Skeletal muscle oxidative metabolism in sedentary humans:: 31P-MRS assessment of O2 supply and demand limitations

Previously, it was demonstrated in exercise-trained humans that phosphocreatine (PCr) recovery is significantly altered by fraction of inspired O-2 ((FI)(), suggesting that in this population under normoxic conditions, O)(O2)(2) (availability limits maximal oxidative rate. Haseler LJ, Hogan ML, and Richardson RS. J Appl Physiol 86: 2013 - 2018, 1999. To further elucidate these population-specific limitations to metabolic rate, we used 31P-magnetic resonance spectroscopy to study the exercising human gastrocnemius muscle under conditions of varied FIO)(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)(O2) ((0.10, 0.21, and 1.00). PCr recovery time constants were significantly longer in hypoxia (47.0 +/- 3.2 s), but there was no difference between hyperoxia (31.8 +/- 1.9 s) and normoxia (30.0 +/- 2.1 s) (mean +/- 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 O)(2) availability spectrum in both sedentary and active populations.