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

TL;DR

Insight is provided into the potential mechanisms of exoskeleton-assisted overhead work and guidance on improving the performance of exoskeletons and may influence the uniformity of monitored muscle activations.

Abstract

Objective: Shoulder exoskeletons can effectively assist with overhead work. However, their impacts on muscle synergy remain unclear. The objective is to systematically investigate the effects of the shoulder exoskeleton on muscle synergies during overhead work.Methods: Eight male participants were recruited to perform a screwing task both with (Intervention) and without (Normal) the exoskeleton. Eight muscles were monitored and muscle synergies were extracted using non-negative matrix factorization and electromyographic topographic 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 AD and MD; while the second synergies were different between conditions, with highest weight of PM and MD, respectively. As for the first synergy in the Intervention condition, the activation profile significantly decreased, and the average recruitment level and activation duration were significantly lower (p<0.05). The regression analysis for the muscle synergies across conditions shows the changes of muscle synergies did not influence the sparseness of muscle synergies (p=0.7341). In the topographic maps, the mean value exhibited a significant decrease (p<0.001) and the entropy significantly increased (p<0.01). Conclusion: The exoskeleton does not alter the number of synergies and existing major synergies but may induce new synergies. It can also significantly decrease neural activation and may influence the heterogeneity of the distribution 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.

Figures (7)

• Figure 1: The overview of the HIT-POSE. (a) The structure diagram of the exoskeleton. (b) The assistive torque profiles with three different PATAs and levels of assistance. Different colored profiles have different PATAs (i.e., 100deg, 120deg, and 140deg, respectively), and profiles of the same color but different transparency have different levels of assistance (i.e., 10N·m, 12N·m, and 14N·m, respectively).
• Figure 2: Location of the EMG sensors and the IMU.
• Figure 3: The variability accounted for (VAF) based on different numbers of synergies for the screwing task.
• Figure 4: Muscle synergies in the Normal and Intervention conditions for the screwing task. (a) The first synergy in the Normal condition. (b) The first synergy in the Intervention condition. (c) The mean of the normalized activation profiles ($H_{norm}$) across all participants in the first synergy during the two conditions. (d) The second synergy in the Normal condition. (e) The second synergy in the Intervention condition. (f) The mean of the normalized activation profiles ($W_{norm}$) across all participants in the second synergy during the two conditions. The bars of the same color represent the same participant, while the gray bars represent the mean value of the participants. The light blue and orange shaded regions represent the standard error of $W_{norm}$ during the Normal and Intervention conditions, respectively.
• Figure 5: The averaged recruitment level ($Recr$) and the percent of activation duration ($Ad$) of each synergy in the Normal and Intervention conditions for the screwing task. (a) $Recr$. (b) $Ad$. * denotes a significant difference ($p$<0.05). N.S. stands for “No Significant”.