Skeletal muscle phosphocreatine recovery in exercise-trained humans is dependent on O-2 availability

In skeletal muscle, phosphocreatine (PCr) recovery from submaximal exercise has become a reliable and accepted measure of muscle oxidative capacity. D uring exercise, O-2 availability plays a role in determining maximal oxidat ive metabolism, but the relationship between O-2 availability and oxidative metabolism measured by P-31-magnetic resonance spectroscopy (MRS) during r ecovery from exercise has never been studied. We used P-31-MRS to study exe rcising human gastrocnemius muscle under conditions of varied fractions of inspired O-2 (FIO2) to test the hypothesis that varied O-2 availability mod ulates PCr recovery from submaximal exercise. Six male subjects performed t hree bouts of 5-min steady-state submaximal plantar flexion exercise follow ed by 5 min of recovery in a 1.5-T magnet while breathing three different F IO2 concentrations (0.10, 0.21, and 1.00). Under each FIO2 treatment, the P Cr recovery time constants were significantly different, being longer in hy poxia [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 p H 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 resul ts demonstrate that PCr recovery is significantly altered by FIO2 and sugge st that, after submaximal exercise, PCr recovery, under normoxic conditions , is limited by O-2 availability.