Skeletal muscle phosphocreatine recovery in exercise-trained humans is dependent on O₂availability

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 by³¹P-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$(F{I}_{O_2}$ ) to test the hypothesis that varied O₂availability 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$F{I}_{O_2}$ concentrations (0.10, 0.21, and 1.00). Under each$F{I}_{O_2}$ 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$F{I}_{O_2}$ and suggest that, after submaximal exercise, PCr recovery, under normoxic conditions, is limited by O₂ availability.