and right MG/SOL. In the alpha band, reflecting Ia afferent and vestibulospinal inputs, intramuscular coherence across all muscles and SOL–SOL intermuscular coherence were significantly higher during unstable than stable surface (p < 0.05). In the beta band, which reflects corticospinal and reticulospinal inputs, MG intramuscular coherence was significantly higher during unstable compared with stable surface (p < 0.05). In addition, alpha- and beta-band CMC, reflecting functional coupling between cortical and muscle, was significantly higher across all muscles during unstable than stable surface (p < 0.05). Taken together, these findings suggest that during postural control on unstable surfaces, neural inputs from Ia afferents, vestibulospinal pathways, and corticospinal pathways are enhanced in the SOL, whereas the MG receives these inputs and a likely increase in reticulospinal drive.