Brief use of a passive shoulder exoskeleton modifies muscle activation after removal

Abstract

Abstract Wearable exoskeletons are increasingly implemented in industrial environments to alleviate physical strain; however, their impact on motor control strategies, particularly following device removal, remains insufficiently understood. This exploratory study examines the short-term post-effects of a passive upper-limb exoskeleton, the Paexo Shoulder, which uses spring-based assistance to support arm elevation. Twenty healthy participants were assigned to either an Exposed group (n = 10), who performed an overhead reaching task while wearing the exoskeleton for 25 minutes, or a Baseline group (n = 10), who completed the same task without assistance. Kinematic and muscular activity were assessed using motion capture and electromyography before and after the intervention. Post-removal analyses revealed limited changes in classical performance and kinematic outcomes during the first 10 minutes after exoskeleton removal. Movement accuracy remained largely unchanged, task completion time showed only a non-robust tendency to decrease, and joint range-of-motion metrics showed directional but non-significant changes after correction for multiple comparisons. In contrast, more detailed indicators of motor strategy suggested short-term neuromuscular reorganization. Estimation plots indicated tendencies toward altered inter-muscle coordination, including changes in anterior deltoid and biceps contribution. The clearest statistically supported effect was observed in biceps activation, which decreased after repeated task execution in the Baseline group but increased after exoskeleton exposure. Overall, these findings suggest that short-term use of a passive shoulder exoskeleton produces limited modifications in end-effector performance and joint range of motion, while more subtle post-removal effects may emerge at the level of muscle activation and coordination strategies. These results highlight the importance of (1) assessing post-use effects of occupational exoskeletons, and (2) complementing classical biomechanical metrics with coordination and electromyographic analyses.