Running Head : Muscle synergies for balance

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 non-negative factorization approach to identify muscle synergies during postural responses in the cat and further, 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 to 15 hindlimb muscles of cats exposed to 16 directions of support surface translation. Results showed that as few as four synergies could account for more than 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.