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Muscle growth, known scientifically as muscular hypertrophy, is literally the increase in the size of muscle cells in a living body. It can be caused by a number of different factors, and occurs in varying degrees throughout life. It is a natural biological process, but can be aided or promoted by physical exercise and nutritional supplements. There are two distinct types of muscle growth, one which mainly increases the size of muscles, and one which mainly increases their strength.
In humans, natural hypertrophy usually ends in the late teens, following puberty. After that point it takes a conscious effort to see continued muscle growth. Short, intense anaerobic exercises — exercises in which the targeted cells are not allowed time to take in fresh oxygen — are the most efficient way of promoting muscle growth. The most obvious example of anaerobic exercise is weightlifting, as compared to an aerobic exercise, such as jogging, in which the muscles are allowed to take in oxygen over the course of the exercise.
Sarcoplasmic hypertrophy, the type of muscle growth that relates mainly to size, involves an increase in the volume of a particular kind of fluid, known as sarcoplasmic fluid, in the muscle. This type of muscle growth is achieved by performing strength exercises that involve lifting lighter weights in a relatively high number of repetitions. Lifting for strength, known as myofibrillar hypertrophy, involves only a small number of repetitions of a given exercise, but with weights totaling nearly one hundred percent of the targeted muscle's capacity. A weightlifter exerting his maximum effort to lift a weight-loaded barbell just once, is a prime example of a myofibrillar exercise.
A leading theory behind the actual development of muscles is known as microtrauma. As muscles are overloaded to lift weights or do other work, tiny tears in the muscle fibers occur. This damage is repaired by the body, which overcompensates by building a stronger fiber in an effort to minimize future damage. Protein is essential to this repair, and to the growth of muscles, and it is estimated that for optimal growth during strength training, 0.03 ounces (about 1 gram) of protein should be consumed daily for each pound (0.45 kilograms) of body weight.
In addition to protein, other natural and synthetic drugs may be taken to promote muscle growth. Testosterone, anabolic steroids, and human growth hormone are just a few of the many chemicals and compounds that naturally, or artificially, increase hypertrophy. Many of these substances are toxic when overused, however, and often have dangerous side effects. In addition, most of them are universally illegal to obtain, or use, without a medical prescription.