The phosphagen system is a quick energy system used by the body during short bursts of intense activity, such as sprinting or weightlifting. It relies on stored adenosine triphosphate (ATP) and creatine phosphate to rapidly regenerate ATP, which is essential for muscle contractions. This system can provide energy for about 10 seconds before it needs to be replenished. During high-intensity exercise, the phosphagen system is the primary source of energy because it works without the need for oxygen. After this short duration, the body shifts to other energy systems, like the glycolytic and oxidative systems, which take longer to produce ATP but can sustain activity for more extended periods.