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[[File:1035px-Glycogen structure.svg.png|right|thumb|200px|A schematic of glycogen, showing a core protein surrounded by strands of glucose.]]
Our bodies store carbohydrate as glycogen, the critical fuel supply for endurance running. 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 "hit the wall”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. * Blood typically contains less than 20 calories of glucose. * Glycogen can also be created from protein via a process called [http://en.wikipedia.org/wiki/Gluconeogenesis gluconeogenesis], but not from fat. * Eccentric exercise, such as [[Downhill Running]], can cause [[[[Delayed Onset Muscle Soreness||DOMS]] and impair glycogen replenishment<ref name="O'Reilly-1987"/>.
=Glycogen Usage=
{| class="wikitable"
[[File:Ventilatory response and glycogen depletion.jpg|none|thumb|400px|This graph <ref name="vent"/> shows the relationship between a cyclist's power output and their breathing rate in normal and glycogen depleted states.]]
=Glycogen Depletion and Muscle Damage=
Muscle biopsies taken after a marathon show damage to muscle fibers, but this damage appears focused on a subset of the fibers<ref name="Warhol-1985"/>. Some fibers show no damage, but adjacent fibers are badly affected. The damaged fibers are depleted of Glycogen and lipids (fat). It seems reasonable to me that this pattern of selective damage is due to the pattern of fibers recruitment, with the fibers that are recruited first becoming both glycogen depleted and damaged. Similar damage can be seen with [[Delayed Onset Muscle Soreness]]. The images below are taken from the gastrocnemius (calf), 24-48 hours after a marathon race.,
{| class="wikitable"
|[[File:MarathonFiberDamage.jpg|none|thumb|400px|The selective pattern of damage, showing the normal upper fiber adjacent to the 'moth eaten' appearance of the damaged lower fiber.]]
|}
=Further reading=
For more details see http://en.wikipedia.org/wiki/Glycogen
<ref name="vent">Effect of glycogen depletion on the ventilatory response to exercise http://jap.physiology.org/content/54/2/470.short</ref>
<ref name="Warhol-1985"> {{Cite journal | last1 = Warhol | first1 = MJ. | last2 = Siegel | first2 = AJ. | last3 = Evans | first3 = WJ. | last4 = Silverman | first4 = LM. | title = Skeletal muscle injury and repair in marathon runners after competition. | journal = Am J Pathol | volume = 118 | issue = 2 | pages = 331-9 | month = Feb | year = 1985 | doi = | PMID = 3970143 }}</ref>
<ref name="O'Reilly-1987"> {{Cite journal | last1 = O'Reilly | first1 = KP. | last2 = Warhol | first2 = MJ. | last3 = Fielding | first3 = RA. | last4 = Frontera | first4 = WR. | last5 = Meredith | first5 = CN. | last6 = Evans | first6 = WJ. | title = Eccentric exercise-induced muscle damage impairs muscle glycogen repletion. | journal = J Appl Physiol | volume = 63 | issue = 1 | pages = 252-6 | month = Jul | year = 1987 | doi = | PMID = 3624128 }}</ref>
</references>