Transspinal stimulation downregulates flexion reflex pathways during walking in healthy humans.

The phase-dependent modulation pattern of the tibialis anterior (TA) flexion reflex was characterized during treadmill walking while transspinal stimulation was delivered at 15, 30, and 50 Hz above and below paresthesia in healthy participants. The flexion reflex was elicited following medial arch foot stimulation with a 30 ms (300 Hz) pulse train. During treadmill walking, the flexion reflex was evoked in the right leg every 3 to 5 steps, and stimuli were randomly dispersed across the step cycle that was divided into 16 equal bins. For each participant, condition and bin of the step cycle, the flexion reflex was measured as the area of the linear EMG envelope starting 20 ms after the end of the pulse train up to 200 ms and was normalized to the maximum locomotor TA EMG activity. The unconditioned flexion reflex was modulated in a phase-dependent manner. Transspinal stimulation, regardless frequency, or intensity produced pronounced flexion reflex depression during walking that coincided with an unchanged slope and intercept, computed from the linear relationship between the flexion reflex and background EMG activity. These findings suggest that transspinal stimulation above and below paresthesia intensities at 15, 30, and 50 Hz downregulates the flexion reflex. Based on our recently reported absent effects on the soleus H-reflex under similar conditions and our current findings we propose that transspinal stimulation downregulates flexion and not extension reflex pathways. More research is needed to delineate whether similar neuromodulation effects are present in flexion and extension reflexes after spinal cord injury in humans.