Muscles are markedly affected by the amouint of use they receive. The structural and biochemical changes in human muscles can be investigated by the technique of needle biopsy, whereby a small piece of muscle is removed for analysis.
Muscles can be increased in size and strength by exercise involving the development of high muscle tensions, such as in isometric exercises or weightlifting. These procedures result in enlargement of the individual fibres in the muscle, including an increase in the quantity of contractile protein in them.
Training of the muscles by endurance exercises, as in distance running for example, results in an appreciable increase in the blood supply to the muscle via proliferation of the blood capillaries. The muscle fibres themselves do not increase in size very much, but there is an increase in the quantities of respiratory enzymes in them. There is also an increase in the amount of connective tissue in the muscles, so that they become less susceptible to minor injuries.
Disuse, such as occurs when a person is confined to bed or subject to prolonged weightlessness in a space station, leads to reduction of the muscle mass and especially of the contractile proteins. Hence the need for specific exercise regimes for invalids and astronauts.
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