Acute hypoxia impairs recovery of voluntary muscle activation after sustained low-intensity contractions

Acute hypoxia has been shown to impair voluntary activation (VA) of muscle and alter excitability of the motor pathway during exercise. However, hypoxia related effects on recovery are relatively unknown. The purpose of this study was to determine how severe acute hypoxia alters neural mechanisms of muscle activation during, and following, a sustained fatiguing contraction. Fifteen participants (25 ± 3.2 years, 6 female) attended two sessions where they were exposed to a hypoxia condition (80% SpO2) and a sham condition. After 15 min of exposure, a 10-min isometric elbow flexion at 20% of maximal torque, and a 6 min recovery phase were examined. Transcranial magnetic stimulation (TMS) and motor point stimulation of the biceps brachii were used to assess VA. No hypoxia-related effects were identified for neuromuscular variables during the fatigue task. However, VA assessed by motor point stimulation was lower during hypoxia than sham at the 4th (sham: 89% ± 7%; hypoxia: 80% ± 12%; P = 0.023) and 6th min (sham: 90% ± 7%; hypoxia: 78% ± 11%; P = 0.040) of recovery. Similarly, VA (P = 0.01) and motor evoked potential area (P = 0.002) in response to cortical TMS were 10% and 11% lower during recovery for hypoxia compared to sham, respectively. Although a hypoxic stimulus of 80% SpO2 did not affect neural activity during the fatiguing task, motor cortical output and corticospinal excitability were reduced during recovery in the hypoxic environment.