Neuromechanics of muscle synergies for posture and movement

Abstract

Recent research suggests that the nervous system controls muscles by activating flexible combinations of muscle synergies to produce a wide repertoire of movements. Muscle synergies are like building blocks, defining characteristic patterns of activation across multiple muscles that may be unique to each individual, but perform similar functions. The identification of muscle synergies has strong implications for the organization and structure of the nervous system, providing a mechanism by which task-level motor intentions are translated into detailed, low-level muscle activation patterns. Understanding the complex interplay between neural circuits and biomechanics that give rise to muscle synergies will be crucial to advancing our understanding of neural control mechanisms for movement.

Figure 1

Muscle synergies allow task-level neural commands to be translated into execution-level muscle activation patterns. This hierarchal structure mirrors that of multisensory integration systems.

Figure 2

The force-production capability of the cat hindlimb is restricted when an identical set of muscle synergies is used for balance control in different postural configurations (adapted from [81•]). A: The gray polygons represent the manifold of possible endpoint forces in a neuromechanical model of the cat hindlimb, given musculoskeletal constraints. From left to right, postural configuration is altered by increasing the “stance distance”, or the anterior-posterior distance between the feet. The most natural, “preferred” postural configuration in the third column is denoted by the cartoon cat. Colored lines denote the force vectors associated with each experimentally-observed muscle synergy. These synergy force vectors rotate with the limb axis as postural configuration changes. The white polygons represent the restricted manifold of possible endpoint forces when the experimentally-identified muscle synergies are used at all postures. B: Manifolds from A are overlaid with recorded postural forces. The “synergy-limited” manifolds predict the systematic rotation of postural forces as stance distance increases.