Comparison of acute responses in spinal excitability between older and young people after neuromuscular electrical stimulation

Purpose: This study aims at comparing acute responses in spinal excitability, as measured by H-reflex, between older and young individuals, following a single session of NMES superimposed onto voluntary isometric contractions of the ankle plantar-flexor muscles (NMES+), with respect to passive NMES (pNMES) and voluntary isometric contractions only (ISO). Methods: Thirty-two volunteers, 16 older (OLDER) and 16 young (YOUNG), were asked to sustain a constant force at 20% of maximal voluntary isometric contraction (MVIC) of the ankle plantar-flexor muscles in the dominant limb during each of the 3 conditions (NMES+ , pNMES and ISO). Fifteen repetitions of 6 s were performed, with a resting interval of 6 s between repetitions. Before and after each condition, soleus H-reflexes were elicited by percutaneous electrical stimulation of the posterior tibial nerve and H-reflex amplitudes recorded by surface EMG. Results: In OLDER, H-reflex amplitude did not change following any experimental condition (ISO: p = 0.203; pNMES: p = 0.542; NMES+: p = 0.431) compared to baseline. On the contrary, in YOUNG, H-reflex amplitudes significantly increased (p < 0.000) and decreased (p = 0.001) following NMES+ and pNMES, respectively, while there was no significant change in reflex responses following ISO (p = 0.772). Conclusion: The lack of change in H-reflex responses following either NMES+ or pNMES might reflect a reduced ability of older people in modulating spinal excitability after the conditions. Specifically, an age-related alteration in controlling mechanisms at presynaptic level was suggested.