Severe acute hypoxia impairs recovery of voluntary activation after a sustained submaximal isometric elbow flexion task

Introduction: Although reduced blood oxygen saturation (SpO2) can cause rapid changes in exercise performance, few studies have examined the direct consequence that hypoxia has on the recovery of the motor system. The purpose of this study was to examine how severe acute hypoxia affects the ability to voluntarily activate muscle during, and following, a sustained submaximal isometric elbow flexion.

Methods: Fourteen individuals (25.3 ± 3.3 yr) were exposed to a hypoxia and sham condition in separate sessions. SpO2 was titrated to 80% SpO2 over 15 min, and remained at this level during testing. The contraction task began after 2 hr of exposure, and consisted of performing a 10 min isometric elbow flexion at 20% MVC. This was followed by a 6 min recovery phase. MVC torque, resting and superimposed twitches, and voluntary activation (VA, motor point and motor-cortical) were assessed every 2 min throughout the contraction protocol, and every 1 min of the recovery phase.

Results: MVC torque, twitch responses, and VA decreased similarly for the sham and hypoxia sessions during the submaximal contraction. However, during the recovery phase motor-point and TMS-evoked superimposed twitches were greater during the hypoxia session (p’s < 0.05). Consequently, motor-point and TMS-based measurements of VA were lower during the recovery phase of the hypoxia session (p’s < 0.05).

Conclusion: Severe acute hypoxia impairs the ability of the motor system to recover after performing sustained submaximal elbow flexions. This finding prompts further investigation into not just exercise, but exercise recovery, after exposure to hypoxic environments.