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1 Department of Medicine, University of California, San Diego, La Jolla, CA, USA
2 Huntington Medical Research Institutes, Pasadena, CA, USA
* To whom correspondence should be addressed. E-mail: L.Haseler{at}griffith.edu.au.
Previously, it was demonstrated in exercise trained humans that phosphocreatine (PCr) recovery is significantly altered by fraction of inspired O2 (FIO2), suggesting that in this population under normoxic conditions, O2 availability limits maximal oxidative rate. To further elucidate these population specific limitations to metabolic rate we used 31P magnetic resonance spectroscopy (MRS) to study the exercising human gastrocnemius muscle under conditions of varied FIO2 in sedentary subjects. To test the hypothesis that PCr recovery from submaximal exercise in sedentary subjects is not limited by O2 availability, but rather by their mitochondrial capacity, 6 sedentary subjects performed 3 bouts of 6 min steady state submaximal plantar flexion exercise followed by 5 min of recovery while breathing 3 different FIO2 (0.10, 0.21, and 1.00). PCr recovery time constants were significantly longer in hypoxia (47.0 ± 3.2s) but there was no difference between hyperoxia (31.8 ± 1.9s) and normoxia (30.0 ± 2.1s) (mean ± SEM). 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 O2 availability in normoxia. Additionally, the significant elongation of PCr recovery in these subjects in hypoxia illustrates the reliance upon O2 supply at the other end of the O2 availability spectrum in both sedentary and active populations.
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