Transcutaneous spinal stimulation in people with and without spinal cord injury: Effect of electrode placement and trains of stimulation on threshold intensity

Harrison T Finn; Elizabeth A Bye; Thomas G Elphick; Claire L Boswell-Ruys; Simon C Gandevia; Jane E Butler; Martin E Héroux

doi:10.14814/phy2.15692

Transcutaneous spinal stimulation in people with and without spinal cord injury: Effect of electrode placement and trains of stimulation on threshold intensity

Physiol Rep. 2023 Jun;11(11):e15692. doi: 10.14814/phy2.15692.

Authors

Harrison T Finn^{1

2}, Elizabeth A Bye^{1

2

3}, Thomas G Elphick^{1

2}, Claire L Boswell-Ruys^{1

2

3}, Simon C Gandevia^{1

3

4}, Jane E Butler^{1

2}, Martin E Héroux^{1

2}

Affiliations

¹ Neuroscience Research Australia, Randwick, New South Wales, Australia.
² School of Biomedical Sciences, University of New South Wales, New South Wales, Kensington, Australia.
³ Prince of Wales Hospital, Randwick, New South Wales, Australia.
⁴ School of Clinical Medicine, University of New South Wales, New South Wales, Kensington, Australia.

Abstract

Transcutaneous spinal cord stimulation (TSS) is purported to improve motor function in people after spinal cord injury (SCI). However, several methodology aspects are yet to be explored. We investigated whether stimulation configuration affected the intensity needed to elicit spinally evoked motor responses (sEMR) in four lower limb muscles bilaterally. Also, since stimulation intensity for therapeutic TSS (i.e., trains of stimulation, typically delivered at 15-50 Hz) is sometimes based on the single-pulse threshold intensity, we compared these two stimulation types. In non-SCI participants (n = 9) and participants with a SCI (n = 9), three different electrode configurations (cathode-anode); L1-midline (below the umbilicus), T11-midline and L1-ASIS (anterior superior iliac spine; non-SCI only) were compared for the sEMR threshold intensity using single pulses or trains of stimulation which were recorded in the vastus medialis, medial hamstring, tibialis anterior, medial gastrocnemius muscles. In non-SCI participants, the L1-midline configuration showed lower sEMR thresholds compared to T11-midline (p = 0.002) and L1-ASIS (p < 0.001). There was no difference between T11-midline and L1-midline for participants with SCI (p = 0.245). Spinally evoked motor response thresholds were ~13% lower during trains of stimulation compared to single pulses in non-SCI participants (p < 0.001), but not in participants with SCI (p = 0.101). With trains of stimulation, threshold intensities were slightly lower and the incidence of sEMR was considerably lower. Overall, stimulation threshold intensities were generally lower with the L1-midline electrode configuration and is therefore preferred. While single-pulse threshold intensities may overestimate threshold intensities for therapeutic TSS, tolerance to trains of stimulation will be the limiting factor in most cases.

Keywords: spinal cord injury; transcutaneous spinal stimulation.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Electrodes
Humans
Muscle, Skeletal / physiology
Quadriceps Muscle
Spinal Cord Injuries* / therapy
Spinal Cord Stimulation* / methods