Peripheral nerve sensory fibers are recruited with less current and greater selectivity by conventional waveforms...

Transcutaneous spinal stimulation aims to target dorsal spinal roots, which deliver sensory information to the spinal cord. However, the stimulation waveform that most effectively recruits sensory fibers at the lowest intensity has not been identified. The purpose of this study was to compare conventional and high-frequency burst-modulated stimulation waveforms in their ability to recruit sensory fibers, as assessed by H-reflex threshold, recruitment characteristics, and motor fiber activation. In participants with intact neurological function (n = 12), soleus H-reflex recruitment curves were recorded for 10 stimulation waveforms: a conventional waveform (400 μs), serving as the reference condition, high-frequency burst-modulated waveforms (2, 5, and 10 kHz) with the same total phase duration, and a longer conventional waveform (1,000 μs), each delivered as biphasic and monophasic pulses. H-reflex threshold was higher for high-frequency waveforms: 2 kHz (biphasic: +63%, monophasic: +35%); 5 kHz (biphasic: +179%, monophasic: +109%); 10 kHz (biphasic: +307%, monophasic: +249%); and lower for the conventional 1,000 μs waveform (biphasic: -39%, monophasic: -44%). Similarly, recruitment curve peak slope (mV/mA) was less steep for high-frequency waveforms: 2 kHz (biphasic: -43%, monophasic: -40%); 5 kHz (biphasic: -77%, monophasic: -40%); 10 kHz (biphasic: -82%, monophasic: -48%); and increased (+38%) for the monophasic conventional 1,000 μs waveform. At stimulus intensity near the H-reflex threshold, M-wave amplitude was larger for high-frequency waveforms. Overall, high-frequency waveforms were less effective than conventional waveforms of equal phase duration at eliciting H-reflexes and showed more motor activation at near-threshold intensities.NEW & NOTEWORTHY Compared with high-frequency burst-modulated waveforms (2, 5, and 10 kHz), conventional waveforms (400 μs) elicited H-reflexes more efficiently, as evidenced by lower thresholds and steeper recruitment curves, and less motor activation, as evidenced by smaller M-waves near threshold and larger H_max/M_max ratios. Monophasic 5 and 10 kHz waveforms also produced lower H-reflex thresholds and steeper slopes than their biphasic versions, suggesting that the anodal phase impairs sensory fiber recruitment with high-frequency stimulation.