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=Glycogen Depletion=
The chart<ref name="selective"/> 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.]]
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Glycogen is the critical fuel supply for endurance running.
=Glycogen Usage=
[[File:Absolute substrate usage against intensity.jpg|none|thumb|400px|The contribution of different energy sources changes<ref name="romijn"/> with exercise intensity. These values were taken after 30 min. of exercise. Note that the total calories available from the blood (free fatty acid and glucose) remains about the same regardless of exercise intensity.]]
At low exercise intensity the majority of the energy comes from free fatty acids in the blood, with a little bit of blood glucose and a little bit of muscle triglyceride. As the exercise intensity increases the contribution of free fatty acids drops. The contribution of blood glucose increases with exercise intensity, but not as dramatically as the contribution of muscle glycogen. At higher intensity muscle glycogen is the major energy source and is critical for performance.
[[File:Substrate usage over time.jpg|none|thumb|400px|Changes in substrate usage<ref name="romijn"/> over 120 min period at 65% [[VO2max|V̇O<sub>2</sub>max]].]]
At 65% [[VO2max|V̇O<sub>2</sub>max]], the usage of different substrates changes over time. The reduced usage of muscle glycogen may be due to a reduction in the availability of the glycogen. Over the two hour period shown, the fat:carbohydrate ratio changes from around 55:45 to 65:35. This change would reduce power output (running speed) at the fixed percentage of [[VO2max|V̇O<sub>2</sub>max]] (see ‘Glycogen Depletion and Breathing’ below).
=Glycogen Depletion=
The chart<ref name="selective"/> 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.]]
=Glycogen Depletion and Breathing=
It requires more oxygen to produce energy from fat than carbohydrate <ref name="vent"/>. This is why when we exercise harder our bodies shift 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.]]
=Useful Glycogen Facts=
* Glycogen is formed primarily from the carbohydrates we consume and is stored in our livers and muscles.
* The glycogen in our livers can leave the liver and flow via our blood to our muscles, brains and other organs.
* 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.
=Further reading=
For more details see http://en.wikipedia.org/wiki/Glycogen
<references>
<ref name="selective">Selective glycogen depletion in skeletal muscle fibers of man following sustained contractions http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1331072/</ref>
<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>
</references>