Muscle synergies in preparation to a step made with obstacle in elderly individuals

BACKGROUND: To evaluate if multi-muscle synergies are comprised of flexible combinations of a small number of postural muscles to stabilize the center of pressure (COP) shift during preparation to making a step in the elderly (self-paced level stepping vs. obstacle crossing stepping). METHODS: Electromyography (EMG) signals of leg and trunk muscles were recorded. Linear combination of integrated indices of muscle activity (M-modes) and their relationship to changes in the COP shift in the anterior-posterior (AP) direction were first determined. Uncontrolled manifold (UCM) analysis was performed to determine the extent to which variance of the M-modes acted to produce a consistent change in the COP displacement. RESULTS: The elderly were capable of stabilizing the COP(AP) coordinate based on co-varied involvement of the M-modes. The synergy index (∆V) changes in the elderly emerged later (100 ms prior to t(0)) and its magnitude was smaller as compared to that reported in younger persons. CONCLUSIONS: Our study reveals that aging is associated with a preserved ability to explore the flexibility of the M-mode compositions but a decrease ability to use multi-M-mode synergies following a predictable perturbation.