Age-related effects on muscle synergies during a standardized exoskeleton-assisted reaching task

Abstract Background Muscle synergy analysis is a relevant approach to characterize motor control in different contexts, including the use of rehabilitative devices. Pilot studies on healthy individuals allow to derive baseline reference synergies to tailor the interventions and assess their effects in patient populations, but challenging levels of inter-subject variability are often reported. Methods Surface EMG was acquired from 10 upper limb muscles of 40 healthy subjects performing a standardized reaching paradigm with and without the assistance of the Float exoskeleton. We computed “free” (unassisted), ”low” (low assistance), ”high” (high assistance), and “whole” (unassisted+assisted) synergies to characterize the modularity of the experimental paradigm. Synergies from single subjects were grouped according to cosine similarity and averaged to obtain reference sets. We assessed their consistency (i.e. their capability to reliably represent single-subject modules) based on number of excluded single-subject modules, cosine similarity and coefficient of variation. We also explored age as a possible factor influencing the modularity of motor control and contributing to inter-subject variability in our study. Subjects were stratified by age to derive reference synergies from age-homogeneous groups and assess their consistency and the presence of age-related effects on the modularity of the task. Results Reference synergies obtained from the entire study population showed a consistency level in line with literature. The assistance of Float appeared to not disrupt the synergistic structure of standardized reaching but to modulate it with a reduced effort of the anterior deltoid and an increased involvement of the trapezius muscle. The reference synergies obtained from age groups highlighted some age-specific response to exoskeleton assistance and showed a small but significant improvement in consistency. Statistical analysis indicated that for older subjects the response to exoskeleton assistance appeared more variable. Discussion The results of this case study suggest that relying on age-matched reference synergies could improve the robustness of synergy-based assessment in rehabilitative applications. The study also identifies relevant challenges in the development of reliable healthy references that should be further explored in future works. Overall, this study supports the utility of muscle synergy analysis in evaluating rehabilitative devices.