Oxygen availability and PCr recovery rate in untrained human calf muscle: evidence of metabolic limitation in normoxia

In contrast to their exercise-trained counterparts, the maximal oxidative rate of skeletal muscle in sedentary humans appears not to benefit from supplemental O-2 availability but is impacted by severe hypoxia, suggesting a metabolic limitation either at or below ambient O-2 levels. However, the critical level of O-2 availability at which maximal metabolic rate is reduced in sedentary humans is unknown. Using P-31 magnetic resonance spectroscopy and arterial oximetry, phosphocreatine ( PCr) recovery kinetics and arterial oxygenation were assessed in six sedentary subjects performing 5- min bouts of plantar flexion exercise followed by 6 min of recovery. Each trial was repeated while breathing one of four different fractions of inspired O-2 ( FIO2) ( 0.10, 0.12, 0.15, and 0.21). The PCr recovery rate constant ( a marker of oxidative capacity) was unaffected by reductions in FIO2, remaining at a value of 1.5 +/- 0.2 min(-1) until arterial O-2 saturation ( Sa(O2)) fell to less than similar to 92%, the average value reached breathing an FIO2 of 0.15. Below this SaO(2), the PCr rate constant fell significantly by 13 and 31% to 1.3 +/- 0.2 and 1.0 +/- 0.2 min(-1) ( P < 0.05) as SaO(2) was reduced to 82 +/- 3 and 77 +/- 2%, respectively. In conclusion, this study has revealed that O-2 availability does not impact maximal oxidative rate in sedentary humans until the O-2 level falls well below that of ambient air, indicating a metabolic limitation in normoxia.