Neuromuscular electrical stimulation (NMES) is a common therapeutic tool for persons with movement disorders. The manner in which NMES generates muscular contractions has traditionally been attributed to the depolarization of motor axons underneath the stimulating electrodes, a purely peripheral mechanism, which does not involve the central nervous system (CNS). During NMES however, sensory axons are also recruited, initiating an afferent volley which can affect both spinal and cortical centers. This thesis is focused on identifying how this afferent volley influences NMES-evoked contractions and CNS excitability. Four projects are described in which NMES was delivered to generate plantar-flexion contractions. The first goal was to establish the influence of stimulus pulse width on the central recruitment of motoneurons. Contrary to previous findings, changing the pulse width did not significantly alter maximal soleus H-reflex amplitudes; however, wider pulses resulted in a leftward shift of the H-reflex recruitment curve and increased H-reflex amplitudes on the ascending limb of the recruitment curve. The second goal was to examine the effect of stimulus pulse-width on electromyographic responses and torque during NMES. During 20 Hz NMES, wide pulse widths depressed motor-waves (M-waves) and enhanced H-reflexes, generating larger contractions with a relatively greater central contribution, than when narrow pulses were used. The third project compared the torque produced during NMES-evoked contractions before and during a complete anesthetic block of the tibial and common peroneal nerves. Results from this project showed that contractions arising from a combination of central and peripheral mechanisms fatigue less than contractions that develop from the recruitment of motor axons alone. The final project investigated how spinal and corticospinal excitability associated with the soleus muscles are affected following NMES, voluntary contractions, or a combination of both. It was found that a combination of voluntary contractions and electrical stimulation induced plastic changes in the spinal circuitry of the stimulated muscle without affecting cortical circuitry or inducing any contralateral effects. Collectively, these experiments highlight that wider pulse widths induce a greater reflexive recruitment of motoneurons which contributes to the evoked torque during NMES, and that the evoked afferent volley reduces fatigue and influences spinal circuitry plasticity in the plantar-flexors. Methods to enhance the afferent volley during NMES are only beginning to be tested in clinical populations and future experiments will determine the potential efficacy for persons with movement disorders.
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