Glycogen during exercise

Hargreaves, M., 1997. Interactions between muscle glycogen and blood glucose during exercise. Exercise and Sport Sciences Reviews 25 21-39. 

Bergstrom, J. Hultman, E. (1967). A study of the glycogen metabolism during exercise in man. Scand. J. Clin. Lab. Invest. 19,218-228. 

Costill, D.L., Coyle, E., Dalsky, G., Evans, W., Fink, W., Hoopes, D. (1977). Effects of elevated plasma FFA and insulin on muscle glycogen usage during exercise. J. Appl. Physiol. 43.695-699. 

Hermansen, L., Hultman, E., Saltin, E. (1967). Muscle glycogen during prolonged severe exercise. Acta Physiol. Scand. 71, 129-139. 

Hultman, E. Nilsson, L.H. (1971). Liver glycogen in man. Effect of different diet and muscular exercise. In Muscle Metabolism During Exercise (Pemow, B. Saltm, B., eds.), pp. 143-151, Plenum Press, New York. 

As is often the case, tissue-specific control mechanisms operate to optimise adaptation to particular conditions. For example, muscle contraction requires an increase in cytosolic calcium ion concentration (see Section 7.2.1, Figure 7.4). During exercise when energy generation needs to be increased, or from a more accurate metabolic point of view, when the ATP-to-ADP ratio falls rapidly, and the accompanying rise in [Ca2 + ] activate (i) glycogen phosphorylase which initates catabolism of... 

Glycogen stores in skeletal muscle are mainly held in reserve to satisfy the organ s need for a burst of energy during exercise, and thus are rapidly depleted upon activity during a fast. 

Thus, it is evident that the mobilization and provision of lipid to muscle during exercise, like during starvation, restricts the usage of carbohydrate. If this did not occur during exercise, glycogen stores within muscle (as well as in the liver) would be depleted more rapidly than normal and may significantly limit the duration of exercise. Hypoglycemia could also result and limit performance. 

Figure 4 - Glycogen depletion from the quadraceps femoris during exercise. Data from Essen (15).

As noted previously, like skeletal muscle, glycogen depletion in liver during endurance exercise is much less in trained animals and in animals who have had free fatty acids artificially elevated. No evidence exists that the mechanism proposed by Randle to account for the inhibition of carbohydrate metabolism in muscle by oxidation of fatty acids is operative in the liver. Thus other factors must be responsible for the slower rate of liver glycogen depletion in these situations. Such factors may include a smaller increase in catecholamine levels, a smaller reduction in insulin levels, and a smaller reduction in blood flow to the liver during exercise (19,20). 

Lemon, P.W.R. Mullin, J.P. (1980) Effect of initial muscle glycogen levels on protein catabolism during exercise. J. Appl. Physiol. 48, 624-629. 

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