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While the human body has sufficient stores of fat to run vast distances, the supply of carbohydrate is quite limited. This carbohydrate store is in the form of Glycogen, a branching chain of glucose molecules.
* Burning glycogen for energy requires less oxygen than fat, making it more efficient. However, the store of glycogen is limited, and when the supply runs low, we "hit the wall".
* Glycogen is stored primarily in the muscles, but that glycogen can only be used by the muscle it's stored in and cannot flow from the muscles through the blood to other places. The glycogen is stored within a muscle fiber, not the overall muscle, so when a fiber gets glycogen depleted, it can't use glycogen from surrounding fibers.
* Some glycogen is stored in the liver where it flows through the blood to all tissues. The human liver typically stores between 90 and 160 grams of Glycogen, or 350 to 650 Calories.
* Blood typically contains less than 20 calories of glucose. (This assumes 5 liters of blood and 100mg/dL of blood glucose, which is 5g of glucose.)
* Eccentric exercise, such as [[Downhill Running]], can cause [[Delayed Onset Muscle Soreness| DOMS]] and impair glycogen replenishment<ref name="O'Reilly-1987"/>.
* Glycogen stores may not be replenished between daily hard runs, such as 10 miles at 80% of [[VO2max|V̇O<sub>2</sub>max]]<ref name="Costill-1971"/>.
* Each gram of Glycogen is stored with between 3-4g of water<ref name="OlssonSaltin1970"/>. This means you can lose a lot of weight rapidly after long runs or brief diets, but this is mostly water weight. This water can provide hydration, offset sweating or increasing urine volumes. Consequently, you may need less fluid earlier in a run compared with later.
* [[High Intensity Interval Training]] may deplete glycogen reserves rapidly. This is anerobic exercise only produces 1/15 the energy from glycogen, so the typical 2,000 calories energy reserve would only give 133 calories!
=Glycogen Usage=
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|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.]]
The depletion of glycogen within [[Muscle| slow twitch fibers]] results in the recruitment of fast twitch fibers<ref name="KrustrupS??Derlund2004"/>.
=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 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>
Similar things have been seen with [[High Intensity Interval Training| HIIT]]. One meta-analysis found that HIIT is the most effective way of depleting glycogen<ref name="Maclin-2019"/>, thought the analysis did not cover many different protocols.==Estimating Glycogen depletion Depletion During HIIT==Assuming that you have 500g of fibers during intermittent exercise at 120% muscle [[VO2max|V̇OGlycogen]], that would normally provide about 2,000 Kcal (Calories) aerobically. However, anaerobically that Glycogen will only provide 1/15 of the energy, which is 133 Kcal. It's estimated that humans use 10.8 ml/min of O2 per watt<subref name="Swain2000"/>2, and a liter of O2 is 5Kcal<ref name=" Janot2005"/sub>max, so that works out to 10.8/1000*5=0.054 Kcal/w/min. If you do intervals at 100w above [[Critical Power]], that would burn through the 133 Kcal in (133Kcal/(0.054*100w), or about 25 minutes. At 150w above Critical Power, that drops to about 16 minutes. This is probably best performed as part of [[High Intensity Interval Training| HIIT]].==Glycogen Restoration after HIITHigh Intensity Exercise==
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="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>
<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>
<ref name=" Janot2005">Janot, Jeffrey M. "Calculating caloric expenditure." ''IDEA Fitness Journal'' 2.6 (2005): 32-33.</ref>
<ref name="Swain2000">David P. Swain, Energy Cost Calculations for Exercise Prescription, Sports Medicine, volume 30, issue 1, 2000, pages 17–22, ISSN [http://www.worldcat.org/issn/0112-1642 0112-1642], doi [http://dx.doi.org/10.2165/00007256-200030010-00002 10.2165/00007256-200030010-00002]</ref>
</references>
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