Intermittent hypoxia enhances voluntary activation and reduces performance fatigability during repeated lower limb contractions.

Abstract

Prior research has highlighted the therapeutic benefits of acute intermittent hypoxia (AIH) in enhancing motor performance after motor incomplete spinal cord injury and in able-bodied individuals. While studies in rodents and humans indicate that AIH may facilitate motor excitability, the relationship between excitability changes and functional performance remains unclear. Additionally, discrepancies in the effects of AIH on excitability in able-bodied individuals merit further investigation. Understanding the concurrent impact of repetitive AIH on voluntary activation and spinal reflex excitability may clarify the functional implications of AIH for muscle force production. High voluntary activation is vital for sustaining torque production during activities that require repeated muscle contractions. We hypothesized that repetitive AIH would attenuate declines in both voluntary activation and maximum torque production typically observed during fatiguing contractions. To test this hypothesis, we examined the effects of four consecutive days of AIH on voluntary activation and torque generation during repeated maximal plantar flexion contractions. We assessed changes in voluntary activation using the central activation ratio (CAR) by calculating the ratio of voluntary torque to the torque produced with supramaximal electrical stimulation. Consistent with our hypothesis, we show that repetitive AIH significantly increases both CAR and peak torque during fatiguing contractions. We did not observe any changes in resting spinal reflex excitability or antagonist muscle coactivation during the fatiguing contractions post-AIH. Together, these findings suggest that repetitive AIH reduces performance fatigability through enhanced descending neural drive. Optimizing voluntary activation is critical for facilitating the recovery of functional walking skills after neurological injury.