ACUTE HYPOXIC EXPOSURE INCREASES PERCEPTION OF FATIGUE PRIOR TO AFFECTING VOLUNTARY ACTIVATION DURING MAXIMAL CONTRACTIONS

Introduction:
Although reducing blood oxygen saturation (SpO2) can cause rapid changes in exercise performance, few studies have examined the direct consequences hypoxia has on the development of fatigue in the motor system. The purpose of this study was to examine how severe acute hypoxia affects an individual’s ability to voluntarily activate muscle, as well as the individual’s perception of fatigue during maximal elbow flexions.

Methods:
Fourteen individuals (23 ± 2.2 yr) were exposed to a hypoxia and a sham intervention. SpO2 was titrated over 15 min and remained at 80% SpO2 during testing. Motor performance was assessed before titration (Pre), 0 hr, 1 hr, and 2 hr after titration. At each time point brief unfatigued elbow flexor MVCs were performed, followed by sustained 20 s MVCs to induced fatigue. Motor point superimposed and resting twitches were obtained from the biceps brachii to calculate level of voluntary activation (VA), and ratings of perceived fatigue were obtained with a modified CR-10 Borg scale.

Results:
In fresh muscle there was no difference in VA between sham and hypoxia. During fatiguing contractions, the perception of fatigue was significantly greater for the hypoxia condition at 0 hr (p = 0.04), 1 hr (p = 0.007), and 2 hr (p < 0.001) compared to sham. VA remained unaffected by hypoxia until the 2 hr timepoint, where VA in the fatigued muscle decreased by 7% compared to sham (p = 0.002).

Conclusion:
Our results suggest that acute hypoxia alters an individual’s perception of fatigue compared to their actual ability to activate muscle during fatiguing maximal contractions.