Modulation of muscle synergies with direction and distance during reaching movements.

Previous studies suggest that muscle synergies are the building blocks used by the central nervous system to construct movements. Here, we investigate how to construct reaching movements across different directions and distances by altering the recruitment of a few muscle synergies. To this end, electromyographic (EMG) signals were recorded from nine upper limb muscles during reaching movements across six directions and three distances in the horizontal plane. Non-negative matrix factorization algorithm was employed to extract muscle synergies. For each one of eleven subjects, three muscle synergies could account for a large fraction (88.85±1.68%) of variations in EMG signals across all conditions. By characterizing the activation of synergies in each direction and distance, we found that two synergies only activated in specific directions, and positive correlations existed between their activation levels and distance in these directions. However, the remaining synergy was almost activated with the same degree in all directions and distances. Overall, our results demonstrate that the control of a repertoire of reaching movements can be achieved by flexibly modulating the activation of several muscle synergies.