Lifting weights before puberty makes children stronger and has not been shown to stunt growth or damage the growth plates in their bones (Pediatrics, November 2010). The older the child, the greater the gain in muscle strength from resistance training. The more and the heavier weights they lift, the stronger they became. A surprising finding was that children did not show a significant increase in strength when they enter puberty, a time when their testosterone levels rise significantly.
The best time for future competitive athletes to start training is before they reach puberty. Having large strong muscles makes you a better athlete, and starting training before puberty enlarges the bones that are used primarily in that sport. Muscles growth is limited by the size of the bones on which they attach. The larger the bone, the stronger the muscle. Children who start to play tennis before they go into puberty have larger bones in the arm that holds the racquet. They also have larger bones in their tennis arm than those who start to play tennis later in life. The larger and stronger your muscles, the harder you can hit a tennis ball. As little as four weeks of hard exercise in growing animals increases bone mass (Medicine & Science in Sports & Exercise, October 2000). This suggests that children who start training while they are still growing will have an advantage over athletes who start training after puberty, because having larger bones allow a person to grow larger muscles.
Lifting weights during growth has not been shown to prevent children from growing to their full potential height. Bones grow from epiphyses, growth centers that are the weakest part of bone, but strength training during growth has not been shown to damage these growth centers. Children who lift weights in supervised programs do not suffer more injuries than adults. With increased strength comes increased speed and increased coordination in movements requiring strength.
In most sports, the strongest athlete wins. Weightlifter Naim Suleymanoglu of Turkey, who won three Olympic gold medals and is probably the greatest weightlifter who ever lived, started lifting weights when he was eight years old. Muscles can only grow to be as strong as the strength of the bones on which they attach, so people with the biggest bones are the ones who can grow the biggest muscles.
Children who lift weights do not grow muscles as large as older people do. Muscles are made up of thousands of individual muscle fibers. Each muscle fiber is innervated by a single nerve, although each nerve can innervate many muscle fibers. When you contract a muscle, you contract only a few muscle fibers at one time. With strength training, children learn to contract more muscle fibers at the same time, so they become stronger primarily by being able to contract more muscle fibers. Adults commonly grow larger muscles.
There is great concern that children may be subjected to unreasonable coaches and inconsiderate parents who place athletic training above the child's own needs and desires. In one study from Southern California, 90 percent of female cross country runners who stated running before they were nine stopped running before they reached high school. In 1967, I started competitive long distance running for young children and was the first national chairman of the age group committee of the Amateur Athletic Union and The Road Runners Club of America. Children came from all over the United States and Canada to compete in age group cross country and track running. Many were coached by experienced runners and trained with the same types of workouts used by the older runners. These children rarely suffered from injuries, and when they were injured, they recovered faster than the older runners. However, the real problem of starting children in competition at an early age is burnout. My own son started serious running when he was five and ran a mile in four minutes and 52 seconds when he was nine. He stopped competitive running when he was eleven.
The concern about serious athletic training for young children is more mental than physical. Children should not begin serious athletic training unless they want to do it. They should take days off from training when they want to, and their coaches and parents must allow them to be children.