Muscle fatigue and postural balance reorganisation during exoskeleton-assisted load handling

Upper-limb exoskeletons are known to reduce local muscle strain, but their impact on global postural regulation remains unclear. This study investigated whether upper-limb exoskeleton usage induces a compensatory trade-off in postural stability or a fundamental reorganisation of motor control strategies during sustained static loading.

Nineteen healthy participants performed a static holding task (10 kg) to exhaustion under two conditions: with and without a passive upper-limb exoskeleton. Neuromuscular fatigue was assessed via surface electromyography (sEMG) spectral analysis and pre-post maximal voluntary contractions (MVC). Postural dynamics were quantified using center-of-pressure (CoP) velocity, Detrended Fluctuation Analysis (DFA), and Multiscale Entropy (MSE) to assess system complexity and adaptability.

Exoskeleton use significantly delayed task failure, nearly tripling time-to-exhaustion (896 s vs 307 s; P < 0.001), while attenuating the decline in EMG median frequency and preserving biceps MVC. Crucially, this endurance gain was accompanied by a strategic shift in postural control: exoskeleton use reduced CoP signal complexity (lower MSE) and increased short-term persistence (higher DFA α1), particularly in the medio-lateral axis. Importantly, this reduced complexity occurred without a concomitant increase in sway velocity.

These findings suggest that exoskeletons do not merely mechanically offload muscles but are associated with a shift toward a "low-complexity" control regime. This reorganisation accompanied the extended endurance and occurred without an increase in sway velocity, consistent with the device assuming part of short-term stability regulation rather than the system compensating through detrimental sway.