Stimulus pulse‐width influences H‐reflex recruitment but not Hmax/Mmax ratio

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Abstract
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 μs pulses in 12 subjects. In contrast to previous findings, changing the pulse‐width did not significantly alter maximal H‐reflex (Hmax) or M‐wave (Mmax) amplitudes or Hmax/Mmax ratios. In fact, the 1000 μs pulses resulted in larger H‐reflexes when the M‐wave was 5% Mmax; smaller M‐waves at Hmax; and lower H‐reflex thresholds compared with 50 μs 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 Hmax/Mmax ratios, suggest that factors other than antidromic collision in motor axons limit Hmax. These results support the idea that 1000 μs 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. Muscle Nerve, 2007