Short-term effects of neuromuscular electrical stimulation during resistance training on the spinal reflex circuit

The purpose of the study was to examine the changes in soleus Hoffmann (H)-reflex and volitional (V)-wave after resistance training (RT) with and without neuromuscular electrical stimulation (NMES). Fourteen participants were randomly allocated to receive RT with NMES (RT+NMES, n = 7) or RT without NMES (RT, n = 7). Each participant trained for 10 sessions of right leg standing calf-raise for 2 weeks. For the RT+NMES group, NMES was applied to the tibial nerve using rectangular pulses (400 µsec duration, 75-Hz trains). Stimulation intensity was set at the maximal tolerable level. The H-reflex was elicited just above motor threshold during rest and during maximal voluntary isometric contraction (MVIC) conditions. The H-reflex and V-wave were normalized to the maximal motor response (Mmax). There was a significant interaction between time (pre/post) and group in the H-reflex during rest and during the MVIC condition (P = 0.05), but no significant difference in plantar torque, Mmax or V-wave. The H-reflex during rest decreased from 0.54 to 0.38 in the RT+NMES group (P = 0.01) but did not change in the RT group (from 0.48 to 0.47, P = 0.79). During MVIC, the H-reflex increased from 0.23 to 0.76 in the RT+NMES group (P = 0.02), but did not change in the RT group (from 0.44 to 0.58, P = 0.12). The results indicate that NMES-induced proprioceptive input during RT has an inhibitory effect in the resting muscle and an excitatory effect in the voluntarily activated muscle via spinal and/or supraspinal pathways.