Muscle synergies involved in shifts of the center of pressure while standing on a narrow support.

We investigated multi-muscle synergies during preparation to push a load forward and their changes with different support conditions. We hypothesized that the subjects show unchanged mode structure and would be able to form multi-mode COP stabilizing synergies while standing on an unstable board. Eight healthy subjects participated in the study. Standing subjects performed load-pushing tasks under two conditions, "normal support" and "ML narrow support". Electromyographic (EMG) signals of 12 postural muscles were recorded and analyzed. The participants also performed standard tasks associated with releasing a load. These trials were used to identify muscle groupings (M-modes) associated with shifts of the center of pressure (COP) and relations between small changes in the M-modes and COP shifts in different support conditions. The subjects showed unchanged mode structure across different support conditions. The framework of the uncontrolled manifold hypothesis was used to partition the EMG variance across load-pushing trials into two components that kept constant and changed the COP coordinates in the anterior-posterior (AP) direction. This analysis has allowed us associate changes in the contribution of muscles with COP shifts under different support conditions. Different time profiles of the synergies were observed related to the COP shifts across conditions. This outcome supports a view that indices of multi-muscle (multi-M-mode) synergies can show anticipatory changes in preparation for a predictable perturbation.