Human muscle performance and PCr hydrolysis with varied inspired oxygen fractions: a P-31-MRS study

The purpose of this study was to use P-31-magnetic resonance spectroscopy t o examine the relationships among muscle PCr hydrolysis, intracellular H+ c oncentration accumulation, and muscle performance during incremental exerci se during the inspiration of gas mixtures containing different fractions of inspired O-2 (FIO2) We hypothesized that lower FIO2 would result in a grea ter disruption of intracellular homeostasis at submaximal workloads and the reby initiate an earlier onset of fatigue. Six subjects performed plantar f lexion exercise on three separate occasions with the only variable altered for each exercise bout being the FIO2 (either 0.1, 0.21, or 1.00 O-2 in bal ance N-2). Work rate was increased (1-W increments starting at 0 W) every 2 min until. exhaustion. Time to exhaustion (and thereby workload achieved) was significantly (P < 0.05) greater as FIO2 was increased. Muscle phosphoc reatine (PCr) concentration, P-i concentration, and pH at exhaustion were n ot significantly different among the three FIO2 conditions. However, muscle PCr concentration and pH were significantly reduced at identical submaxima l workloads (and thereby equivalent rates of respiration) above 4-5 W durin g the lowest FIO2 condition compared with the other two FIO2 conditions. Th ese results demonstrate that exhaustion during all Fret occurred when a par ticular intracellular environment was acheived and suggest that during the lowest FIO2 condition, the greater PCr hydrolysis and intracellular acidosi s at submaximal workloads may have contributed to the significantly earlier time to exhaustion.