Factors affecting changes in muscle glycogen concentration during and after prolonged exercise.

Muscle glycogen concentration decreases continuously during prolonged severe exercise. The rate of depletion increases with increasing exercise intensity. During heavy exercise at 65-75% of VO2 max, time till exhaustion correlates with the pre-exercise muscle glycogen concentration and exhaustion coincides with empty glycogen stores. A substantial number of type I fibres are depleted within 60 min of mild exercise (43% of VO2 max). These results suggest that glycogen depletion may contribute to muscle fatigue even during light exercise. When glycogen depletion becomes present in an increasing number of type I fibres, glycogen breakdown occur in an increasing number of type II fibres, indicating that these fibres take over more of the force development. It is well documented that a high carbohydrate diet is a prerequisite for a rapid post-exercise muscle glycogen resynthesis. Recent studies indicate that there is an increasing rate of glycogen resynthesis with increasing oral glucose intake up to a certain level. A further increase in glucose intake gives no further increase in the rate of resynthesis. However, glucose infusion is reported to induce a 2-3 times higher rate of resynthesis. This higher rate of resynthesis may be caused by the higher blood glucose concentration which can occur during glucose infusion. Preliminary results indicate that muscle glycogen is resynthesized at equal rates when the blood glucose concentration is kept at the same level, irrespective of whether the glucose is administered orally or by infusion. Muscle glycogen resynthesis seems to be slower during fructose ingestion than glucose ingestion.(ABSTRACT TRUNCATED AT 250 WORDS)