Enhanced somatosensory feedback modulates cutaneous reflexes in arm muscles during self-triggered or prolonged stimulation

Somatosensory feedback plays important roles in regulating all animal movement. The effects of sensory feedback on spinally mediated neural excitability are widely studied using cutaneous electrical stimulation paradigms. Cutaneous reflex amplitudes are reduced when stimulation is self-triggered instead of externally triggered. Altered spinal excitability and motor output are also observed following sustained stimulation with various parameters. Our purpose was to probe for interactions between mode and duration by investigating muscle responses following enhanced cutaneous stimulation. Fifteen neurologically intact participants were recruited. Cutaneous reflexes in the extensor carpi radialis (ECR) were evoked with brief (15 ms, 300 Hz) or sustained (300 ms, 50 Hz) stimulation trains. Stimulation was applied to the superficial radial or median nerve at the wrist and triggered by: (1) a computer program (random-triggered); (2) muscle contraction (EMG-triggered); (3) the participant pressing a button themselves (button-triggered). During each condition, isometric contractions were performed with ECR muscle activity maintained at 10, 25, 35, and 50% of maximal voluntary contraction. Stronger inhibitory reflexes were found following brief superficial radial nerve stimulation was EMG-triggered suggesting that modulation of cutaneous reflex excitability is specific to the timing when sensory ‘cues’ are applied during muscle contraction. No difference was observed following sustained stimulation applied to the superficial radial nerve meaning that brief and sustained stimulation affect the cutaneous pathways differentially. Nerve-specific responses were found between superficial radial and median nerve stimulation, such that greater inhibition was induced by EMG-triggered sustained stimulation to the median nerve. These observations are critical in moving beyond pathway phenomenology toward targeted sensory enhancement and amplified motor output in rehabilitation and training.