Contribution of M-waves and H-reflexes to contractions evoked by tetanic nerve stimulation in humans Authors

Tetanic neuromuscular stimulation evokes contractions by depolarizing motor axons beneath the stimulating electrodes. However, we have shown that extra torque can develop due to the discharge of spinal neurons recruited by the evoked sensory volley. The present experiments investigated whether extra torque in the ankle plantarand dorsiflexors was associated with enhanced H-reflexes. The tibial and common peroneal nerves were stimulated using 7s trains (20Hz for 2s, 100Hz for 2s, 20Hz for 3s). Extra torque was defined as significantly more torque during 20Hz stimulation after the 100Hz burst (time2) than before it (time1). In 9 of 11 subjects, extra plantarflexion torque developed during stimulation just above motor threshold. In these 9 subjects, torque increased from 8-13% MVC (time1 to time2), the soleus H-reflex increased from 13-19% Mmax and the M-wave of ~2% Mmax did not change significantly. To evoke extra dorsiflexion torque, greater stimulation intensities were required. In 6 of 13 subjects, extra torque developed at intensities that evoked an M-wave of 5-20% Mmax at time1. In these 6 subjects, torque doubled from 2-4% MVC (time1 to time2) while tibialis anterior (TA) H-reflexes and Mwaves did not change significantly (H-reflex from 0.8-2% Mmax; M-wave from 12-14% Mmax). In 7 of 13 subjects, extra torque developed at higher stimulation intensities (35-65% Mmax). In these 7 subjects, torque increased from 13-20% MVC, while TA H-reflexes and M-waves were not significantly different (H-reflex from 0.7-1% Mmax; M-wave from 49-54% Mmax). Thus, enhanced H-reflexes contributed to extra plantarflexion, however, other factors generated extra dorsiflexion. Page 2 of 35