Changes

The chart<ref name="Gollnick-1974"/> 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 [[Muscle|Muscle 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|V̇O₂max]]) medium (64 %[[VO2max|V̇O₂max]]) and low (31 %[[VO2max|V̇O₂max]]) for each of Slow Twitch and Fast Twitch muscle fibers.]]

=Glycogen Depletion and Breathing Rate=

It requires more oxygen to produce energy from fat than carbohydrate<ref name="vent"/>. This may be why higher intensity exercise harder 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 [[Heart Rate Drift| increased demand for oxygen]] can often be seen in the [[Running Efficiency Calculator| calculated running efficiency]]. In addition, the amount of O₂ that is extracted from the air is lower with glycogen depletion, probably because breathing rate is driven by CO₂ concentrations<ref name="KyrPullinen2000"/>.

<ref name="BrooksGaesser1980">G. A. Brooks, G. A. Gaesser, End points of lactate and glucose metabolism after exhausting exercise, Journal of Applied Physiology, volume 49, issue 6, 1980, pages 1057–1069, ISSN [http://www.worldcat.org/issn/8750-7587 8750-7587], doi [http://dx.doi.org/10.1152/jappl.1980.49.6.1057 10.1152/jappl.1980.49.6.1057]</ref>

<ref name="Hatta-1990">H. Hatta, Oxidative removal of lactate after strenuous exercise., Ann Physiol Anthropol, volume 9, issue 2, pages 213-8, Apr 1990, PMID [http://www.ncbi.nlm.nih.gov/pubmed/2400462 2400462]</ref>

</references>

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