Non-Invasive Interventions for Respiratory Recovery in Chronic Spinal Cord Injury

Acute intermittent hypercapnic-hypoxia (15 episodes of 1.5 minute O2= 9%, CO2=4% and balance N2, interspaced with 1.5 min room air

An open-loop continuous tSCS stimulation \[30 min duration, low frequency (30 Hz), tolerable intensity (\~10-15 mA), 400 µsec pulse width, biphasic symmetric waveform\] will be delivered using 2 channels: Channel 1- C4-C5 spine; Channel 2- T4 spine; returning electrodes at ASIS.

Respiratory strength training will be conducted using a standard, spring-loaded threshold device while the participant is receiving either tSCS or Sham tSCS. Participants will complete one warm-up set at a pressure threshold approximately 40% of their pre-test outcomes (obtained on day 1 of the corresponding intervention block) for maximal inspiratory and expiratory pressure generation. Following the warm-up, participants will train at a pressure threshold approximately 70% of their pre-test outcomes and will complete three sets of 6-12 repetitions for both inspiratory and expiratory strength training. A training breath will consist of a \~1-2-second sustained effort through the device, separated by 5-10 seconds of quiet breathing. The order of the three sets each of inspiratory or expiratory training will alternate for each of the five daily sessions; the order on day 1 of training will be randomly determined.