Spatial-Temporal Dynamics of Evoked Action in Finger Flexors: Implications for Optimizing Transcutaneous Nerve Stimulation

Objective: Transcutaneous nerve stimulation (TNS) is a promising approach for the neurorehabilitation of hand function; however, its effects on muscle activation patterns remain poorly understood. To investigate the spatial and temporal distributions of H-reflexes and M-waves in finger flexor muscles using multichannel TNS and high-density electromyography. Methods: Fifteen healthy participants underwent stimulation of the median and ulnar nerves, and the muscle activity and finger forces were recorded. Recruitment curves and spatial activation maps were constructed for the H-reflexes and M-waves across stimulation intensities and locations. Results: Considerable inter-individual variability was observed in the recruitment patterns and spatial distributions. Higher spatial congruence between the H-reflex and M-wave activation patterns in extrinsic arm muscles than in intrinsic hand muscles was associated with more efficient force production. The relationship between spatial activation patterns and force outputs varied across fingers, with earlier recruitment of index finger muscles. Conclusion: This study provides new insights into the complex interplay between the afferent and efferent pathways in hand motor control. The associations between spatial congruence, recruitment patterns, and force production efficiency enhance our understanding of the neuromuscular activation mechanisms. Significance: These findings have implications for optimizing TNS protocols in neurorehabilitation and developing personalized interventions for individuals with impaired hand function.