The average size of a person's muscles is determined to a great extent by heredity plus the level of testosterone secretion, which, in men, causes considerably larger muscles than in women.
With training, however, the muscles can become hypertrophied perhaps an additional 30 to 60 per cent. Most of this hypertrophy results from increased diameter of the muscle fibers rather than increased numbers of fibers, but this probably is not entirely true, because a very few greatly enlarged muscle fibers are believed to split down the middle along their entire length to form entirely new fibers, thus increasing the number of fibers slightly.
Read: How to Increase Muscle Mass Anywhere On Your Body
The changes that occur inside the hypertrophied muscle fibers themselves include (1) increased numbers of myofibrils, proportionate to the degree of hypertrophy; (2) up to 120 per cent increase in mitochondrial enzymes; (3) as much as 60 to 80 per cent increase in the components of the phosphagen metabolic system including both ATP and phosphocreatine; (4) as much as 50 per cent increase in stored glycogen; and (5) as much as 75 to 100 per cent increase in stored triglyceride (fat). Because of all these changes, the capabilities of both the anaerobic and the aerobic metabolic systems are increased, increasing especially the maximum oxidation rate and efficiency of the oxidative metabolic system as much as 45 per cent.
In Layman's Words, with training, your muscles can become like Arnold Schwarzenegger's anatomy.
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