What is the primary source of energy during high-intensity exercise?

During high-intensity exercise, the body primarily relies on glycogen as its main source of energy. Glycogen is the stored form of carbohydrates, primarily found in the liver and muscles. When engaging in intense physical activities, the demand for quick energy increases, and glycogen can be rapidly broken down into glucose to fuel muscle contractions. This process allows for sustained energy output during activities such as sprinting, weightlifting, or other vigorous exercises.

While carbohydrates are an essential part of the energy system, glycogen specifically is the immediate substrate used during high-intensity efforts. In contrast, fat becomes a more significant energy source during lower intensity activities, and protein typically serves more of a structural role rather than a primary energy source, particularly during short bursts of intense activity. Thus, glycogen's ability to provide rapid and readily accessible energy makes it the correct answer for high-intensity exercise scenarios.