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From Fellrnr.com, Running tips
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* The glycogen in our muscles can only be used by those muscle fibers.
* Glycogen can also be created from protein via a process called [http://en.wikipedia.org/wiki/Gluconeogenesis gluconeogenesis], but not from fat.
=Glycogen Depletion=
The chart below shows that muscles do not become glycogen depleted at the same time. At all intensities shown, slow twitch fibers become depleted before fast twitch. The depletion within a fiber type is also not equivalent, with some fibers becoming depleted while others are fully loaded. This pattern implies a pattern of fiber recruitment, where a subset of muscle fibers are recruited until they become exhausted, at which point other fibers are then used. As the slow twitch fibers become exhausted, fast twitch fibers are used in turn.
[[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.]]
=Further reading=
For more details see http://en.wikipedia.org/wiki/Glycogen
