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The Effect of Transcutaneous Electrical Stimulation on Spinal Motor Neuron Excitability in People Without Known Neuromuscular Diseases: The Roles of Stimulus Intensity and Location

SGP Hardy, PT, PhD, is Professor, Department of Physical Therapy, School of Health Related Professions, and Department of Anatomy, School of Medicine, The University of Mississippi Medical Center, 2500 N State St, Jackson, MS 39216 (USA) (phardy{at}shrp.umsmed.edu).
TB Spalding, PT, MPT, and H Liu, PT, PhD, were graduate students, Department of Physical Therapy, School of Health Related Professions, The University of Mississippi Medical Center, at the time this study, which was undertaken in partial fulfillment of the requirements for their Master of Physical Therapy degrees
TG Nick, PhD, is Associate Professor, Department of Physical Therapy, School of Health Related Professions, The University of Mississippi Medical Center
RH Pearson, PT, EdS, is Assistant Professor, Department of Physical Therapy, School of Health Related Professions, The University of Mississippi Medical Center
AV Hayes, R EDT/EPT, is Neurophysiology Technologist, Neurophysiological Research Laboratories, Center for Neuroscience and Neurological Recovery, The Methodist Rehabilitation Center, Jackson, Miss
DS Stokic, MD, is Director of the Neurophysiological Research Laboratories, Center for Neuroscience and Neurological Recovery, The Methodist Rehabilitation Center

Address all correspondence to Dr Hardy

Background and Purpose. The Hoffmann reflex (H-reflex) is widely acknowledged as an indirect indicator of spinal motor neuron excitability. The purpose of this study was to determine whether transcutaneous electrical stimulation (TES), applied over the dorsiflexors or plantar flexors of the ankle, would alter the soleus muscle's H-reflex. Attention was focused on the roles of stimulus intensity and location. Subjects. Thirty-two volunteers without known neuromuscular diseases (17 women [53%]; mean years of age=27, SD=7.3, range=21–48) were studied. Methods. Subjects were randomly assigned to 1 of 4 groups, and TES was administered for 15 minutes. Stimulation site and intensity varied according to group assignment. H-reflexes were recorded before and for 10 minutes after TES. Results. H-reflex amplitudes increased following TES at sensory threshold, whereas H-reflex amplitudes did not change following TES at 1.5 times motor threshold. The site of stimulation did not influence the resulting H-reflexes. Discussion and Conclusion. Low-intensity TES increases H-reflex amplitudes (and presumably the excitability of spinal motor neurons to Ia afferent input) in subjects without known neuromuscular diseases. High-intensity TES had little influence on H-reflex amplitudes.

Key Words: H-reflex • Motor neuron excitability • Transcutaneous electrical stimulation