Acute intermittent hypoxia (AIH) involves short (~1-2min) bouts of breathing low oxygen air to stimulate spinal neuroplasticity. Studies in rodents and humans indicate that AIH improves motor function after spinal cord injury (SCI). This study will use a double blind, cross-over design to test if the combination of AIH and respiratory strength training improves breathing function more than either approach alone in adults with chronic SCI.

Spinal cord injury (SCI) disrupts neural pathways to respiratory motor neurons, causing muscle paralysis and decreased breathing capacity. Since respiratory impairment is the major cause of illness and death with SCI, it is critical to devise new strategies to restore breathing function.

Repetitive exposure to brief episodes of low oxygen (acute intermittent hypoxia or AIH) has demonstrated to increase respiratory function in humans with chronic SCI. Additionally pre-clinical studies demonstrate that AIH-induced functional benefits are enhanced by combining AIH with task-specific training. The investigator's central hypothesis is that combined daily AIH (dAIH) and respiratory strength training will elicit greater and more sustained gains in respiratory function than either treatment alone in people with chronic SCI.

This study is a double-blind, randomized, cross-over design where participants will complete 4 random-ordered blocks, consisting of a 5-day intervention followed by a 3-week washout period, during which 1-day, 3-day, and 1-week post-tests will be conducted. Participants will include 53 adults with chronic, incomplete SCI with >20% respiratory impairment based on maximal inspiratory or expiratory pressure generation.

Inclusion Criteria:

• Chronic incomplete SCI for ≥ 1 year at or below C-1 to T-12
• Incomplete SCI based on residual sensory and motor function below the level of the injury or injury classification of B, C, D at initial screening according to the American Spinal Injury Association Impairment Classification and the International Standards for the Neurological Classification of SCI

-OR-

Chronic complete SCI for ≥ 1 year at or below C-4 to T-12

• Complete SCI based on the absence of residual sensory or motor function below the level of injury or injury classification of A at initial screening according to the American Spinal Injury Association Impairment Classification and the International Standards for the Neurological Classification of SCI.
• Medically stable with clearance from physician
• SCI due to non-progressive etiology
• >20% impairment in maximal inspiratory or expiratory pressure generation, relative to established normative values.

Exclusion Criteria:

• Current diagnosis of an additional neurologic condition such as multiple sclerosis, Parkinson's disease, stroke, or brain injury
• Presence of unstable or uncontrolled medical conditions such as cardiovascular disease, myocardial infarction, <1 year lung disease, infections, hypertension, heterotopic ossification.
• Severe neuropathic pain
• Known pregnancy
• Severe recurrent autonomic dysreflexia
• History of seizure disorder < 1 year

Participants and the clinical evaluator will be informed of possible interventions but will be blinded to the gas mixture (hypoxic vs normoxic air) used during each AIH intervention.