About 60%-70% of the creatine in skeletal muscle is phosphorylated, thereby preventing migration across the plasma membrane, and essentially trapping the molecule within the muscle cell.
Studies have clearly documented the benefits of dietary creatine supplementation, which is theorized to cause a substantial increase of phosphocreatine levels in type II (fast-twitch) skeletal muscle fibers.
Creatine plays a key role in maintaining a high ATP:ADP ratio by phosphorylation of ADP1, thereby delaying muscle fatigue and allowing for prolonged high-intensity exercise:
is one of the basic muscle energy stores, particularly in . During exercise, phosphorylated creatine (PCr) is thought to be an immediate source for high energy phosphate groups with which to replenish . There is some evidence to suggest that creatine only contributes significantly to ATP replenishment for the first few seconds of intense activity. Since PCr is generated from Cr by ATP, an ATP shortage will inhibit PCr synthesis.
This balance can be altered slightly by creatine ingestion. Within a few days, intracellular Cr levels reach a new equilibrium level. A much smaller fraction of this additional creatine appears to be stored in the high-energy phosphocreatine form, however. Finally: the body appears to have a target level for circulating creatine, and ingestion or supplementation reduces synthesis by the body.