Stimulus pulse-width influences H-reflex recruitment but not H(max)/M(max) ratio.

It has been proposed that pulse-widths of 0.5-1.0 ms should be used to evoke H-reflexes in humans; however, the influence of pulse-width on H-reflex recruitment over a range of stimulus intensities has not been well characterized. We constructed soleus H-reflex vs. M-wave recruitment curves using 50, 200, 500, and 1000 micros pulses in 12 subjects. In contrast to previous findings, changing the pulse-width did not significantly alter maximal H-reflex (H(max)) or M-wave (M(max)) amplitudes or H(max)/M(max) ratios. In fact, the 1000 micros pulses resulted in larger H-reflexes when the M-wave was 5% M(max); smaller M-waves at H(max); and lower H-reflex thresholds compared with 50 micros pulses. These differences reflect a leftward shift in the H-reflex vs. M-wave recruitment curve when using wide vs. narrow pulses and, combined with no change in the H(max)/M(max) ratios, suggest that factors other than antidromic collision in motor axons limit H(max). These results support the idea that 1,000 micros pulses should be used to evoke H-reflexes and suggest that wider pulses may be beneficial to generate contractions with a greater reflex contribution when using neuromuscular stimulation for rehabilitation.