O2 EXTRACTION DURING EXERCISE IN NORMOXIA AND HYPOXIA

Abstract

382 Exercise performance is impaired in ambient hypoxia. To test whether this impairment is due to inadequate muscle oxygenation, we studied small muscle exercise where blood flow would not be limited centrally by cardiac output. Five subjects completed rhythmic handgrip exercise (5 sec contraction / 5 sec rest) for 3 min at 17%, 33% and 50% of maximal voluntary contraction (MVC), while breathing 21% O2 or 10% O2 in a randomized, repeated measures fashion. Near-infrared spectroscopy measured relative concentrations of oxy- and deoxy-Hb+Mb in the flexor digitorum profundus. Values were normalized to the signal range observed from rest (defined = 100) and during 10 min of resting ischemia (defined = 0). We used the magnitude of the pre- to post-contraction signal change during a contraction cycle as an index of fractional O2 extraction. Mean ± SD values for the oxy-Hb+Mb signal were: (* different from normoxia, P < 0.05) TableTableHandgrip force was not different between conditions and did not decline over any 3 min exercise bout, suggesting that total muscle VO2 at a fixed%MVC was the same in normoxia and hypoxia; yet O2 extraction was reduced by ≈ 40%. We conclude that even in severe hypoxia, total oxygen uptake must be maintained in small muscle exercise by increasing muscle blood flow to compensate for decreased fractional oxygen extraction.