Title: Effects of Perturbation-Based Balance Training and Transcutaneous Spinal Cord Stimulation on Postural Balance Control in Healthy Subjects
Description: The purpose of this dissertation was to explore methods for generating neuroplastic changes in healthy individuals using transcutaneous spinal cord stimulation (TSCS) and perturbation-based training in order to improve balance performance. This was done to gain an understanding of their effects on healthy individuals, which could then be used in designing treatments for both healthy and motor-impaired subjects. Three studies were undertaken. First, we set out to show that the Robotic Upright Stand Trainer (RobUST) could generate improvements in balance after perturbation balance training (PBT). In this same study, we showed that the assist-as-needed support of RobUST generates postural control improvements. Balance performance metrics including (i) margin of stability (MOS), (ii) metrics based on the center of pressure (COP) and center of mass (COM) excursions, (iii) postural muscle activations, (iv) balance strategy selection (between ankle and hip strategies) were used in this study.

Electromyographic data were also collected from 11 subjects who participated in this study. Subjects were split into a RobUST assisted group (FF) and a non-assisted group (NF). An analysis of variance (ANOVA) was carried out to identify the main effects of the two factors, i.e., training and grouping. We also studied the interaction effects between the two factors in the performance variables. After training, the threshold of the forces that destabilize balance increased for all participants. In addition, the area within which they could withstand perturbations without falling also increased. Muscle activation decreased in most muscles for subjects in both groups indicating that subjects improved balance while demonstrating more energetically efficient strategies. The post-training behavior of the two groups differed in the following way: the NF group adapted towards faster reactions to perturbations, greater use of the hip strategy, and more use of the erector spinae muscle, while the FF group adapted towards slower responses and less MOS.

These results show that although balance adaptations with RobUST-assisted PBT are not the same as without RobUST, it is still a platform capable of improving balance performance. Second, the effect of TSCS as a means of boosting neuroplasticity and a replacement for epidural stimulation were tested. Eight subjects were given TSCS for 30 mins while lying supine, and their neurophysiological and balance performance measures were tested before and after the intervention. T-tests were used to assess the difference in performance, and it was found that TSCS caused hypopolarisation of the sensory neurons, which increased the synaptic efficacy of sensory afferent–motoneuron synapses. This change was evidenced by increased H-reflex recovery and a leftward shift of the H-reflex recruitment curve. No improvement in fall frequency was observed, although balance adjustments were made that reduced muscle activity. This experiment showed that TSCS could be used to modulate the excitability of the spinal cord in healthy subjects.

Third, TSCS was combined with a training intervention in order to study how these two sources of plasticity interact. TSCS was applied to eleven subjects while they underwent a training intervention in which they played a game in virtual reality (VR) while their balance was perturbed by forces applied by RobUST. Balance characteristics were measured both with and without TSCS, before and after the intervention. It was found that TSCS initially caused an increase in muscle activity and an increase in fall frequency for perturbations in the forward direction. With more practice, though, muscle activity decreased. It was postulated that the CNS adjusted to the initial elevated levels of muscle activity caused by TSCS by suppressing muscle activity in order to ensure successful motor control. These results suggest that TSCS can be used to elevate the resting potential of neurons in the dorsal (close to the back of the body) root, making them more easily excited by cortical signals. These changes induced by TSCS can be beneficial to spinal cord injury patients.