EFFECTS OF F1O2 ON LEG VO2 DURING CYCLE ERGOMETRY IN SEDENTARY SUBJECTS

In a recent study of completely sedentary normal young subjects, leg V O2max was reduced by hypoxia in proportion to mean capillary PO2 as F1 O2 was reduced from 0.15 to 0.12. However. the increase in VO2max from F1O2 = 0.15 to 0.21 was less than expected for the increase in mean c apillary PO2. This finding has led us to hypothesize that in sedentary subjects breathing room air, VO2max is not limited by O-2 supply but rather by oxidative capacity of mitochondria. The present study sought to obtain further evidence for or against this hypothesis in sedentar y subjects by assessing leg VO2max (VO(2)leg) breathing 100% O-2, as w ell as in normoxia and hypoxia. Data from 18 subjects studied at F1O2 = 0.12, 0.15, and 0.21 and from six more studied at 0.12, 0.15, and 1. 00 were analyzed. In all 24 we measured VO(2)leg by arterial and venou s blood sampling and thermodilution leg blood flow during maximal cycl e ergometry at each F1O2. VO(2)leg was not increased by room air or 10 0% O-2 breathing relative to that observed at F1O2 = 0.15, but it was reduced while breathing 12% O-2. The data at F1O2 = 0.12 and 0.15 conf ormed to the predictions of O-2 supply Limitation of maximal VO2max as previously. These results confirm and extend our prior observations t hat in sedentary, as opposed to trained subjects,muscle VO2max is O-2 supply limited only in hypoxia.