Influence of phase duration and waveform on the relative recruitment of motor and sensory axons in a human peripheral nerve

Electrical stimulation can be used to stimulate human peripheral nerves, for some applications sensory axons are the targets, for others the targets are motor axons. Presently, we assessed the influence of two stimulus parameters, phase duration and waveform, on the relative recruitment of sensory versus motor axons. Four monophasic pulses (0.1, 0.5, 1.0, and 2.0 ms phase durations), two square biphasic pulses (0.125 and 0.5 ms phase durations) and two sinusoidal biphasic kilohertz frequency alternating current pulses (KFAC: 0.1, 0.5 ms phase durations), were tested in twenty participants. Pulses were delivered to generate soleus M-wave versus H-reflex recruitment curves (<i>n</i> = 40 stimuli) and, in separate trials, to produce M-waves that were ∼5% of the maximal M-wave (<i>n</i> = 20 stimuli). Changes in the amplitude of H-reflexes, relative to M-waves, between pulses provided measures of the relative recruitment of sensory versus motor axons. There was a significant effect of phase duration, but not waveform, on most of outcome measures that was consistent with a preferential recruitment of sensory over motor axons when using pulses with longer phase durations. The phase duration of a stimulus pulse, but not its waveform, influenced the relative recruitment of sensory and motor axons in the human tibial nerve. For monophasic, biphasic, and sinusoidal KFAC waveforms, short phase durations preferentially target motor axons, and longer phase durations target sensory axons. Our findings may help clinicians to better understand the impact of phase duration and waveforms on ES and to design more rational stimulation strategies.