Muscle synergies in preparation to a step made with and without obstacle

To study multi-muscle synergies during preparation in making a step (self-paced level stepping vs. obstacle crossing stepping). The uncontrolled manifold hypothesis was used to explore the organization of leg and trunk muscles into groups (M-modes) and co-variation of M-mode involvement (M-mode synergies) during stepping tasks. Subjects performed two tasks: (1) making a comfortable step from quiet stance (STCS), (2) stepping over an obstacle of 15 % body height from quiet stance, STOS. Electromyographic (EMG) signals of 10 postural muscles were recorded and analyzed. Principal component analysis was used to identify M-modes within the space of integrated indices of muscle activity. Variance in the M-mode space across stepping trials was partitioned into two components, one that did not affect the average value of center of pressure (COP) shift and the other that did. An index (Delta V) corresponding to the normalized difference between two components of variance was computed. Under the two tasks, strong multi-M-mode synergies stabilizing trajectories of the COP in the anterior-posterior direction were found. Despite the significant differences in the COP shifts and EMG patterns of postural adjustments, the synergies showed only minor differences across the conditions. These findings demonstrate the robustness of multi-M-mode synergies across different manners of making a step.