Characteristics of posterior root reflexes evoked by transcutaneous spinal stimulation with various pulse durations

Posterior root reflexes elicited by transcutaneous spinal stimulation (TSS) are useful for assessing spinal excitability and guiding neuromodulation interventions. Although various stimulation parameters have been extensively studied, the effect of pulse duration on reflex characteristics has not been thoroughly examined. This study systematically characterized posterior root reflexes across eight pulse durations ranging from 50 to 2,000 µs in 12 healthy participants using unipolar lumbosacral TSS (cathode over T11-12 processes, bilateral anode paraumbilically). In addition, in a subgroup of six participants, the repeatability of reflex characteristics over 2-3 mo was evaluated, and differences between the unipolar and bipolar configurations were examined. Recruitment curves in the major leg muscles reached similar plateau amplitudes across the pulse durations but shifted toward higher stimulation intensities with shorter pulses. The strength-duration curves for the motor threshold intensity were similar across muscles, with an average rheobase of 44.4 mA and a chronaxie of 362.9 µs. The strength-duration curves corresponding to 90% of the recruitment plateau revealed a 24.8 mA higher rheobase and only a 46.9 µs shorter chronaxie. Onset latencies of amplitude-matched reflexes increased 0.81 ms from 50 to 2,000 µs. Paired-pulse suppression demonstrated minimal dependency on pulse duration, although some muscle-specific variations were observed. The two ancillary experiments demonstrated good test-retest repeatability of the unipolar configuration and higher rheobase without significant differences in chronaxie with the bipolar configuration. We conclude that a wide range of pulse durations can produce posterior root reflexes when the stimulation intensity is properly adjusted. These findings offer a framework for selecting stimulation parameters for electrical neuromodulation.<b>NEW & NOTEWORTHY</b> The study demonstrates the impact of pulse duration on posterior root reflex characteristics across the dynamic range of the recruitment curve. It shows that strength-duration parameters-rheobase and chronaxie-are specific to the transcutaneous spinal stimulation settings and cannot be generalized from peripheral nerve H-reflex studies. The adequate reliability of the unipolar configuration is relevant for longitudinal studies, whereas the similarity between the unipolar and bipolar configurations indicates that they are complementary.