Stimulus pulse-width influences H-reflex recruitment but not Hmax/Mmax 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 mu s 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 mu s 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 mu s 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 1000 mu s 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.