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Back in the 1970s, researchers in Japan studied child laborers and discovered that, among their many misfortunes, the juvenile workers tended to be abnormally short. Physical labor, the researchers concluded, with its hours of lifting and moving heavy weights, had stunted the children’s growth. Somewhat improbably, from that scientific finding and other similar reports, as well as from anecdotes and accreting myth, many people came to believe “that children and adolescents should not” practice weight training, said Avery Faigenbaum, a professor of exercise science at the College of New Jersey. That idea retains a sturdy hold in the popular imagination. As a recent position paper on the topic of children and resistance training points out, many parents, coaches and pediatricians remain convinced that weight training by children will “result in short stature, epiphyseal plate” — or growth plate — “damage, lack of strength increases due to a lack of testosterone and a variety of safety issues.”

Kids, in other words, many of us believe, won’t get stronger by lifting weights and will probably hurt themselves. But a major new review just published in Pediatrics, together with a growing body of other scientific reports, suggest that, in fact, weight training can be not only safe for young people, it can also be beneficial, even essential.

In the Pediatrics review, researchers with the Institute of Training Science and Sports Informatics in Cologne, Germany, analyzed 60 years’ worth of studies of children and weightlifting. The studies covered boys and girls from age 6 to 18. The researchers found that, almost without exception, children and adolescents benefited from weight training. They grew stronger. Older children, particularly teenagers, tended to add more strength than younger ones, as would be expected, but the difference was not enormous. Over all, strength gains were “linear,” the researchers found. They didn’t spike wildly after puberty for boys or girls, even though boys at that age are awash in testosterone, the sex hormone known to increase muscle mass in adults. That was something of a surprise. On the other hand, a reliable if predictable factor was consistency. Young people of any age who participated in resistance training at least twice a week for a month or more showed greater strength gains than those who worked out only once a week or for shorter periods.

Over all, the researchers concluded, “regardless of maturational age, children generally seem to be capable of increasing muscular strength.”

That finding, which busts one of the most pervasive myths about resistance training for young people — that they won’t actually get stronger — is in accord with the results and opinions of most researchers who have studied the subject. “We’ve worked with kindergartners, having them just use balloons and dowels” as strength training tools, “and found that they developed strength increases,” said Dr. Faigenbaum, a widely acknowledged expert on the topic of youth strength training. (His most recent book is in fact titled “Youth Strength Training.”)

But interestingly, young people do not generally add muscular power in quite the same way as adults. They rarely pack on bulk. Adults, particularly men but also women, typically add muscle mass when they start weight training, a process known as muscular hypertrophy (or, less technically, getting buff). Youths do not add as much or sometimes any obvious muscle mass as a result of strength training, which is one of the reasons many people thought they did not grow stronger. Their strength gains seem generally to involve “neurological” changes, Dr. Faigenbaum said. Their nervous systems and muscles start interacting more efficiently. A few small studies have shown that children develop a significant increase in motor-unit activation within their muscles after weight training. A motor unit consists of a single neuron and all of the muscle cells that it controls. When more motor units fire, a muscle contracts more efficiently. So, in essence, strength training in children seems to liberate the innate strength of the muscle, to activate the power that has been in abeyance, unused.

And that fact, from both a physiological and philosophical standpoint, is perhaps why strength training for children is so important, a growing chorus of experts says. “We are urban dwellers stuck in hunter-gatherer bodies,” said Lyle Micheli, M.D., the director of sports medicine at Children’s Hospital Boston and professor of orthopedic surgery at Harvard University, as well as a co-author, with Dr. Faigenbaum, of the National Strength and Conditioning Association’s 2009 position paper about children and resistance training. “That’s true for children as well as adults. There was a time when children ‘weight trained’ by carrying milk pails and helping around the farm. Now few children, even young athletes, get sufficient activity” to fully strengthen their muscles, tendons and other tissues. “If a kid sits in class or in front of a screen for hours and then you throw them out onto the soccer field or basketball court, they don’t have the tissue strength to withstand the forces involved in their sports. That can contribute to injury.”

Consequently, many experts say, by strength training, young athletes can reduce their risk of injury, not the reverse. “The scientific literature is quite clear that strength training is safe for young people, if it’s properly supervised,” Dr. Faigenbaum says. “It will not stunt growth or lead to growth-plate injuries. That doesn’t mean young people should be allowed to go down into the basement and lift Dad’s weights by themselves. That’s when you see accidents.” The most common, he added, involve injuries to the hands and feet. “Unsupervised kids drop weights on their toes or pinch their fingers in the machines,” he said.

In fact, the ideal weight-training program for many children need not involve weights at all. “The body doesn’t know the difference between a weight machine, a medicine ball, an elastic band and your own body weight,” Dr. Faigenbaum said. In his own work with local schools, he often leads physical-education class warm-ups that involve passing a medicine ball (usually a “1 kilogram ball for elementary-school-age children” and heavier ones for teenagers) or holding a broomstick to teach lunges safely. He has the kids hop, skip and leap on one leg. They do some push-ups, perhaps one-handed on a medicine ball for older kids. (For specifics about creating strength-training programs for young athletes of various ages, including teenagers, and avoiding injury, visit strongkid.com, a Web site set up by Dr. Faigenbaum, or the Children’s Hospital Boston sports medicine site.)

As for the ideal age to start weight training, Dr. Faigenbaum said: “Any age is a good age. But there does seem to be something special about the time from about age 7 to 12. The nervous system is very plastic. The kids are very eager. It seems to be an ideal time to hard-wire strength gains and movement patterns.” And if you structure a program right, he added, “it can be so much fun that it never occurs to the kids that they’re getting quote-unquote ‘strength training’ at all.”