In skeletal muscle, phosphocreatine (PCr) recovery from submaximal exercise has become a reliable and accepted measure of muscle oxidative capacity. During exercise, O₂ availability plays a role in determining maximal oxidative metabolism, but the relationship between O₂ availability and oxidative metabolism measured by ³¹P-magnetic resonance spectroscopy (MRS) during recovery from exercise has never been studied. We used ³¹P-MRS to study exercising human gastrocnemius muscle under conditions of varied fractions of inspired O₂ (FI_O2) to test the hypothesis that varied O₂ availability modulates PCr recovery from submaximal exercise. Six male subjects performed three bouts of 5-min steady-state submaximal plantar flexion exercise followed by 5 min of recovery in a 1.5-T magnet while breathing three different FI_O2 concentrations (0.10, 0.21, and 1.00). Under each FI_O2 treatment, the PCr recovery time constants were significantly different, being longer in hypoxia [33.5 ± 4.1 s (SE)] and shorter in hyperoxia (20.0 ± 1.8 s) than in normoxia (25.0 ± 2.7 s) (P  0.05). End-exercise pH was not significantly different among the three treatments (7.08 ± 0.01 for 0.10, 7.04 ± 0.01 for 0.21, and 7.04 ± 0.02 for 1.00). These results demonstrate that PCr recovery is significantly altered by FI_O2 and suggest that, after submaximal exercise, PCr recovery, under normoxic conditions, is limited by O₂ availability.

oxidative capacity; mitochondria; intracellular oxygenation; 31-phosphorus-magnetic resonance spectroscopy; fraction of inspired oxygen

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