Many people are under the misconception that any exercise makes muscles grow larger and stronger. That's not true. Back in 1925 German scientists discovered that to acquire large muscles you must increase the intensity of work done within a given time. That means that it doesn't matter how much work you do. What counts is how fast you do it. This discovery has come to be known as the overload principle. Perhaps the most famous experiment which demonstrates it involves rats trained to run at different speeds for varying lengths of time. Rats that ran at 6 meters per minute for 195 miles had smaller muscles that rats that ran at 26 meters per minute, but for only 58 miles. In another study all rats ran the same speed. One group ran for an hour per day for three months for a total of 35 miles. Another ran at the same speed for 3 hours a day for six months, a total of 207 miles. When the experiment ended, the size of the muscles in both the groups remained the same.
The principle of overload also explains why sprinters have bigger and larger muscles that distance runners. Although it's more work to run a mile than it is to run 100 yards, the sprinter is doing more work per second. Consequently, his muscles will become larger.
A muscle is composed of muscle fibers, the number of which vary from person to person in the same muscle. The muscle fibers in the upper arm of one man may number 40,000 while the calf muscle may contain 1,120,000 fibers. Another person may have only 946,000 fibers. Heredity controls the number of muscle fibers present, and it will not change.
It it these muscle fibers which determine how large a muscle can grow. In 1897 an Italian scientist named Morpurgo showed that even though exercise had produced a 50 per cent increase in the size of a muscle, the number of muscle fibers stayed the same. The reason for the change was the increase in diameter - about 40 per cent - of the muscle fibers. Thus, he developed the principle now commonly accepted that strength development has a definite limitation depending on the number of muscle fibers present at birth and on the fixed maximum size to which any muscle fiber may grow. That means that people who want to grow stronger cannot expect unlimited development. Heredity has already drawn the line. But, through proper exercise they may reach their heredity boundaries.
(IronMan Magazine March 1976 Vol. 35 No. 3)