During high-intensity workouts, the body utilizes glycogen as a primary energy source. Glycogen, stored in muscles and the liver, is broken down to glucose when immediate energy is required. This process is essential for sustaining energy levels during short bursts of intense activity, such as sprinting or heavy lifting. Once glycogen stores are depleted, the body may switch to fat utilization, which is slower and less efficient for quick energy demands. Proper nutrition maintains sufficient glycogen levels for optimal performance and recovery.