How Intensity and Duration of Exercise Affect Glycogen Use
During exercise, muscle glycogen levels fall progressively as a semilogarithmic function of time (Hultman, 1971), as shown in Exercises 3.4. The rate of glycogen breakdown is greatest during the first 15 to 20 minutes of exercise, after which, for reasons that will become clear later, the rate falls. The rate of glycogen breakdown is critically dependent on the intensity of exercise, so that the higher the exercise intensity (expressed as percent V02max), the more rapid the rate of glycogen utilization (Saltin & Karlsson, 1971).
Exercises 3.4 also shows that severe muscle glycogen depletion does not occur during continuous exercise at very high intensities (greater than 90% V02max). This is because at such high exercise intensities, hydrogen (protons) rapidly accumulates within the muscle cell, which causes a marked fall in muscle pH, which ultimately prevents muscle contraction.
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However, if exercise of high intensity is performed intermittently (dotted lines in Exercises 3.4), which would allow for the metabolism of the excess protons during recovery, then a very marked degree of muscle glycogen depletion can occur within a very short period of time.
Continuous exercise at 70 to 85% V02max that is sustained for prolonged periods of time (longer than 2 hours) causes the greatest degree of muscle glycogen depletion. Therefore, at least theoretically, only performance in exercises such as long-distance running, cycling, or swimming events that last more than 2 hours is likely to be aided by manipulations aimed at increasing initial preevent muscle-glycogen stores or at slowing the rate of glycogen utilization during exercise.
Researchers generally agree that during prolonged exercise at 70 to 90% V02max, glycogen depletion occurs to the greatest extent in the ST muscle fibers (Thomson et al, 1979), somewhat less in the FTa fibers, and least in the FTb fibers (Andersen & Sjogaard, 1976; Gollnick et al, 1973, 1974; Vollestad & Blom, 1985). After a subject runs a standard marathon race, his or her ST and FTa fibers are completely glycogen depleted, with only the FTb fibers retaining some glycogen (W.M. Sherman et al, 1983). During supramaximal (>100% V02max) exercise, all three fiber types become glycogen depleted, although
Exercises 3.4 Rates of liver and muscle glycogen utilization as a function of exercise intensity depletion is greatest in the FTb fibers, least in the ST fibers, and intermediate in the FTa fibers (Essen, 1978; Thomson et al, 1979). At lower exercise intensities (40 to 60% V02max), the greatest glycogen breakdown occurs in the ST fibers (Vollestad & Blom, 1985).
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