Muscle synergy analysis of short-term adaptation to arm-support exoskeletons during pseudo-static and dynamic...

Occupational arm-support exoskeletons (ASEs) can reduce shoulder muscle activity during overhead work, but their effects on muscle synergy structure and temporal activation remain limited. We examined the effects of using three different exoskeletons on muscle synergies during simulated overhead tasks. Muscle activity from 18 participants (gender-balanced) performing both pseudo-static and dynamic tasks across 24 conditions (three ASEs and a control condition) was analyzed using non-negative matrix factorization to extract synergy number, structure, and activation coefficients. Dynamic tasks recruited more muscle synergies (interquartile range: 2-5) than pseudo-static tasks (interquartile range: 1-3), with some task combinations showing modest increases with ASE use compared to the control condition. Synergy structure and temporal activation were generally similar across interventions (mean cosine similarity 0.74-0.92), but certain ASE-task combinations produced significant local changes in synergy structure. Using exoskeletons generally altered muscle weightings, shifting from primary arm-elevating and shoulder-stabilizing muscles toward modules involving neck and back muscles, suggesting compensatory strategies for device-imposed biomechanical demands. Activation time courses remained highly similar across most interventions during pseudo-static tasks, though dynamic tasks showed reduced peak magnitude with exoskeleton use. Our results indicate that while modular motor control is largely preserved with ASE use, device- and task-specific adaptations in synergy structure and temporal activation can occur. Future research should explore how ASE design features influence neuromuscular strategies and assess long-term adaptation of muscle synergies in occupational settings.