The primary energy system used during short bursts of high-intensity activity is the anaerobic system, specifically the ATP-PCr pathway. This system provides immediate energy for activities lasting about 10 seconds, such as sprinting or heavy lifting. It relies on stored adenosine triphosphate (ATP) and phosphocreatine (PCr) within muscles. Although this system is effective for quick bursts of power, it is limited and quickly fatigues. After depleting ATP and PCr, the body transitions to anaerobic glycolysis for sustained higher intensity efforts.