Multi-muscle coordination during a challenging stance

Purpose: This study aimed to identify the muscle synergies during standing under various sensory contexts in healthy young adults. Methods: Sixteen healthy young adults participated in this study. The 4-min stance task was conducted under vision (eyes open or eyes closed) and proprioception (standing on ground or narrowed blocks) manipulated contexts. Electromyography (EMG) of 8 muscles around the right side of the trunk and leg were recorded and submitted to principal component analysis (PCA) to extract the muscle synergies. Two-way ANOVA with repeated measures was employed to test the effect of sensory contexts on the muscle synergies. Results: PCA extracted three muscle synergies that accounted for the variance of standing EMG, including the push-back (composed of medial gastrocnemius, vastus medialis and biceps femoris), push-forward (composed of tibialis anterior and rectus femoris) and proximal mixed (composed of rectus abdominis, rector spinae, rectus femoris and biceps femoris) synergies. Block-standing increased the contribution of the push-back synergy while decreased the contribution of the push-forward synergy. In addition, contribution of the proximal mixed synergy was higher under ground-standing with eyes open than under block-standing with eyes open. Conclusion: Three muscle synergies were identified during standing in healthy young adults, and the synergies were affected by proprioception but not visual disturbance. The push-back and push-forward synergies showed the opposite response to proprioceptive disturbance, which may result from their antagonism role. Whether this control regime is used for elderly adults or patient populations with movement disorder needs to be further investigated.