Skeletal muscle phosphocreatine recovery in exercise-trained humans is dependent on O2 availability

In skeletal muscle, phosphocreatine (PCr) recovery from submaximal exercise has become a reliable and accepted measure of muscle oxidative capacity. During exercise, O-2 availability plays a role in determining maximal oxidative metabolism, but the relationship between O-2 availability and oxidative metabolism measured by P-31-magnetic resonance spectroscopy (MRS) during recovery from exercise has never been studied. We used P-31-MRS to study exercising human gastrocnemius muscle under conditions of varied fractions of inspired O-2 (FIO2) to test the hypothesis that varied O-2 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 FIO2 concentrations (0.10, 0.21, and 1.00). Under each FIO2 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 less than or equal to 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 FIO2 and suggest that, after submaximal exercise, PCr recovery, under normoxic conditions, is limited by O-2 availability.