The ATP-PC system provides ATP very quickly and efficient but the amount of PC in muscles is limited. Energy can be produces by the ATP-PC system for approximately 10 seconds, and it is restored approximately 2 minutes if resting. The ATP-PC system as the primary source of energy is used in high-intensity activity lasting for 10 seconds or less such as football. This allows for the activity to be repeated in intense, short bouts, without immediate exhaustion. The only way that PC can be restored is during recovery and both ATP and PC are stored in the contractile tissue of muscles. This system represents the most readily available source of ATP for use by the muscles.