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[[File:Glycogen depletion ST FT.jpg|none|thumb|800px|Glycogen depletion in human muscle fibers. The bars are colored with black indicating high glycogen content through to white indicating glycogen depletion. Three different intensities are shown; high (84% [[VO2max|V̇O<sub>2</sub>max]]) medium (64 %[[VO2max|V̇O<sub>2</sub>max]]) and low (31 %[[VO2max|V̇O<sub>2</sub>max]]) for each of Slow Twitch and Fast Twitch muscle fibers.]]
=Glycogen Depletion and BreathingRate=It requires more oxygen to produce energy from fat than carbohydrate <ref name="vent"/>. This is may be why when we higher intensity exercise harder our bodies shift shifts to burning more carbohydrate. When our muscles become depleted of glycogen, muscles are forced to burn more fat. At any given exercise intensity we will use more oxygen when we are glycogen depleted. This means our heart rate will be higher and out breathing will be deeper and faster. It also means our perceived exertion is much higher for a given pace when glycogen depleted. This effect is most noticeable at the end of a long run or a marathon race, and it becomes much harder to stay on target pace. In fact, it can become up to 20% harder and this can be the difference between relaxed easy breathing and panting for breath. This increased demand for oxygen can often be seen in the [[Running Efficiency Calculator|calculated running efficiency]].
[[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. 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.]]
|[[File:MarathonFiberDamage2.jpg|thumb|400px|Here you can see extreme damage, with only the Z band of the fiber remaining (marked Z). Adjacent fibers show far less damage.]]
|}
=Useful Glycogen Facts=
* Glycogen is formed primarily from the carbohydrates we consume and is stored in our livers and muscles.
<ref name="romijn">Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration http://ajpendo.physiology.org/content/265/3/E380.short</ref>
<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>
</references>