CoachMuscle fibres come in two main types. Slow fibres contract slowly and weakly but tirelessly: rich in mitochondria and in capillaries, red with an oxygen-storing protein, they run on respiration and dominate in postural muscles and in endurance athletes. Fast fibres contract quickly and powerfully but tire within a minute: poorer in mitochondria, paler, they rely on the phosphate store and fermentation and dominate in sprinters. The proportion in a given muscle is largely inherited; training changes the size and the equipment of the fibres more than their number.
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Example 9.10 (Reading a muscle biopsy)
A stain for mitochondrial enzymes on a thin slice of thigh muscle shows a mosaic: dark fibres (slow) and pale fibres (fast). A marathon runner’s sample is 80% dark; a sprinter’s 30%; an untrained person’s about half and half. The same stain on the same runner after a year of sprint training shows the pale fibres enlarged but not more numerous.