Muscle Glycogen Resynthesis after Short Term, High Intensity Exercise and Resistance Exercise

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Abstract
Typical rates of muscle glycogen resynthesis after short term, high intensity exercise (15.1 to 33.6 mmol/kg/h) are much higher than glycogen resynthesis rates following prolonged exercise (≈2 mmol/kg/h), even when optimal amounts of oral carbohydrate are supplied (≈8 mmol/kg/h). Several factors differ during post-exercise recovery from short term, high intensity exercise compared with prolonged exercise. The extremely fast rate of muscle glycogen resynthesis following short term, high intensity exercise may originate from these differences. First, peak blood glucose levels range from 6.6 to 8.9 mmol/L during recovery from short term, high intensity exercise. This is markedly higher than the blood glucose values of 2 to 3.4 mmol/L after prolonged exercise. In response to this elevation in plasma glucose levels, insulin levels increase to ≈60 μU/ml, a 2-fold increase over resting values. Both glucose and insulin regulate glycogen synthase activity, and higher levels of them improve muscle glycogen synthesis. Secondly, high intensity exercise produces high levels of glycolytic intermediates in muscle, as well as high lactate levels ([La]) in muscle and blood. Finally, fast-twitch glycolytic muscle fibres are more heavily used in short term, high intensity exercise. This promotes greater glycogen depletion in the fast-twitch fibres, which have a higher level of glycogen synthase activity than slow-twitch fibres. While the exact contribution of each of these factors is unknown, they may act in combination to stimulate rapid muscle glycogen resynthesis rates. Muscle glycogen resynthesis rates following resistance exercise (1.3 to 11.1 mmol/kg/h) are slower than the rates observed after short term, high intensity exercise. This may be caused by slightly lower muscle and blood [La] after resistance exercise. In addition, a greater eccentric component in the resistance exercise may cause some interference with glycogen resynthesis.