Spinal reflex in human lower leg muscles evoked by transcutaneous spinal cord stimulation

The H-reflex is one of the most common and useful techniques in the field of motor control. However, the H-reflex technique also involves difficulty in data interpretation when stimulus intensity is high enough to stimulate both sensory and motor fibers (antidromic current). On the other hand, transcutaneous stimulation applied on the spinous processes is able to stimulate the dorsal root, resulting in selective stimulation of only sensory fibers without evoking a direct motor response and antidromic current on the motor fibers. The purpose of this study was to examine the maximal reflex response that can be elicited in the lower leg muscles using transcutaneous spinal stimulation. Seven subjects participated in the study. EMG signals were recorded from triceps surae (SOL, MG, LG) in the prone position. Transcutaneous stimulation was applied both to the spinous process (between T11 and T12, spinal stimulation, SS) and to the popliteal fossa (peripheral stimulation, PS). Using SS and PS, Hmax amplitudes of triceps surae muscles were measured and standardized with Mmax. Hmax values in MG and LG by SS (31% and 41%) were significantly greater than those by PS (20% and 23%, respectively). Although not significant, Hmax amplitude in SOL by SS (76%) was also greater than that by PS (60%). It is suggested that transcutaneous stimulation is able to evoke H-reflex without a direct motor response. Hmax amplitudes traditionally measured by stimulation applied to a mixed nerve may underestimate the potential connectivity between the sensory and motor systems in humans.