The primary source of energy used by the body during short, intense bursts of activity is ATP (adenosine triphosphate) and creatine phosphate. These compounds provide immediate energy for activities like sprinting or weightlifting. The body relies on anaerobic metabolism during such efforts, which allows for quick energy release without the need for oxygen. This energy system can sustain maximal efforts for only a short duration, typically around 10 seconds, before the body begins transitioning to other energy sources.