Final answer:
The body's choice of energy system during physical activity depends on the intensity and duration of the activity. There are three main energy systems in the body: immediate energy system, glycolytic system, and aerobic system.
Step-by-step explanation:
The body's choice of which energy system to use during physical activity is dictated by the intensity and duration of the activity. There are three main energy systems in the body: the immediate energy system (ATP-PCr system), the glycolytic system, and the aerobic system. The immediate energy system provides energy for short bursts of high-intensity activity, such as a sprint. The glycolytic system provides energy for medium-intensity activities, such as a 400-meter run. The aerobic system provides energy for low-intensity, long-duration activities, such as a marathon.