Modularity in the motor system: decomposition of muscle patterns as combinations of time-varying synergies

The question of whether the nervous system produces movement through the combination of a few discrete elements has long been central to the study of motor control. Muscle synergies, i.e. coordinated patterns of muscle activity, have been proposed as possible building blocks. Here we propose a model based on combinations of muscle synergies with a specific amplitudeand temporal structure. Time-varying synergies provide a realistic basis for the decomposition of the complex patterns observed in natural behaviors. To extract time-varying synergies from simultaneous recordingof EMG activity we developed an algorithm which extends existing nonnegative matrix factorization techniques.