• Change in 10 Meter Walk Test [ Time Frame: Session 1 (baseline test, initial visit), After 5 intervention sessions (Post Test), 1 week post (1 week follow up Post) ]
  This test will examine the patient's gait speed. Patients will be directed to walk at their preferred maximum but safe speed.
• Change in 6 Minute Walk Test Using VO2 Analysis [ Time Frame: Session 1 (baseline test, initial visit), After 5 intervention sessions (Post Test), 1 week post (1 week follow up Post) ]
  The 6 Minute Walk Test (6MWT) measures the distance a subject can walk indoors on a flat, hard surface in a period of 6 minutes, using assertive devices, as necessary. The test is a reliable and valid evaluation of functional exercise capacity and is used as a sub-maximal test of aerobic capacity and endurance. The test will be used to determine participant's gait efficiency at baseline and at study completion. The walk test is patient self-paced and assesses the level of functional capacity. Patients are allowed to stop and rest during the test, however, the timer does not stop. If the patient is unable to complete the time, the time stopped is noted and reason for stopping prematurely is recorded. This test will be administered while wearing a portable gas analysis system called the Cosmed K4B2 to measure oxygen consumption. This system measures oxygen consumption (VO2) and Carbon-dioxide production (VCO2) in a breath by breath fashion.

• Change in 10 Meter Walk Test [ Time Frame: Session 1 (baseline test, initial visit), Session 16 (Midpoint, 4 weeks), Session 30 (Post intervention, week 8), Session 33 (3 month follow-up), Session 35 (6 month follow-up), Session 37 (12 month follow-up) ]
  This test will examine the patient's gait speed. Patients will be directed to walk at their preferred maximum but safe speed.
• Change in 6 Minute Walk Test Using VO2 Analysis [ Time Frame: Session 1 (baseline test, initial visit), Session 16 (Midpoint, 4 weeks), Session 30 (Post intervention, week 8), Session 33 (3 month follow-up), Session 35 (6 month follow-up), Session 37 (12 month follow-up) ]
  The 6 Minute Walk Test (6MWT) measures the distance a subject can walk indoors on a flat, hard surface in a period of 6 minutes, using assertive devices, as necessary. The test is a reliable and valid evaluation of functional exercise capacity and is used as a sub-maximal test of aerobic capacity and endurance. The test will be used to determine participant's gait efficiency at baseline and at study completion. The walk test is patient self-paced and assesses the level of functional capacity. Patients are allowed to stop and rest during the test, however, the timer does not stop. If the patient is unable to complete the time, the time stopped is noted and reason for stopping prematurely is recorded. This test will be administered while wearing a portable gas analysis system called the Cosmed K4B2 to measure oxygen consumption. This system measures oxygen consumption (VO2) and Carbon-dioxide production (VCO2) in a breath by breath fashion.

OBJECTIVES:

• To determine whether acute intermittent hypoxia therapy (AIH) combined with transcutaneous spinal cord stimulation during ambulation training modulates spinal locomotor networks in individuals with spinal cord injury
• To determine whether acute intermittent hypoxia therapy (AIH) combined with transcutaneous spinal cord stimulation during ambulation training improves locomotor function in individuals with spinal cord injury
• To determine whether acute intermittent hypoxia therapy (AIH) combined with transcutaneous spinal cord stimulation during ambulation training improves symmetry of gait in individuals with spinal cord injury
• To determine whether acute intermittent hypoxia therapy (AIH) combined with transcutaneous spinal cord stimulation during ambulation training improves standing posture and balance in individuals with spinal cord injury
• To determine whether ambulation efficiency (improved cardiovascular conditioning) improves with a combination of AIH, transcutaneous spinal cord stimulation and ambulation training in individuals with spinal cord injury

• Spinal Cord Injuries
• Spinal Cord Diseases

• Device: Acute Intermittent Hypoxia + Non Invasive Spinal Cord Stimulation + Gait Training
  Subjects will receive up to 45 minutes of AIH prior to receiving transcutaneous electrical spinal cord stimulation delivered by a Transcutaneous Spinal Cord Neurostimulator while performing locomotor activities.
  Other Name: AIH + tSCS+ Gait
• Other: Sham Acute Intermittent Hypoxia + Non Invasive Spinal Cord Stimulation + Gait Training
  Subjects will perform locomotor activities with transcutaneous electrical spinal cord stimulation after receiving sham acute intermittent hypoxia
  Other Name: SHAM AIH + tSCS + Gait
• Other: Sham Acute Intermittent Hypoxia + Sham Non Invasive Spinal Cord Stimulation + Gait Training
  Subjects will perform locomotor activities with sham transcutaneous electrical spinal cord stimulation after receiving sham acute intermittent hypoxia
  Other Name: SHAM AIH + SHAM tSCS + Gait

