Carbohydrate depletion is one of the main reasons why fatigue develops.
The subjects were endurance trained cyclists who had fasted overnight. Early in the exercise energy was supplied almost equally by carbohydrate and fat. Gradually, as muscle glycogen levels declined, the contribution of blood glucose to the energy supply increased. Liquid carbohydrate feedings provided at two hours into the cycling session maintained normal blood glucose levels and allowed carbohydrate to continue as a major part of the fuel mix within working muscles. Fatigue occurred after four hours of continuous cycling exercise.