Transcutaneous Spinal Cord Stimulation Combined With Arm Bike for Cardiovascular Recovery in SCI

Spinal cord injury (SCI) can make it hard for the body to self-regulate some of its automatic functions like blood pressure, breathing, and heart rate. This can also make it hard for those living with SCI to exercise or complete their usual daily activities. The goal of this randomized trial is to test combinatory therapy of moderate arm-crank exercise paired with non-invasive transcutaneous spinal cord stimulation (tSCS) for cardiovascular recovery in adults aged 21-65 following chronic motor-complete spinal cord injury (SCI) at or above the thoracic sixth spinal segment (≥T6). The main questions the study aims to answer are: * Conduct tSCS mapping to determine the most effective location and stimulation intensity for BP control in individuals with motor-complete SCI ≥ T6. * Evaluate the effects 8 weeks of targeted tSCS paired with arm-crank exercise compared to sham stimulation with exercise on improving cardiovascular function in individuals with motor-complete SCI ≥T6. * Evaluate the dosage-response of 8 weeks vs. 16 weeks of targeted tSCS paired with arm-crank exercise on cardiovascular function in individuals with motor-complete SCI ≥T6. * Explore the mechanisms involved in cardiovascular recovery with long-term tSCS paired with arm-crank exercise. Participants will: * Receive either transcutaneous spinal cord stimulation or "sham" spinal cord stimulation while exercising on an arm-crank bicycle in the first 8 weeks. * Come in for approximately 60 visits over a 6-month period. This includes 2, 8-week periods where the investigators will ask participants to come in 3x per week for spinal cord stimulation and exercise. * During assessment visits the researchers will perform a variety of exams including a neurologic, cardiovascular, pulmonary, physical, and autonomic exam, and will ask questions about quality of life and functioning. Assessment procedures also include stool sample and nasal swab collection. Researchers will compare those who receive tSCS and