Excitability of posterior root reflex evoked by transcutaneous spinal stimulation is distinct from the H-reflex in the soleus muscle: a pulse duration study.

Transcutaneous spinal stimulation studies of the lumbosacral posterior root reflex (PRR) have commonly used a 1-ms pulse duration adopted from soleus Hoffmann reflex (H-reflex) protocols. However, it remains unknown whether the excitability characteristics of the PRR and H-reflex vary with pulse duration. Twelve neurologically intact participants underwent tibial nerve stimulation to evoke soleus M waves and H-reflexes, and lumbosacral transcutaneous spinal stimulation to evoke soleus PRRs. Recruitment curve data were collected across eight pulse durations (50-2,000 µs). Stimulation intensities corresponding to 50% of the maximum H-reflex were used to construct strength-duration curves for each response type. Rheobase and chronaxie were estimated using the Weiss-Lapicque equation. Maximum peak-to-peak amplitudes of the M wave, H-reflex, and PRR did not differ significantly across pulse duration (with increased stimulation intensity at shorter pulse durations). However, when normalized to the maximum M wave, the PRR was 12.3% larger than the H-reflex. The strength-duration curves were well described by a hyperbolic function, with the PRR exhibiting a substantially higher rheobase (50.8 mA) and shorter chronaxie (338 µs) than the H-reflex (6.7 mA; 874 µs) and M wave (12.5 mA; 545 µs). The maximum soleus PRR and H-reflex do not differ across pulse durations from 50 to 2,000 µs, although shorter pulses require higher stimulation intensities. The differences in PRR and H-reflex rheobase and chronaxie values are likely due to specific electrode configurations, tissue properties, and sites of activation. The excitability characteristics of the soleus PRR and H-reflex have implications for selecting stimulators and their parameters.NEW & NOTEWORTHY Direct comparison of the soleus PRR and H-reflex excitability reveals that the PRR activates a larger proportion of the motoneuron pool and exhibits a markedly higher rheobase and shorter chronaxie, reflecting differences in tissue properties and stimulation geometry. The shorter PRR chronaxie implies that PRR can be efficiently elicited with a pulse duration of less than 1 ms, as adopted from H-reflex protocols.