The primary energy source used during high-intensity workouts is carbohydrates. These are readily available in the form of glucose, which is derived from glycogen stored in muscles. As exercise intensity increases, the body relies more heavily on carbohydrates since they can provide quick energy compared to fats. This reliance is particularly evident during short, explosive activities, such as sprinting or heavy lifting, where anaerobic energy systems kick in, producing energy without oxygen but leading to fatigue more rapidly.