[2013][2014][2015][2016][2017][2018] 1999. To further elucidate these population-specific limitations to metabolic rate, we used 31 P-magnetic resonance spectroscopy to study the exercising human gastrocnemius muscle under conditions of varied FI O 2 in sedentary subjects. To test the hypothesis that PCr recovery from submaximal exercise in sedentary subjects is not limited by O 2 availability, but rather by their mitochondrial capacity, six sedentary subjects performed three bouts of 6-min steady-state submaximal plantar flexion exercise followed by 5 min of recovery while breathing three different FI O 2 (0.10, 0.21, and 1.00). PCr recovery time constants were significantly longer in hypoxia (47.0 Ϯ 3.2 s), but there was no difference between hyperoxia (31.8 Ϯ 1.9 s) and normoxia (30.0 Ϯ 2.1 s) (mean Ϯ SE). End-exercise pH was not significantly different across treatments. These results suggest that the maximal muscle oxidative rate of these sedentary subjects, unlike their exercise-trained counterparts, is limited by mitochondrial capacity and not O 2 availability in normoxia. Additionally, the significant elongation of PCr recovery in these subjects in hypoxia illustrates the reliance on O 2 supply at the other end of the O2 availability spectrum in both sedentary and active populations. oxidative capacity; mitochondria; exercise; intracellular oxygenation; 31-phosphorous-magnetic resonance spectroscopy THERE IS A GROWING APPRECIATION of the concept that O 2 supply and demand limitations to maximal metabolic rate are dependent on the population studied (4, 26, 31). However, it should be noted that the preponderance of data have been collected from physically active or endurance-trained subjects who reveal O 2 supply dependence at maximal O 2 uptake (V O 2 max ). The supply of O 2 and its utilization by skeletal muscle is a tightly interwoven process that cannot easily be assessed by a single measurement. Although 31 P-magnetic resonance spectroscopy (MRS) measurements of phosphocreatine (PCr) recovery provide an accurate measure of skeletal muscle oxidative metabolism (2,11,15,17), alone this methodology is unable to discern limitation caused by O 2 supply from that of mitochondrial O 2 demand.However, the combination of PCr recovery measurement under conditions of altered O 2 availability, manipulated by varying the inspired O 2 fraction (FI O 2 ), has been utilized to demonstrate that, in exercise-trained humans, under normoxic conditions, O 2 availability limits maximal oxidative rate (7). The practical implication of these data is that PCr recovery measurements alone should be interpreted with caution because differences in PCr recovery between subjects may not be due to metabolic limitations (1, 5, 19) but rather to limitations in O 2 supply (30). To solidify this unique approach of combining manipulations in O 2 availability with PCr recovery, it is important to demonstrate that this methodology is able to determine the difference between mitochondrial limitation vs. O 2 supply limitation across subje...