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Muscle Metabolism

Muscle cells have varied and flexible requirements for fat and carbohydrate substrates. Switching between the two depends on the available supply and is governed by the Randle Cycle which ensures continued function – especially of the heart– under diverse nutritional circumstances. The efficiency with which human muscles can convert food energy to work ranges from about 20-25%.

In skeletal muscle, Type I (slow twitch) fibres are heavily perfused by capillaries and contain many mitochondria necessary for the oxidative conversion of food energy that allows the muscle to do physical work. Type II fibres are classified in several sub-types according to their speed of contraction. The fastest fibres contain few mitochondria and can sustain only short anaerobic bursts.

The major skeletal muscles contain mixtures of Type I and Type II fibres. The relative proportions are genetically regulated but can be altered by training. Training for endurance events increases the proportion of Type I slow twitch fibres and their ability to oxidise fat.

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