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

ABSTRACT 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 × 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.