A Limited Set of Muscle Synergies for Force Control During a Postural Task

TLDR

Computational techniques have reduced complex muscle activation patterns to simple synergies, offering new insights into the long‑standing degrees‑of‑freedom problem in motor control. The study aimed to identify muscle synergies during cat postural responses using nonnegative factorization and test whether neural simplification is matched by a biomechanical reduction in degrees of freedom. EMG from 8–15 hindlimb muscles was recorded while cats were subjected to 16 directions of support‑surface translation, and nonnegative factorization was applied to extract synergies. Four synergies accounted for over 95 % of the automatic postural response, each activated by specific perturbation directions and correlated with a unique endpoint‑force vector, suggesting that postural synergies encode a neural command specifying limb endpoint force.

Abstract

Recently developed computational techniques have been used to reduce muscle activation patterns of high complexity to a simple synergy organization and to bring new insights to the long-standing degrees of freedom problem in motor control. We used a nonnegative factorization approach to identify muscle synergies during postural responses in the cat and to examine the functional significance of such synergies for natural behaviors. We hypothesized that the simplification of neural control afforded by muscle synergies must be matched by a similar reduction in degrees of freedom at the biomechanical level. Electromyographic data were recorded from 8-15 hindlimb muscles of cats exposed to 16 directions of support surface translation. Results showed that as few as four synergies could account for >95% of the automatic postural response across all muscles and all directions. Each synergy was activated for a specific set of perturbation directions, and moreover, each was correlated with a unique vector of endpoint force under the limb. We suggest that, within the context of active balance control, postural synergies reflect a neural command signal that specifies endpoint force of a limb.