Impact of total creatine and accessible phosphate pool on endurance exercise: implications for muscle fibers bioenergetic specificity and creatine supplementation.

PURPOSE: To determine the effect of the total creatine (tCr) and accessible phosphate (aP) pool (tCaP) on the kinetic variables (maximal muscle oxygen consumption, V̇O2max, transition time of the muscle V̇O2 on-kinetics, t0.63, time to exhaustion, tlim, critical power, CP and work over CP in the power-duration relationship, W') of intense moderate-term endurance exercise.

METHODS: A dynamic computer model of the skeletal muscle bioenergetic system, involving the each-step-activation mechanism of system stimulation during work transitions and Pi-double-threshold mechanism of muscle fatigue is used.

RESULTS: A decrease in tCaP significantly increases V̇O2max, tlim and CP, moderately increases W' and shortens t0.63. Significantly enough decrease in tCaP moves the system (at a determined power output) from the severe to heavy (and further to moderate) exercise intensity domain. An increase in tCr alone has an insignificant effect on the system.

CONCLUSIONS: A decrease in tCaP, especially in aP, increases fatigue resistance of muscle during endurance exercise through attenuation of Pi increase during exercise. An increase in tCr alone has little impact on fatigue resistance. In the sequence of muscle cell types: glycolytic fast-twitch type II skeletal muscle fibers → oxidative slow-twitch type I skeletal muscle fibers → heart muscle fibers the tCaP, especially aP, pool size decreases, which can contribute to the increasing fatigue-resistance in this sequence. While Cr supplementation, elevating tCr, increases the strength and force of acute maximal-intensity very-short-term exercise, it has no significant effect on endurance exercise, at least through the creatine kinase system.