We studied muscle activation patterns in response to perturbations of posture (sudden changes in the external force applied to the thorax) during two time intervals corresponding to pre-programmed postural reactions and voluntary corrections of posture. A hypothesis was tested that a set of postural muscles could be used to form stable groups (M-modes) whose composition changes in different time intervals after a perturbation. Perturbations were applied at the sternum level to standing subjects at an unexpected time. Principal component analysis with factor extraction allowed to identify sets of three factors (M-modes) during the two time intervals, 80-180 ms (T1) and 250-450 ms (T2) after the perturbation. The composition of M-modes was similar within each time interval across subjects and perturbations but differed significantly between T1 and T2. In particular, M-modes during T1 were characterized by more co-contraction patterns. The results suggest that the neural controller is able to rearrange M-mode composition in real time based on a safety-efficacy trade-off. The results also support the idea that M-modes represent synergies in the muscle space, while they may be used as elemental variables to form synergies at a higher hierarchical level to produce desired mechanical effects.
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