During prolonged exercise, the body primarily utilizes energy sourced from glycogen and fat reserves. Initially, glycogen stored in muscles and the liver serves as the primary energy substrate for quick bursts of activity. As the duration of exercise lengthens, the body gradually shifts to utilizing fat as a more sustainable fuel source. This transition is vital for endurance performance, as fat stores are more abundant. Additionally, maintaining hydration and electrolyte balance is essential during prolonged exertion to support cellular function and overall performance. Understanding these energy dynamics can help athletes optimize their training and recovery regimens.