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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 O<sub>2</sub> that is extracted from the air is lower with glycogen depletion, probably because breathing rate is driven by CO<sub>2</sub> concentrations<ref name="KyrPullinen2000"/>.
[[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 HIITat High Intensities=[[High Intensity Interval Training]] (anerobic) exercise can deplete Glycogen rapidly. This is because glucose gives 2 ATP anaerobic anaerobically rather than 30 ATP aerobicaerobically, producing only 1/15<sup>th</sup> the energy. It seems that glycogen is restored after HIITexercise, but this may be from muscle breakdownrather than a reverse conversion of lactate to glycogen.
==Glycogen Depletion during HIIT==
The charts below are based on two HIIT sessions, one using 7x (3 minutes at 120% [[VO2max|V̇O<sub>2</sub>max]] + 10 minutes rest) the other using 8 x (1 minute at 150% [[VO2max|V̇O<sub>2</sub>max]] + 10 minutes rest) <ref name="Gollnick-1974"/>. Notice that the Glycogen depletion reflects the anerobic metabolism, resulting in far less energy (ATP) than would be produced aerobically. The lactate level increased dramatically during the training (it might have been lower if some easy exercise had been used instead of rest as this would have metabolized the lactate.) This is particularly striking given the rest periods, suggesting that the lactate was not converted back to glucose or Glycogen in that time period, possibly due to oxygen debt. The slow twitch fibers are depleted to a similar level to those at 120 minutes of 30% [[VO2max|V̇O<sub>2</sub>max]], 60 minutes at 64% [[VO2max|V̇O<sub>2</sub>max]], or 40 minutes at 83% [[VO2max|V̇O<sub>2</sub>max]]. The fast twitch fibers have greater glycogen depletion than any of the sub-maximal intensities at any time.
<gallery widths=300px heights=300px class='center">
File:Glycogen depletion at super-maximal intensities.jpg
File:Per Fiiber glycogen depletion at super-maximal intensities.jpg
</gallery>
The glycogen depletion during HIIT seems largely due to the incomplete metabolism of glucose, resulting in lactate accumulation while producing far fewer calories than complete metabolism. The implications of this depend on how much glycogen is replenished from the conversion of lactate back to Glycogen.
* A study found that 2 minutes of intense exercise resulted in a large drop in Glycogen, but over the following 30 minutes, much of this was restored<ref name="HarrisBergström1971"/>.<br/> [[File:Glycogen depletion after recovery from supermaximal exercise.jpg|center|thumb|300px| Glycogen depletion after recovery from supermaximal exercise]]
<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>
<ref name="KrustrupS??Derlund2004">Peter Krustrup, Karin S??Derlund, Magni Mohr, Jens Bangsbo, Slow-Twitch Fiber Glycogen Depletion Elevates Moderate-Exercise Fast-Twitch Fiber Activity and O2 Uptake, Medicine & Science in Sports & Exercise, volume 36, issue 6, 2004, pages 973–982, ISSN [http://www.worldcat.org/issn/0195-9131 0195-9131], doi [http://dx.doi.org/10.1249/01.MSS.0000128246.20242.8B 10.1249/01.MSS.0000128246.20242.8B]</ref>
<ref name="Maclin-2019">Macklin, Ian; Wyatt, Frank; Ramos, Malaeni; and Ralston, Grant (2019) "Muscle Glycogen Depletion and Replenishment: A Meta-Analytic Review," ''International Journal of Exercise Science: Conference Proceedings'': Vol. 2 : Iss. 11 , Article 10.
Available at: https://digitalcommons.wku.edu/ijesab/vol2/iss11/10</ref>
</references>
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