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* 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"/>.
* [[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=
{| class="wikitable"
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="selectiveGollnick-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.]]
=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 720% 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 HIIT=
[[High Intensity Interval Training]] can deplete Glycogen rapidly. This is because glucose gives 2 ATP anaerobic rather than 30 ATP aerobic, producing only 1/15<sup>th</sup> the energy. It seems that glycogen is restored after HIIT, but this may be from muscle breakdown.
==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>
Glycogen depletion of fibers during intermittent exercise at 120% [[VO2max|V̇O<sub>2</sub>max]].
==Glycogen Restoration after HIIT==
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]]
* A study of six subjects of wildly varying training status were evaluated for glycogen status after HIIT<ref name="MacDougallWard1977"/>. The HIIT protocol was 1 minute at 140% [[VO2max|V̇O<sub>2</sub>max]] with 3-minute rests, repeated until the 140% could not be maintained for 30 seconds. The intervals reduced Glycogen stores to 28% of the starting value, and after two hours (fasted) the glycogen stores had only rebounded by ~10% (visual estimate from the chart.) After five hours, glycogen stores continued to be replenished, even in athletes who were fasted for that period. However, it seems unlikely this would be from lactate, and it's possible it was from protein (muscle) breakdown. (Interestingly, the highest trained subjects were able to maintain the intervals the shortest time, but of course were working much harder in absolute terms.)
* A study used a HIIT protocol of 6 seconds @ around 240% [[VO2max|V̇O<sub>2</sub>max]] with 30 second rests to exhaustion<ref name="Balsom-1999"/>. On a low carbohydrate diet the glycogen dropped from 181 pre-workout to 64 post, a ~65% drop. In a high carbohydrate diet, it dropped from 540 to 151, a ~72% drop. There was a clear relationship between pre-workout glycogen levels and time to exhaustion.
* Seven healthy subjects performed "heavy exercise" which reduced muscle glycogen by 50%, of which 35% was restored during the 60-minute recovery period<ref name="AstrandHultman1986"/>. The authors concluded that 50% of the lactate was converted to Glycogen in this period, but the study assumes the source of the glycogen is the lactate without tracing the path or considering protein as the source.
* Without sufficient oxygen, lactate rises, but even after oxygen levels are normalized, there's direct evidence from animal models that the lactate is not converted back to glucose or glycogen. In rats, the lactate produced during a 2-3-minute burst of intense exercise is oxidized rather than being converted back to glucose/glycogen<ref name="Hatta-1990"/>. Another study on rats found that less than 20% of lactate gets converted to Glycogen<ref name="BrooksGaesser1980"/>. In a distinctly grim experiment, rats were exercised by tying a weight to their tail and having them swim until too exhausted to stay above the water (~3 minutes) <ref name="NikolovskiFaulkner1996"/>. Some rats had their livers removed prior to the exercise to see how this changed lactate and glycogen responses to the exercise. Glycogen dropped to about 25% (visual estimate from the graph), with the liver-less rats having non-significantly lower glycogen levels. After 30 minutes, both types of rats had restored some of their Glycogen, with the liverless rats being about the same at 30 minutes as the intact rats immediately after exercise. What's all this mean? It suggests that glycogen restoration occurs even without the liver, though having a liver helps. However, it's hard to evaluate the results from tortured animals to voluntary exercise in humans.
=Glycogen Depletion and Muscle Damage=
Muscle biopsies taken after a marathon show damage to muscle fibers, but this damage appears focused on a subset of the fibers<ref name="Warhol-1985"/>. Some fibers show no damage, but adjacent fibers are badly affected. The damaged fibers are depleted of Glycogen and lipids (fat). It seems reasonable to me that this pattern of selective damage is due to the pattern of fibers recruitment, with the fibers that are recruited first becoming both glycogen depleted and damaged. Similar damage can be seen with [[Delayed Onset Muscle Soreness]]. The images below are taken from the gastrocnemius (calf), 24-48 hours after a marathon race,
<references>
<ref name="ShioseYamada2018">Keisuke Shiose, Yosuke Yamada, Keiko Motonaga, Hideyuki Takahashi, Muscle glycogen depletion does not alter segmental extracellular and intracellular water distribution measured using bioimpedance spectroscopy, Journal of Applied Physiology, volume 124, issue 6, 2018, pages 1420–1425, ISSN [http://www.worldcat.org/issn/8750-7587 8750-7587], doi [http://dx.doi.org/10.1152/japplphysiol.00666.2017 10.1152/japplphysiol.00666.2017]</ref>
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