Muscle synergy stability and human balance maintenance (Journal of NeuroEngineering and Rehabilitation)

Background: The signals that the central nervous system (CNS) produces and sends to the muscles to effect movementare not entirely understood. Muscle synergy theory suggests that the central nervous systemproduces a small number of signals that pass through a network that distributes combinations of thesesignals to the muscles. Though these synergies are rather stable over time, some variability is present. Methods: Here, we investigated the variability of muscle synergy and defined a synergy stability index (SSI) toquantify it. We measured the activity of muscles responsible for maintaining lateral balance in humansstanding on a platform that was subjected to lateral disturbance from the platform. We then calculatedmuscle synergies attributed to postural reflex and automatic response by using non-negative matrixfactorization (NMF). Finally, from the calculated muscle synergies, we obtained SSI. Results: We observed a positive proportional relation between balance performance and SSI. Participants whowere adept at maintaining balance were found to have invariant muscle synergies, and non-adeptparticipants showed variable muscle synergies. Conclusions: These results suggest that SSI can be used to quantitatively evaluate balance maintenance ability.