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Physiology of Creatine

Phosophocreatine (PCr) and adenosine triphosphate (ATP) supply most of the energy for short term, maximal work. PCr availability is important for performance during brief, high intensity exercise because the depletion of PCr prevents ATP from being resynthesized at the rate required.
Approximately 60% of muscle creatine is in the form of PCr. Muscle creatine is replenished at the rate of about 2 gm/day. Creatine supplementation does not increase muscle ATP levels, but it increases PCr (phosphocreatine) and the capacity to maintain ATP during high intensity exercise, especially, fast twitch muscle fibers (Type II). It is also suspected that the free creatine may stimulate muscle mitochondria to resynthesize more phosphocreatine after exercise when stores are depleted.

Cell Volumizing Effects

Creatine increases the rate of protein synthesis by drawing more water into the muscle cell. This is known as "cell volumizing". This increased hydration is very important to muscle hypertrophy and strength increases. Creatine increases strength, which allows for more intensity to be generated, which in turn, facilitates growth. Hydration is vital to protein synthesis and flexibility. By changing the volume of the muscle cell you also change the rate of glycogen synthesis which has a direct metabolic effect.