Low Oxygen Therapy to Enhance Walking Recovery After SCI.

Inclusion Criteria:

* 18 to 80 years of age
* medically stable with medical clearance from study physician to participate
* SCI at or below C1 and at or above L2 with at least some sensory or motor function preserved below the neurologic level
* non-progressive etiology of spinal injury
* American Spinal Injury Association (ASIA) scores of C-D at initial screen
* ambulatory (able to complete the 10-meter walk test without support from another person)
* chronic injury (define as \> 12 months post-injury) to avoid potential for spontaneous neurological plasticity and recovery

Exclusion Criteria:

* severe concurrent illness or pain, including unhealed decubiti, severe neuropathic or chronic pain syndrome, severe infection (e.g., urinary tract), hypertension, cardiovascular disease, pulmonary disease, severe osteoporosis, active heterotopic ossification in the lower extremities, severe systemic inflammation
* \< 24 on Mini-Mental Exam
* severe recurrent autonomic dysreflexia
* history of severe cardiovascular/pulmonary complications including hypertension (systolic blood pressure \> 150 mmHg)
* pregnancy because of unknown effects of AIH or tSTIM on a fetus (individuals of childbearing potential will not otherwise be excluded)
* botulinum toxin injections in lower extremity muscles within the prior three months
* history of tendon or nerve transfer surgery in the lower extremity
* untreated severe sleep-disordered breathing characterized by uncontrolled hypoxia and sleep fractionation that may impact the outcome of this study.
* active implanted devices (e.g., intrathecal baclofen pump)
* receiving concurrent electrical stimulation
* motor threshold evoked by transcutaneous spinal stimulation \>200 mA