Acute intermittent hypoxia-induced increases maximal motor unit discharge rates in people with chronic incomplete spinal cord injury

Acute intermittent hypoxia (AIH) is an emerging technique for enhancing neuroplasticity and function in respiratory and limb musculature. Thus far, AIH-induced improvements in strength have been reported for upper and lower limb muscles after chronic incomplete cervical spinal cord injury (iSCI) but the underlying mechanisms have been elusive. We used high-density surface electromyography (HDsEMG) to determine if motor unit discharge behaviour is altered after 15 x 60 s exposures to 9% inspired oxygen interspersed with 21% inspired oxygen (AIH), compared to breathing only 21% air (SHAM). We recorded HDsEMG from the biceps and triceps brachii of seven individuals with iSCI during maximal elbow flexion and extension contractions, and motor unit spike trains were identified using convolutive blind source separation. After AIH, elbow flexion and extension torque increased by 54% and 59% from baseline (p = 0.003), respectively, whereas there was no change after SHAM. Across muscles, motor unit discharge rates increased by ~4 pulses per second (p = 0.002) during maximal efforts, from pre to post AIH. These results suggest that excitability and/or activation of spinal motoneurons are augmented after AIH, providing a mechanism to explain AIH-induced increases in voluntary strength. Pending validation, AIH may be helpful in conjunction with other therapies to enhance rehabilitation outcomes due to these enhancements in motor unit function and strength. ### Competing Interest Statement The authors have declared no competing interest. ### Clinical Trial NCT05513911 ### Funding Statement This study was funded by the National Institute on Disability, Independent Living, and Rehabilitation Research ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: IRB of Northwestern University gave ethical approval for this work I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors