Effects of F1O2 on leg VO2 during cycle ergometry in sedentary subjects

In a recent study of completely sedentary normal young subjects, leg VO2max was reduced by hypoxia in proportion to mean capillary PO2 as F1O2 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 capillary 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 sedentary subjects by assessing leg VO2max (VO(2)leg) breathing 100% O-2, as well 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 venous blood sampling and thermodilution leg blood flow during maximal cycle ergometry at each F1O2. VO(2)leg was not increased by room air or 100% 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 conformed to the predictions of O-2 supply Limitation of maximal VO2max as previously. These results confirm and extend our prior observations that in sedentary, as opposed to trained subjects,muscle VO2max is O-2 supply limited only in hypoxia.