Effects of Muscle Synergy during Overhead Work with a Passive Shoulder Exoskeleton: A Case Study.

Objective: Shoulder exoskeletons can effectively assist with overhead work. However, their impacts on muscle synergy remain unclear. The objective of this study is to investigate the effects of muscle synergies during exoskeleton-assisted overhead work.

Methods: Eight participants performed a screwing task both with and without the exoskeleton. Eight muscles were monitored and muscle synergies were extracted using non-negative matrix factorization and electromyographic maps.

Results: The number of synergies extracted was the same (n = 2) in both conditions. Specifically, the first synergies in both conditions were identical, with the highest weight of anterior deltoid and middle deltoid; while the second synergies were different between conditions, with highest weight of pectoralis major and middle deltoid, respectively. Compared to the condition without the exoskeleton, the first row of the activation coefficient matrix (i.e., activation profile) showed a significant decrease, with the corresponding average recruitment level and activation duration were significantly lower (p<0.05). The regression analysis shows the changes of muscle synergies did not influence the sparseness of muscle synergies (i.e., the degree of concentration of muscles within synergies, p = 0.7341). The mean value exhibited a significant decrease (p<0.001) and the entropy which measured the uniformity of muscle activation significantly increased(p<0.01).

Conclusion: The exoskeleton did not alter the number of synergies and existing major synergies. It can significantly decrease muscle activity and may influence the uniformity of monitored muscle activations.

Significance: This study provides insights into the potential mechanisms of exoskeleton-assisted overhead work and guidance on improving the performance of exoskeletons.