Multichannel Nerve Stimulation for Diverse Activation of Finger Flexors

Objective: Neuromuscular electrical stimulation (NMES) is a common approach to restore muscle strength of individuals with a neurological injury but restoring hand dexterity is still a challenge. This study sought to quantify the diversity of finger movements elicited by a multichannel nerve stimulation technique. Methods: A 2 \times 8 stimulation grid, placed on the upper arm along the ulnar and median nerves, was used to activate different finger flexors by automatically switching between randomized bipolar electrodes. The forces from each individual finger as well as the high-density electromyogram (HDEMG) of the intrinsic and extrinsic flexors were recorded. The elicited finger forces were categorized using hierarchical clustering, and the 2D correlation of the spatial patterns of muscle activation was also calculated. Results: A wide range of movement patterns were identified, including multi-finger and single-digit movements. Additionally, a number of electrode pairs elicited similar finger movements. The muscle activation patterns showed similar and distinct spatial patterns, signifying activation redundancy. Conclusion: These results revealed the diversity of elicitable finger movements and muscle activations. The system redundancy can be explored to compensate for system instability due to fatigue or electrode shift. The outcomes can also enable the development of an automatic calibration of the stimulation.