Muscle synergies underlying control of taking a step during support surface translation

We investigated the muscle activation patterns and the center of pressure (COP) displacement in stepping behavior to determine the relations between anticipatory synergy adjustments (ASAs) and anticipatory postural adjustments (APAs) during support surface translation.

Surface muscle activity of eleven leg and trunk muscles was analyzed to identify sets of four muscle modes (M-modes). Linear combination of M-modes and their relationship to changes in the COP shift in the anterior–posterior (AP) direction were then determined. Uncontrolled manifold (UCM) analysis was performed to determine variance components in the M-mode space and indices of M-mode synergy stabilizing the COP shift.

Prior to the step initiation, synergies stabilizing COP were seen in both conditions. The synergy index started to drop before a change in the averaged activation levels across trials in postural muscles. The magnitude of synergy index was significantly larger under the perturbation condition.

Results of the study have revealed that the central nervous system is able to prepare multi-muscle synergies when a step is performed during support surface translation. Prior to APAs, ASAs reduce stability of COPAP coordinate that is to be adjusted during the APAs. These findings may help get closer to understanding of physiological mechanism of postural preparation to external perturbation.