Differential modulation of trunk muscle activation using thoracic epidural spinal stimulation

<i>Objective</i>. Epidural spinal stimulation (ESS) has demonstrated promising functional improvements in trunk control following spinal cord injury (SCI). However, previous ESS studies targeting trunk muscle activation have been limited to stimulation over the eleventh thoracic to first lumbar vertebral levels, which may not be optimal based on anatomical evidence regarding trunk muscle innervation. In this light, the objective of this study was to investigate trunk muscle activity in response to ESS at varying stimulation locations above the thoracic spine.<i>Approach.</i>An electrode array was implanted above the thoracic spine of 13 participants. ESS-evoked responses in trunk muscles were quantified while stimulation location along the rostrocaudal and mediolateral axes of the spine was systematically manipulated.<i>Main results.</i>Ipsilateral ESS between the T6 and T10 vertebrae induced responses in all trunk muscles, resulting in average motor thresholds (MTs) and latencies of abdominal muscles ranging from 1.5 to 2.0<i>µ</i>C and 7.4 to 9.2 ms, respectively; however, stimulation between the T8 and T10 vertebrae demonstrated lower MTs and shorter latencies. Ipsilateral stimulation resulted in 2.4 times greater maximum response amplitudes, 30% lower MTs, and 0.9 ms shorter latencies compared to contralateral stimulation.<i>Significance</i>. Our study provides quantitative evidence on the differential effects of ESS amplitude and location on trunk muscle activity while also suggesting that both afferent and efferent pathways contribute to ESS-evoked muscle activation. The results enhance our fundamental understanding of ESS-induced trunk muscle activity and have the potential to guide electrode placement for future therapeutic or restorative applications toward improving trunk control following SCI.