Vollestad and Blom (1985) also found that the rate of glycogen breakdown is the same in all ST fibers for the duration of exercise; the authors found that the rate of breakdown in FTa fibers increases after the first few minutes of exercise and thereafter stays the same in all fibers, whereas the rate of glycogen breakdown in FTb fibers is quite variable.
Liver glycogen levels also fall during exercise, but this rate of decrease has not been studied in the same depth as has that of muscle glycogen. This is because the biopsy technique used to collect samples of liver for glycogen analysis is far more painful and potentially more dangerous than is the technique for muscle biopsy; thus both researcher and subject are much more reluctant to undertake this procedure!
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However, indirect evidence suggests that the rate of liver glycogen utilization increases linearly with increasing exercise intensity and averages 0.9 to 1.1 g/ min at exercise intensities of 70 to 85% V02max. Exercises 3.4 shows that if the total liver glycogen stores are 135 g before a marathon race (see Exercises 3.3), these stores will be completely depleted within 140 to 215 minutes of exercise at 70 to 85 % V02max if the subject takes no carbohydrate by mouth during that period. This means that during prolonged exercise at 70 to 75 % V02max, liver glycogen depletion is likely to occur before muscle glycogen depletion in the athlete who takes no carbohydrate by mouth during exercise.
In summary, the extent of muscle and liver glycogen depletion during exercise depends on the intensity and duration of the exercise and will be greatest after prolonged exercise at standard marathon (85 % V02max) or ultramarathon (70 to 75% V02max) pace. However, at exercise intensities of 70 to 75% V02max, depletion of liver glycogen stores is likely to precede depletion of muscle glycogen stores. The importance of this is discussed next.
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