Generalizability of Muscle Synergies in Isometric Force Generation Versus Point-to-Point Reaching in the Human Arm Workspace



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Muscle synergies have been used to investigate a variety of motor tasks to uncover the fundamental knowledge of neuromotor control and to develop novel therapeutic methods for patients with motor impairments. From our literature review, not many studies have analyzed the generalizability of muscle synergies between different motor conditions, more specifically motor tasks in the upper extremity. Our aim of this study was to classify the muscle synergies in isometric force generation and point-to-point reaching and to identify muscle synergies that may be shared between the two motor tasks. To do so, we recruited 10 right-handed able-bodied participants to perform center-out target matching in 12 different directions and at four different starting arm locations using the isometric force generation device and the KINARM Exoskeleton for the isometric force generation and point-to-point reaching task, respectively. Surface electromyographic (EMG) electrodes were placed on nine arm muscles and four back muscles to measure muscle activation throughout the experiment. The non-negative matrix factorization algorithm was then used to extract the muscle synergies and their activation profiles. From our results, five and six muscle synergies were found to reconstruct the EMG data of the isometric force generation and point-to-point reaching task, respectively. The muscle synergies across starting arm locations were mostly conserved within the same task. Across the motor tasks, more than half of the muscle synergies were shared, although task-specific differences in the synergy composition and activation profiles were observed. This work is beneficial for future studies to determine the abnormal muscle synergies and to identify effective rehabilitation methods in the upper extremity of stroke survivors.



Muscle synergy, Upper extremity, Neuromotor control, EMG, Isometric, Dynamic