Bipolar transcutaneous spinal stimulation evokes short-latency reflex responses in human lower limbs alike standard unipolar electrode configuration.

Noninvasive electrical stimulation targeting the posterior lumbosacral roots has been applied recently in reflexes studies and as a neuromodulation intervention for modifying spinal cord circuitry after an injury. Here, we characterized short-latency responses evoked by four bipolar electrode configurations placed longitudinally over the spinal column at different vertebral levels from L1 to T9. They were compared with the responses evoked by the standard unipolar (aka monopolar) electrode configuration (cathode at T11/12, anode over the abdominal wall). Short-latency responses were recorded in the rectus femoris, medial hamstrings, tibialis anterior, and soleus muscles, bilaterally, in 11 neurologically intact participants. The response recruitment characteristics (maximal amplitude, motor threshold) and amplitude-matched onset latencies and paired-pulse suppression (35-ms interstimulus interval) were assessed with 1-ms current-controlled pulses at intensities up to 100 mA. The results showed that short-latency responses can be elicited with all bipolar electrode configurations. However, only with the cathode at T11/12 and the anode 10 cm cranially (∼T9), the maximum response amplitudes were statistical equivalent (P < 0.05) in the medial hamstrings, tibialis anterior, and soleus but not the rectus femoris, whereas motor thresholds were not significantly different across all muscles. The onset latency and paired-pulse suppression were also not significantly different across the tested electrode configurations, thereby confirming the reflex nature of the bipolar short-latency responses. We conclude that the bipolar configuration (cathode T11/12, anode ∼T9) produces reflex responses that are ostensibly similar to those evoked by the standard unipolar configuration. This provides an alternative approach for neuromodulation intervention.NEW & NOTEWORTHY Transcutaneous spinal stimulation with the identified bipolar electrode configuration may offer several advantages for neuromodulation interventions over commonly used unipolar configurations: there are no associated abdominal contractions, which improves the participant's comfort; additional dermatomes are not stimulated as when the anode is over the abdominal wall or iliac crest, which may have unwanted effects; and, due to a more localized electrical field, the bipolar configuration offers the possibility of targeting cord segments more selectively.