The body's energy system varies during different exercises based on intensity and duration. For short bursts of activity, such as sprinting, the body primarily relies on the ATP-PC system, utilizing stored energy for immediate power. For moderate to high-intensity activities lasting from a few minutes to around 30 seconds, the glycolytic system kicks in, breaking down carbohydrates for energy. In longer-duration exercises, the aerobic system predominates, using oxygen to convert fat and carbohydrates into energy. Understanding these systems helps in selecting appropriate training methods for specific fitness goals.