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Our bodies store carbohydrate as glycogen, the critical fuel supply for endurance running. =Introduction=While the human body has sufficient stores of fat to run vast distances, the supply of carbohydrate is quite limited. This carbohydrate store is in the form of Glycogen, a branching chain of glucose molecules. * Burning glycogen for energy requires less oxygen than fat, making it more efficient. However, the store of glycogen is limited, and when the supply runs low we "hit the wall". * Glycogen is stored primarily in the muscles, but that glycogen can only be used by the muscle it’s stored in and cannot flow through the blood to other places. * Some glycogen is stored in the liver where it flows through the blood to all tissues.* The human liver typically stores between 90 and 160 grams of Glycogen, or 350 to 650 Calories.

* Eccentric exercise, such as [[Downhill Running]], can cause [[[[Delayed Onset Muscle Soreness||DOMS]] and impair glycogen replenishment<ref name="O'Reilly-1987"/>.* Glycogen stores may not be replenished between daily hard runs, such as 10 miles at 80% of [[VO2max|V̇O₂max]]<ref name="Costill-1971"/>.