During heavy exercise lasting less than two hours, the primary fuel for muscular work is

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During heavy exercise lasting less than two hours, the primary fuel for muscular work is muscle glycogen. Glycogen is stored glucose in the muscles, which can be broken down quickly to provide energy for exercise. Blood glucose and plasma FFA (free fatty acids) are also used for energy during exercise, but to a lesser extent than muscle glycogen. Amino acids are not a primary fuel during exercise, as they are used primarily for building and repairing muscles after exercise.

During Heavy Exercise Lasting Less Than Two Hours, the Primary