While strength training was once doubted to benefit kids, a new research review confirms that children and teenagers can boost their muscle strength with regular workouts.

The findings, researchers say, support recent recommendations from the National Strength and Conditioning Association (NSCA) that kids strength-train two to three times a week - though only under professional supervision.

In years past, there were concerns that school-age children and teenagers might run a high risk of injuring themselves through strength training, which can be performed using free weights, exercise machines, elastic bands or the body’s own resistance.

However, studies in recent years have shown that kids’ risk of injury from strength training is no greater than - and is often less than - that from other types of exercise or sports. And experts now say that the potential benefits of such training - such as increased bone density, decreased body fat and boosting performance and curbing injury risk in sports - generally outweigh any risks.

The new study, reported in the journal Pediatrics, looked at age and other factors that might influence the effectiveness of strength training for kids.

Dr. Michael Behringer and colleagues at the German Sport University Cologne combined the results of 42 previously published studies that involved a total of 1,728 children and teenagers who were randomly assigned to perform supervised strength training or serve as a control group.

In most of the studies, kids used free weights or resistance-training machines, anywhere from one to five times a week, for an average of 40 minutes per session. The duration of the training ranged from one month to just over a year.

Overall, Behringer’s team found, the training was effective at boosting kids’ strength, with gains being greater among older kids versus prepubertal children (typically about age 10 or younger). And, not surprisingly, a few weekly sessions worked better than one, while a longer training duration was more effective than a short one.

The average strength gain varied widely among the studies, but in the majority the kids improved their strength by 20 percent to 40 percent of their starting levels.

Exercises involving what are known as isotonic contractions - bicep curls, squats and bench presses, for example - appeared to be most effective.

“Since resistance training in children and adolescents is known to be safe and to be associated with several health benefits, children and adolescents should be generally encouraged to participate in a resistance-training program,” Behringer told Reuters Health in an email.

He added that “our data underlined once again that it is effective over all phases of maturity.”

According to Behringer’s team, the findings are in line with 2009 guidelines from the NSCA suggesting that kids strength train two to three times a week. According to the group, children who are old enough for sports - around age 7 or 8 - are generally ready for some strengthening exercises.

But the NSCA also cautions that children should only perform such exercises under the supervision of someone with professional training - as part of school physical education or an athletic training program, for example.

“If qualified supervision, age-appropriate exercise equipment, and a safe training environment are not available, youth should not perform resistance exercise due to the increased risk of injury,” the NSCA writes in its guidelines.

Behringer agreed that parents should consider professional supervision a must.

SOURCE:  Pediatrics, online October 25, 2010.