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

Area of Science:

• Neuroscience
• Biomedical Engineering
• Motor Control

Background:

• Understanding finger force control is vital for neuroprosthetic design and rehabilitation.
• Spinal reflex pathways and biomechanical factors significantly impact hand movements.

Purpose of the Study:

• To investigate how spinal reflex engagement, electrode placement, and anatomy affect finger interdependence during force control.
• To explore the relationship between spinal reflex strength and force production across different fingers.

Main Methods:

• Combined transcutaneous nerve stimulation (ulnar/median nerves) with high-density electromyography (HD-EMG) and multi-digit force measurements.
• Evoked H-reflexes in extrinsic finger flexors, quantifying spinal excitability via H-reflex/M-wave (HM) ratios.
• Recorded force outputs across individual fingers and pairs under varying stimulation conditions.

Main Results:

• Index and middle fingers naturally produce more force than ring and pinky fingers.
• Spinal reflex strength showed a trend with greater force production, highlighting individual neuromuscular strategies.
• The middle finger exerted the strongest influence on neighboring finger movements, indicating neural crosstalk and biomechanical coupling.
• Different electrode configurations yielded varied force distribution patterns, from single-finger to multi-finger responses.

Conclusions:

• Multi-digit force coupling during nerve stimulation is influenced by spinal excitability, electrode-dependent nerve recruitment, and anatomical variations.
• Isolating reflex-mediated force dynamics advances motor control models.
• Personalized stimulation protocols are essential for optimizing neuroprosthetic interfaces and post-stroke rehabilitation.