A multifactorial assessment based on muscle synergies to evaluate the effects of hip exoskeleton assistance in functional tasks.

Wearable devices such as lower-limb exoskeletons are increasingly proposed to support mobility, assist activities of daily living, and aid rehabilitation. However, quantifying the quality of interaction between human and robot remains a critical challenge, particularly when considering neuromuscular coordination during functional movements. In this study, we investigated user adaptation to a commercially available wearable hip exoskeleton by combining muscle synergy analysis with subjective perception during functional tasks. Seventeen healthy adults performed four mobility tasks (overground walking, sit-to-stand, stair ascent, and descent) in a free condition and under three assistance modalities: transparent (non-assisting), low assistance, and high assistance. Surface electromyography (EMG) and kinematic data were recorded to extract muscle synergies and assess changes in neuromuscular coordination. Subjective questionnaires were also included to evaluate participants' perceptions of the quality of interaction with the device. Results showed that the transparent modality preserved both the spatial and temporal structure of muscle synergies across all tasks, indicating that the device alone did not alter motor control. In contrast, increasing assistance levels led to task-specific changes, with significant reductions in spatial similarity in propulsion-related synergies during walking (p < 0.001) and stair ascent (p = 0.002), while stabilization-dominated tasks remained largely unaffected. Notably, these findings were consistent with participants' subjective perception, with higher assistance reported as significantly less natural during locomotion tasks (p < 0.001). Overall, our results suggest that synergy-based analysis provides a sensitive and objective method for evaluating human-robot interaction, reflecting not only changes in motor control but also in the user's subjective experience.