Delayed fatigue in finger flexion forces through transcutaneous nerve stimulation

Objective. Weakness of the hand is a major impairment which limits independent living. Neuromuscular electrical stimulation (NMES) is a common approach to help restore muscle strength. Traditional NMES directly over the muscle often leads to a rapid onset of muscle fatigue. In this study, we investigated the force sustainability of finger flexor muscles using a transcutaneous nerve stimulation approach. Approach. Finger flexion forces and high-density electromyogram (HD EMG) signals were obtained while electrical stimulation was applied to the ulnar and median nerve bundles through a stimulation grid on the proximal arm segment. Stimulation was also applied to the finger flexor muscle belly targeting the motor point, serving as a control condition. The force produced from the two stimulation approaches were initially matched, and muscle fatigue was subsequently induced with 5 min of continuous stimulation. The rate of decay of the force and EMG amplitude were quantified, and the spatial distribution of the muscle activation during the sustained contraction was also evaluated. Main results. The proximal nerve stimulation approach induced a slower decay in both force and EMG, compared with the stimulation at the motor point. The spatial distribution of the elicited muscle activation showed that the proximal nerve stimulation led to a distributed activation across the intrinsic and extrinsic finger flexor muscles and also activated a wider area within the extrinsic muscle. Significance. Our findings demonstrated that the stimulation of the proximal nerve bundles can elicit sustained force output and delayed decrease in the rate of force decline. This is potentially due to a spatially distributed activation of the muscle fibers, compared with the traditional motor point stimulation. Future development of our nerve stimulation approach may enable prolonged usage during rehabilitation or assistance for better functional outcomes.