Exploring the Muscle-Tendon Biomechanics and Load Mitigation of a Passive Shoulder Exoskeleton During Overhead Work

The biomechanical mechanisms, assistance-level effects, and load-mitigation effects of shoulder exoskeletons during overhead work remain insufficiently understood. This study aims to explain experimental outcomes using simulation-derived muscle-tendon metrics, compare assistive torque profiles within the tested range, and evaluate their biomechanical effects on load mitigation.

OpenSim musculoskeletal simulations were conducted for eight participants performing overhead work. Partial least squares (PLS) regression was performed using EMG data from a previously published experimental dataset to characterize multivariate associations between simulation-derived muscle-tendon metrics and experimentally observed muscular outcomes. In addition, five participants were newly recruited in the present study for independent validation for the prediction from simulation, i.e. the muscle activation reduction under 0%, 30%, 50% and 60% conditions.

PLS analyses showed muscle- and condition-dependent latent associations between simulation-derived muscle-tendon metrics and experimental outcomes. Force- and tendon-related variables showed higher contributions in several fitted PLS models; these contributions were interpreted within the shared covariance structure of the mechanically coupled predictors rather than as independent physiological effects. Simulation results showed that 70% assistance produced the lowest mean total shoulder muscle activation, while no statistically significant differences were detected among the 50%, 60%, and 70% conditions. The 60% condition was retained as a representative experimentally tested condition for subsequent biomechanical-demand analyses. Under this condition, the cumulative activation index, force-weighted activation index, and cumulative joint loading index demonstrated reductions in cumulative muscle activation, force-weighted muscular demand, and shoulder joint loading, respectively. Experimental EMG decreased monotonically with increasing assistance up to 60%, which was the highest experimentally tested assistance level.

This study demonstrates that simulation-derived muscle-tendon metrics can characterize experimental responses and support evidence-based comparison of assistive strategies for reducing biomechanical demand and shoulder loading during the tested overhead work.

The proposed simulation-experiment pipeline supports evidence-based exoskeleton design for reducing the biomechanical load during overhead work.