Neuromuscular Electrical Stimulation Does Not Influence Spinal Excitability in Multiple Sclerosis Patients

(1) Background: Neuromuscular electrical stimulation (NMES) has beneficial effects on physical functions in Multiple sclerosis (MS) patients. However, the neurophysiological mechanisms underlying these functional improvements are still unclear. This study aims at comparing acute responses in spinal excitability, as measured by soleus Hoffmann reflex (H-reflex), between MS patients and healthy individuals, under three experimental conditions involving the ankle planta flexor muscles: (1) passive NMES (pNMES); (2) NMES superimposed onto isometric voluntary contraction (NMES+); and (3) isometric voluntary contraction (ISO). (2) Methods: In total, 20 MS patients (MS) and 20 healthy individuals as the control group (CG) took part in a single experimental session. Under each condition, participants performed 15 repetitions of 6 s at 20% of maximal voluntary isometric contraction, with 6 s of recovery between repetitions. Before and after each condition, H-reflex amplitudes were recorded. (3) Results: In MS, H-reflex amplitude did not change under any experimental condition (ISO: p = 0.506; pNMES: p = 0.068; NMES+: p = 0.126). In CG, H-reflex amplitude significantly increased under NMES+ (p = 0.01), decreased under pNMES (p < 0.000) and was unaltered under ISO (p = 0.829). (4) Conclusions: The different H-reflex responses between MS and CG might reflect a reduced ability of MS patients in modulating spinal excitability.