• Experimental: Acute Intermittent Hypoxia + Non Invasive Spinal Cord Stimulation + Gait Training

  May receive up to 45 minutes of AIH prior to up to 50 min of locomotion training with transcutaneous spinal cord stimulation. However, the amount of time spent in side-lying locomotion training, treadmill training and overground training will depend on individual tolerance and progression.

  Intervention: Device: AIH prior to Noninvasive spinal stimulation during gait training

  Intervention: Device: Acute Intermittent Hypoxia + Non Invasive Spinal Cord Stimulation + Gait Training
• Sham Comparator: Sham Acute Intermittent Hypoxia + Non Invasive Spinal Cord Stimulation + Gait Training
  May receive up to 45 minutes of Sham AIH prior to up to 50 min of locomotion training with transcutaneous spinal cord stimulation. However, the amount of time spent in side-lying locomotion training, treadmill training and overground training will depend on individual tolerance and progression.
  Intervention: Other: Sham Acute Intermittent Hypoxia + Non Invasive Spinal Cord Stimulation + Gait Training
• Sham Comparator: Sham Acute Intermittent Hypoxia + Sham Non Invasive Spinal Cord Stimulation + Gait Training
  May receive up to 45 minutes of Sham AIH prior to up to 50 min of locomotion training with sham transcutaneous spinal cord stimulation. However, the amount of time spent in side-lying locomotion training, treadmill training and overground training will depend on individual tolerance and progression.
  Intervention: Other: Sham Acute Intermittent Hypoxia + Sham Non Invasive Spinal Cord Stimulation + Gait Training

Inclusion Criteria:

• Participants have been diagnosed with a spinal cord injury below level C2
• ASIA Impairment Scale Grade A-D
• Participants are 18 years of age or older
• Participants are at least 6 months post spinal cord injury
• Participants with paraplegia or tetraplegia secondary to a single spinal cord injury
• Participants are able to provide informed consent
• Participants are not currently receiving regular physical therapy services
• Participants who demonstrate at least a 10% change in isometric or isokinetic plantarflexion, dorsiflexion or knee extension force above baseline after a single bout of Acute Intermittent Hypoxia (AIH) during screening

Exclusion Criteria:

• Individuals less than 18 years of age
• Individuals less than 6 months post spinal cord injury
• Individuals with ataxia
• Individuals with multiple spinal cord injury history
• Modified Ashworth score of 3 or greater in lower extremity
• Pregnancy or nursing
• Pacemaker or anti-spasticity implantable pumps
• Active pressure sores
• Unhealed bone fractures
• Peripheral neuropathies
• Painful musculoskeletal dysfunction due to active injuries or infections
• Severe contractures in the lower extremities
• Active urinary tract infection
• Clinically significant depression, psychiatric disorders, or ongoing drug abuse
• Diagnosed with any of the following medical conditions: congestive heart failure, cardiac arrhythmias, uncontrolled hypertension, uncontrolled diabetes mellitus, chronic obstructive pulmonary disease, emphysema, severe asthma, previous myocardial infraction, or known carotid/intracerebral artery stenosis
• Individuals with a tracheostomy or who utilize mechanical ventilation.
• Individuals who are currently enrolled in another interventional research study or in therapy related to upper extremity function.
• Participants will be excluded if they have had a botulinum toxin injection to lower extremity musculature within the last 3 months. Participants will need to refrain from lower extremity botulinum toxin injections for the duration of the study. If participants wean off antispasticity medications to successfully complete the responsiveness to AIH screening session, they will need to refrain from the medications for the duration of the study.
• Documented sleep apnea.
• Orthopedic injuries or surgeries that would impact an individual's ability to use the lower extremity.
• Traumatic brain injury or other neurological conditions that would impact the study.
• Blood hemoglobin levels less than 10g/dL.

We will not include the following populations:

• Adults unable to consent, unless accompanied by a legally authorized representative.
• Individuals who are not yet adults (infants, children, teenagers)
• Pregnant women
• Prisoners