The Effect of Exoskeletal-assisted Walking Combined With Transcutaneous Spinal Cord Stimulation on Bone Strength.

Background: Immobilization results in bone loss that predisposes to osteoporosis and fracture, which may be complicated by non-union, infection, and deep venous thrombosis. Reduced muscular contraction after SCI and the elevated release of cortisol contribute to a catabolic state, resulting in a loss of lean tissue mass (LTM) below the level of lesion. Six months after motor-complete SCI, the average muscle cross-sectional area (CSA) significantly decreases at the quadriceps, hamstrings, and hip adductors (14-16%), and 12% and 24% at the soleus and gastrocnemius, respectively. Following SCI, the quadricep muscles generate less total force and force per unit area when evoked by surface electrode electrical stimulation. This loss of muscle CSA and strength in the lower extremities limits the ability to stand, ambulate, and preserve bone - even if neural regenerative strategies could be implemented in the future. In addition to the marked skeletal muscle atrophy, persons with non-ambulatory motor-complete SCI also experience a precipitous loss of bone mineral content (BMC) and bone mineral density (BMD) by as much as 1% per week below the level of lesion. In individuals with motor-incomplete lesions who have not reached their ambulatory potential, there is still considerable bone loss due to immobilization that can reach the fracture threshold years after injury. This rapid bone loss during the first two years after SCI results in volumetric BMD (vBMD) at the DF and PT decreasing by \~ 50% and 26% at the trabecular and cortical compartments, respectively. During the chronic phase of SCI bone loss continues more slowly throughout the individuals lifetime. This loss in muscle and bone places individuals with SCI at high risk for fragility fracture. More than 50% of individuals with SCI experience a fragility fracture over the course of their lifetimes. Objectives: Aim 1: To compare the effects of 108 sessions of EAW + sham tSCS versus EAW + active tSCS on the muscle-bone unit in wheelchair-dependent chronic SCI participants. Aim 2 (exploratory): To determine the acute time-course responses for serum/plasma biomarkers of bone resorption and formation, muscle contractile activity, and the mRNA profiles of circulating exosomes collected prior to (time 0), and again 30, 60, 120, 180, minutes and 24, and 48 hours following an acute session of both the EAW + active tSCS and EAW + sham tSCS training interventions. Setting: Participant enrollment, the clinical trial intervention (EAW + sham tSCS versus EAW + active tSCS), EMG data collection, dual energy X-ray absorptiometry (DXA), peripheral quantitative computed tomography (pQCT), magnetic resonance imaging (MRI) to measure the cross-sectional area of the mid-thigh, and the time-course responses for serum/plasma biomarkers of bone resorption and formation and muscle contractile activity will be performed at the Kessler Foundation and the James J. Peters VA Medical Center. Design: After meeting eligibility criteria, wheelchair users with chronic SCI will be block randomized into the EAW + active tSCS group or the EAW + sham tSCS group (n=12 in each group). Both groups will receive 60 minutes of EAW overground training per session for a total of 108 sessions (3 X week for 36 weeks). In addition to the EAW training, the EAW + active tSCS group will receive simultaneous lumbosacral tSCS targeted to activate the locomotor central pattern generator. Participants: Twenty-four participants (12 participants/group) with SCI will be recruited over a 4-year period and randomly assigned to an EAW + sham tSCS or EAW + active tSCS group. At the end of the first year, approximately 3 participants will have completed the protocol. Outcome measures: At baseline, the investigators will perform imaging to measure bone density and strength, surface EMG to assess muscle contractility, and a time-course response for serum muscle and bone biomarkers following an acute bout of EAW. The investigators will capture these same data again after \~54 training sessions (mid-point), and after 108 training sessions (month 9 timepoint). In addition, MRI of both legs for muscle CSA will be performed at the baseline and month 9 time point.