Increased H-reflexes boost muscle contractions during tetanic stimulation of tibial nerves in neurologically-intact humans

Tetanic neuromuscular electrical stimulation (at 100 Hz) over ankle dorsiflexors and plantarflexors in persons with and without spinal cord injury can evoke contractions, which are considered spinal of origin, extra to those due to stimulating motor axons directly. Presently, we determined whether H-reflexes contribute to “extra” plantarflexion contractions, evoked in relaxed neurologically-intact persons by stimulating the tibial nerve at just above motor threshold: at 20 Hz for 2 s – 100 Hz (burst) for 2 s – 20 Hz for 3 s (1-ms pulses). Extra contractions were generated if isometric ankle torque was larger after the 100-Hz burst (at 6 s) than before it (at 1 s). The peak-to-peak amplitude of each soleus H-reflex and M-wave was quantified during 20-Hz stimulation, and mean amplitudes were also compared at 1 s and 6 s (average of responses within 400 ms of selected time). Overall (N=6), subsequent to the first H-reflex (~50% Mmax), reflexes were typically depressed during the initial 20-Hz stimulation. From before to after the 100-Hz burst, the mean H-reflex increased from 12 to 21 % Mmax (P<0.05) and the corresponding mean torque increased from 4 to 10 % MVC (P<0.01), while the mean M-wave of 2 % Mmax was unchanged. We conclude that increased spinal reflex responses contribute to the extra plantarflexion. Since motoneurons are reflexively recruited, presumably in a normal physiological order, the implication is that such contractions may be more fatigue resistant than those due to directly stimulating motor axons.