Acute hypoxic exposure increases perception of fatigue prior to affecting quantitative measures of fatigue during maximal contractions

BACKGROUND AND AIM:
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. Therefore, 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 the titration period. At each time point brief (~3 s) elbow flexor MVCs were performed, followed by sustained (~20 s) MVCs. TMS was used to elicit motor evoked potentials (MEP area) in the biceps brachii to assess corticospinal excitability. 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: Perception of fatigue was enhanced immediately after reducing blood oxygen concentration to 80% SpO2 (P < 0.001). After 2 hr of hypoxic exposure, MEP area increased (P = 0.03) and VA of the biceps brachii decreased (P = 0.012). However, these changes in MEP area and VA were only seen when performing the brief MVCs and not during the sustained MVCs.

CONCLUSIONS: Pathways responsible for fatigue perception are more sensitive to changes in tissue oxygenation, and that redistribution of blood flow to active areas of the motor cortex and active muscle is responsible for the lack of change seen in muscle activation during the sustained protocol.