Transcutaneous Spinal Stimulation and Exercise for Locomotion

Inclusion Criteria: * Has spinal cord injury (T12 or higher level) of at least 1-year duration * Is between 21 and 70 years of age * Has difficulty with leg functions and mobility in activities of daily living (e.g. walking, transferring surface to surface, standing) * Has stable medical condition without cardiopulmonary disease or frequent autonomic dysreflexia that would contraindicate participation in lower extremity rehabilitation or testing activities * Is capable of performing simple cued motor tasks * Has ability to attend up to 5 sessions per week physical therapy sessions and testing activities * Has adequate social support to be able to participate in training and assessment sessions, up to 5 sessions per week, for the duration of up to 11 months within the study period * Is volunteering to be involved in this study * Cleared for gait training by the primary physician of the subject * Has the ability to read and speak English Exclusion Criteria: * Has autoimmune etiology of spinal cord dysfunction/injury * Has history of additional neurologic disease, such as stroke, multiple sclerosis, traumatic brain injury, etc. * Has peripheral neuropathy (diabetic polyneuropathy, entrapment neuropathy, etc.) * Has rheumatic diseases (rheumatoid arthritis, systemic lupus erythematosus, etc.) * Has significant medical disease; including uncontrolled systemic hypertension with values above 170/100 mmHg; cardiac or pulmonary disease; uncorrected coagulation abnormalities or need for therapeutic anticoagulation. * Has active cancer * Has cardiovascular or musculoskeletal disease or injury that would prevent full participation in physical therapy intervention * Has unhealed fracture, contracture, pressure sore, urinary tract infection, or other illnesses that might interfere with lower extremity rehabilitation or testing activities * Has any condition that would render the patient unable to safely cooperate with the study tests as judged by the screening physician * is pregnant * is dependent on ventilation support * Has implanted stimulator (e.g. epidural stimulator, vagus nerve stimulator, pacemaker, cochlear implant, etc) * Has depression or anxiety based on the Center for Epidemiologic Studies Depression Scale (CESD) (score \>16/60) and General Anxiety Disorder-7 item Questionnaire (score \>9/21), respectively * Has alcohol and/or drug abuse * Has cognitive impairment based on Short Portable Mental Status Questionnaire (SPMSQ) (score \>2/10) * is unable to read and/or comprehend the consent form * is unable to understand the instructions given as part of the study * Has established osteoporosis and taking medication for osteoporosis treatment * Has bone mineral density T scores ≤ -3.5 or Z scores ≤ -2.5 in the anteroposterior lumbar spine and/or proximal femur measured by a DEXA scan for subjects weight bearing and ambulating outside of a body-weight supporting exoskeleton (if participants are clear for other criteria and have not been examined by Dual Energy Xray Absorptiometry (DEXA) at the screening, we will ask participants to undergo DEXA to reveal any risk of weight bearing activity) * Has low-energy fracture history before or after spinal cord injury * History of orthopedic surgery in lower extremities that may be a confounding factor for interpretation of the results (such as tendon transfer, tendon or muscle lengthening for spasticity management, arthrodesis, etc.) * fixed lower extremity joint contractures * is taller than 80 inches and/or more than 350 pounds body weights, due to the limitation of the experimental equipment such as the body weight support system * lacks the ability to fully comprehend and/or perform study procedures in the investigator's opinion/judgement * has history of severe allergy (i.e. allergic reaction that could not be treated with antihistaminic medication) * has malabrosption syndrome, primary hyperthyroidism, and/or hypogonadism * has a history of seizures * has chronic headaches or migraines * has deep brain stimulators, metallic devices in the head such as aneurysm clips/coils and stents, vagus nerve stimulators