Direction modulation of muscle synergies in a hand-reaching task

Functional tasks of the upper extremity can be executed by a variety of muscular patterns, independent of the direction, speed and load of the task. This large number of degrees of freedom imposes a significant control burden on the CNS. Previous studies suggested that the human cortex synchronizes a discrete number of neural functional units within the brainstem and spinal cord, i.e. muscle synergies, by linearly combining them to execute a great repertoire of movements. Further exploring this control mechanism, we aim to study whether a single set of muscle synergies might be generalized to express movements in different directions. This was implemented by using a modified version of the non-negative matrix factorization algorithm on EMG data sets of the upper extremity of healthy people. Our twelve participants executed hand-reaching movements in multiple directions. Muscle synergies that were extracted from movements to the center of the reaching space could be generalized to synergies for other movement directions. This finding was also supported by the application of a weighted correlation matrix, the similarity index and the results of the K-means cluster analysis. This might reinforce the notion that the CNS flexibly combines a single set of small number of synergies in different amplitudes to modulate movement for different directions.