Transcutaneous Nerve Stimulation Uncovers Spinal Reflex Contributions to Finger Force Coupling Patterns

Understanding how spinal reflex pathways interact with biomechanical constraints to govern finger force control is critical for advancing neuroprosthetic design and neurorehabilitation. This study combined transcutaneous stimulation of the ulnar/median nerves with high-density electromyography (HD-EMG) and multi-digit force measurements to investigate how spinal reflex engagement (via H-reflex activation), electrode placement, and neuroanatomical variability shape finger interdependence. We evoked H-reflexes in the extrinsic finger flexors and quantified spinal excitability using H-reflex/M-wave (HM) ratios while recording force outputs across individual fingers and pairs. The index and middle fingers naturally generate more force than the ring and pinky fingers. When plotted, spinal reflex strength exhibited a slight upward trend, suggesting a potential relationship with greater force production and emphasizing the significance of individual neuromuscular strategies. The middle finger showed the strongest influence over neighboring finger movements, demonstrating how biomechanical coupling and neural crosstalk between fingers dominate the coordination of multi-finger actions. Different stimulation electrode configurations produced varying patterns of force distribution across the fingers, with some pairs enabling selective single-finger activation, while others triggered broader multi-finger responses. These results demonstrate that multi-digit force coupling during nerve stimulation arises from spinal excitability, electrode-dependent nerve recruitment, and individual anatomical differences. By isolating reflex-mediated force dynamics, this study advances hierarchical motor control models and underscores the need for personalized stimulation protocols to optimize neuroprosthetic interfaces and post-stroke rehabilitation.