The primary source of energy used by the body during short, intense bursts of activity is adenosine triphosphate (ATP). ATP provides immediate energy through a process called anaerobic respiration, which does not require oxygen. This source of energy is crucial during activities like sprinting or heavy lifting, typically lasting only a few seconds. Following this, the body may switch to other energy systems, such as glycolysis and oxidative phosphorylation, to sustain longer durations of physical exertion.