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 (31)P 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) (FI(O(2))) (0.10, 0.12, 0.15, and 0.21). The PCr recovery rate constant (a marker of oxidative capacity) was unaffected by reductions in FI(O(2)), remaining at a value of 1.5 +/- 0.2 min(-1) until arterial O(2) saturation (Sa(O(2))) fell to less than approximately 92%, the average value reached breathing an FI(O(2)) of 0.15. Below this Sa(O(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 Sa(O(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.