Phosphocreatine System(ATP-PC) also known as the anaerobic alactic system is used during the first 10 seconds of sustained muscle contraction. It is anaerobic because it doesn’t require oxygen to function and alactic because it doesn’t produce lactic acid.
To make ATP, phosphocreatine transfers a phosphate molecule to ADP. The newly-formed ATP is now available to be used for muscle contraction. The leftover portion is creatine, which is processed by the liver into creatinine and eliminated through the kidneys and urine. The body doesn’t recycle this creatine into phosphocreatine
Muscle fatigue is noticeable by the end of the phosphocreatine cycle. After 10 seconds of muscle contraction, the muscles are able to use anaerobic glycolysis and aerobic glycolysis pathways to provide energy. Phosphocreatine is regenerated by the liver, pancreas, and kidneys.
Fast-twitch skeletal muscles store more phosphocreatine to have available for short periods of intense muscle contraction. The slow-twitch skeletal muscles and heart store less.