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 do moderate arm-crank exercise to those who receive a sham stimulation and do moderate arm-crank exercise to see if tSCS is effective at improving cardiovascular and autonomic functioning in those with SCI.

Inclusion Criteria:

• Are between 21-65 years of age.

• Chronic SCI (non-progressive, with complete motor paralysis) at or above the T6 spinal cord segment.

• Greater than ten months post injury or diagnosis, at least 6 months from any spinal surgery.

• American Spinal Injury Association Impairment Scale (AIS) A, B for SCI.

• Have stable medical condition without cardiopulmonary disease that would contraindicate participation in intensive training or testing activities.

• Cleared for strenuous exercise with arm bike by a primary physician of the subject (the clearance for intensive training with arm bike is obtained verbally or through a written form by a primary physician.)

• Willing and able to comply with all clinic visits and study-related procedures.

• Able to understand and complete study-related questionnaires (must be able to understand and speak English or have access to an appropriate interpreter as judged by the investigator).

• Have no painful musculoskeletal dysfunction, unhealed fracture, pressure sore, or active infection that may interfere with testing.

• Are with stable management of spinal cord related clinical issues (i.e., spasticity management).

• Women of childbearing potential must not be intending to become pregnant, currently pregnant, or lactating. The following conditions apply:

  -----Women of childbearing potential must agree to use adequate contraception during the period of the trial and for at least 28 days after completion of treatment. Effective contraception includes abstinence.

• Sexually active males with female partners of childbearing potential must agree to use effective contraception during the period of the trial and for at least 28 days after completion of treatment.

• Are volunteering to be involved in this study.

• Has adequate social support to be able to participate in training and assessment sessions, up to 3 sessions per week, for the duration of up to 6 months within the study period.

• Must provide informed consent.

Exclusion Criteria:

• Have autoimmune etiology of spinal cord dysfunction/injury
• Have history of additional neurologic disease, such as stroke, MS, traumatic brain injury, etc.
• Have peripheral neuropathy (diabetic polyneuropathy, entrapment neuropathy, etc.)
• Have rheumatic diseases (rheumatoid arthritis, systemic lupus erythematosus, etc.)
• Are ventilator dependent.
• Have clinically significant, unmanaged, depression, ongoing alcohol and/or drug abuse.
• Use any medication or treatment that in the opinion of the investigators indicates that it is not in the best interest of the participant to participate in this study.
• Have Intrathecal baclofen pump.
• Have cardiovascular, respiratory, bladder, or renal disease unrelated to SCI or presence of hydronephrosis or presence of obstructive renal stones.
• Have severe acute medical issue that in the investigators' judgement would adversely affect the participant's participation in the study. Examples include, but are not limited to acute urinary tract infections, active heterotopic ossification, newly changed antidepressant medications [tricyclics], debilitating muscle pain, pressure sores, or unstable diabetes.
• Have pacemakers, stimulators, medication pumps in the trunk, deep brain stimulators, metallic devices in the head such as aneurysm clips/coils and stents, vagus nerve stimulators.
• Take more than 40 mg of Baclofen per day
• Have severe anemia (Hgb<8 g/dl or Hgb,80 g/L) or hypovolemia as measured by hematocrit via blood test in the last six months.
• Are a member of the investigational team or his/her immediate family.
• Have undergone electrode implantation surgery.
• Have signs of lower motor neuron impairment (i.e., concomitant conus medullaris/cauda equina injury).
• Have history of severe allergy (i.e. allergic reaction that could not be treated with antihistaminic medication)
• Have malabsorption syndrome, primary hyperthyroidism, and/or hypogonadism.
• Have a history of seizures.
• Have chronic headaches or migraines.