MUSCLE TRAINING
One type of muscle can transform itself into another type through training. Weightlifters and bodybuilders can build up intermediate muscle fibres from fast-twitching biceps and other muscles. The proportion of different muscle fibres in a group of muscles can change according to the training that they undergo. The proportion of fast and slow muscle fibres is genetically determined but the proportion of intermediate muscle fibres to fast fibres can increase due to athletic training.
In humans most muscle fibres are mixed and therefore appear pink. The back and calf muscles are dominated by slow muscle fibres and are therefore red in colour and they maintain posture. Eye and hand muscles, which carry out brisk movements, are white in colour as they contain fewer blood vessels and less myoglobin.
Repeated muscle training leads to the development of more mitochondria, larger glycogen reserves and a higher concentration of enzymes and proteins in muscles. All these increase the muscle bulk in general and muscles get hypertrophied or bulky. The number of muscle fibres does not change as that is genetically determined, but their contents (proteins, glycogen, enzymes, mitochondria) may change. Some people remain skinny no matter how much they exercise or how much they eat. Their muscle bulk may marginally improve. This is due to an individual's constitution. Sumo wrestlers build up huge muscle bulk and fat reserves through training and diet.
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