Time-dependent effects of neuromuscular electrical stimulation on changes in spinal excitability are dependent on stimulation frequency: A preliminary study in healthy adults

Neuromuscular electrical stimulation (NMES) can be used as treatment for spasticity. The present study examined differences in time-dependent effects of NMES depending on stimulation frequency. Forty healthy subjects were separated into four groups (no-stim, NMES of 50, 100, and 200?Hz). The un-conditioned H-reflex amplitude and the H-reflex conditioning-test paradigm were used to measure the effectiveness on monosynaptic Ia excitation of motoneurons in the soleus (SOL) muscle, disynaptic reciprocal Ia inhibition from tibialis anterior (TA) to SOL, and presynaptic inhibition of SOL Ia afferents. Each trial consisted of a 30-min period of NMES applied to the deep peroneal nerve followed by a 30-min period with no stimulation to measure prolonged effects. Measurements were performed periodically. Stimulation applied at all frequencies produced a significant reduction in monosynaptic Ia excitation of motoneurons in the SOL muscle, however, only stimulation with 50?Hz showed prolonged reduction after NMES. NMES frequency did not affect the amount of disynaptic reciprocal Ia inhibition and presynaptic inhibition of Ia afferents. The results show a frequency-dependent effect of NMES on the monosynaptic Ia excitation of motoneurons. This result has implications for selecting the optimal NMES frequency for treatment in patients with spasticity.