Generalizability of Muscle Synergies during Isometric Reaching in the Human Arm Workspace

The ability to create complex movements using the human upper arm plays a big role in performing activities of daily living. Unfortunately, hemiparetic stroke survivors often struggle in performing the same motor tasks due to the emergence of abnormal muscle synergies, or intermuscular co-activation patterns, caused by damage in the neural networks from the motor cortex. Isometric training is an alternative training method that could be beneficial for severely impaired stroke survivors since it does not require large ranges of arm movements. However, the characteristics of isometric reaching remain unclear, especially in the context of muscle synergies between different workspace locations. To investigate the generalizability of muscle synergies across various arm positions in healthy subjects during isometric reaching, we collected EMG data from the arm muscles of 12 young, healthy subjects doing isometric force generation on the 2-D horizontal plane with 5 different arm positions. At each arm position, the subject did 5 repetitions of the movement in 12 different target directions. The EMG signals were then processed and analyzed using non-negative matrix factorization to identify the muscle synergies. Overall, we found that 4 muscle synergies predicted the muscle activation patterns and the composition of each synergy was consistent across all arm locations. These findings provide the foundation in determining the potential abnormal muscle synergies in the upper extremity of stroke survivors.