Differential Modulation of Trunk Muscle Activation using Thoracic Epidural Spinal Stimulation
Creators: Schadan, Sydney M
Publisher: name: University of Alberta Library
Year: 2024

<p>Trunk stability and control in a world of gravitational forces and perturbations requires coordinated trunk muscle activation through involvement of control mechanisms within the neuromuscular system. Individuals with spinal cord injury (SCI) often suffer under impaired trunk control and posture, which may lead to physical limitations, including the inability to sit upright and perform common reaching tasks. Such impairment can also result in secondary health complications, particularly pressure sores, kyphosis, or compromised internal organ function. Epidural spinal stimulation (ESS) delivers electrical current to the dorsal spine to selectively activate motor pools within the spine, allowing for targeted muscle activation and improved functional outcomes following SCI. ESS has been used to evoke trunk muscle responses and demonstrated promising functional improvements in trunk control and posture following SCI, including improved trunk extension, and forward and lateral reaching. Current anatomical knowledge suggests that trunk muscles are innervated within the upper and lower thoracic regions of the spine. In spite of this, previous studies employing ESS for the purpose of trunk muscle activation have been limited to stimulation above the twelfth thoracic (T12) to first lumbar (L1) vertebral levels. Moreover, the majority of work to date has focused on characterizing functional outcomes, with less emphasis given to advancing our systematic understanding of the muscle responses to ESS. Therefore, the objective of this thesis was to further our understanding of the relationship between epidural spinal stimulation and activation of spinal circuitry and muscle activation in the domain of trunk stability and control. Within this domain, a study was performed to investigate trunk muscle activation in response to ESS at varying stimulation locations above the thoracic spine. A 16- or 32- electrode array was implanted above two adjacent vertebra levels within the fourth thoracic (T4) to tenth thoracic (T10) vertebral range of 13 participants. Data from 11 participants were analyzed. Electromyography data were collected bilaterally from the external obliques, internal obliques, and rectus abdominis, as well as unilaterally from the erector spinae muscles at one thoracic (T7) and one lumbar level (L3). The amplitude and timing of evoked muscle responses were quantified while stimulation location along both the rostrocaudal and mediolateral axes of the spine was systematically manipulated. ESS delivered between the T6 and T10 vertebrae evoked responses in all trunk muscles resulting in average motor thresholds and onset latencies of abdominal muscles from ipsilateral stimulation ranging from 1.5 to 2.0 µC and 7.4 to 9.2 ms, respectively; however, stimulation between the T8 and T10 vertebrae demonstrated lower motor thresholds and shorter onset latencies. ESS evoked responses in both ipsilateral and contralateral muscles; however, on average 2.4 times greater maximum response amplitudes, 30% lower motor thresholds, and 0.9 ms shorter onset latencies were seen for ipsilateral stimulation compared to contralateral stimulation. All abdominal muscles demonstrated constant onset latencies with increasing charge applied, which is consistent with activation of afferent pathways. The presence of shorter onset latencies at higher applied charges in the responses recorded in the erector spinae muscles provided evidence for activation of both afferent and efferent pathways. The results from this study demonstrate the influence of stimulation location on trunk muscle activation via thoracic ESS. The results may guide electrode placement for future rehabilitative applications aiming to improve trunk control and posture following SCI via ESS. Future work is required to determine the transferability of results to less-invasive spinal stimulation approaches. </p>