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Pint-sized tots are pumping iron at the CrossFit Calgary gym. Kids as young as 6 are doing curls with two-pound dumbbells, while their older peers, 8 and 9, are doing the same with five-pound weights under the watchful eyes of CrossFit Kids' certified instructors. No parent has ever complained, and no child has ever been hurt. In fact, there's only one problem with pint-sized tots lifting weights, says Chantal Theberge, the gym's kids and teens program director.

"If anything, we have to hold them back a little bit because they always want to go a little bit faster and harder than anyone else," she says.

New fitness guidelines released last week by the Canadian Society for Exercise Physiology recommend that children and youth aged 5 to 17 get at least one hour of moderate to vigorous intensity physical activity every day, including muscle- and bone-strengthening activities at least three days a week. That doesn't mean CSEP is suggesting five-year-olds should take up weightlifting per se. But despite a widespread belief that lifting weights is harmful to children, CSEP experts say it's perfectly safe, provided children have proper supervision.

It was once thought that kids who lift weights might damage their epiphyseal plates, growth plates found in bones, resulting in stunted growth.

There "isn't good evidence that doing weight training stunts growth," says Ian Janssen, an assistant professor in the school of kinesiology and health studies at Queen's University whose research contributed to the CSEP guidelines.

That myth, which developed because of the relatively short stature of many gymnasts, is due to a "selection issue," he says. Kids who are shorter than their peers are more likely to be streamed into gymnastics because it favours a low centre of gravity.

David Behm, a professor in the school of human kinetics and recreation at Memorial University and co-author of the CESP position paper on resistance training in children and adolescents, agrees with Dr. Janssen on the safety of underage lifting.

"If you look at the relative incidence of injuries of everything Canadian kids do, playing hockey, skateboarding, skiing etc., the incidence of injury for those sports is much higher than any children who have ever been on a strength training program," he says.

Lifting weights helps prevents injuries, he adds. "You're going to have a child whose bones are denser, you're going to have muscles that are stronger, and you're going to have tendons and connective tissue that is also denser and stronger."

Dr. Behm says no child is too young to start. "Physiologically, there is absolutely no reason why a kid can't resistance train," he says.

Of course, learning proper form and not over-exerting themselves are essential, which is why kids at CrossFit Calgary can go for months before they even try a bicep curl."Kids don't get a weight in their hand until they have the movement down perfectly," Ms. Theberge says. "If we have a nine-year-old or an eight-year-old who has the movement down perfectly, they're going to get five-pound dumbbells."

Kids who are 6 and have the movement down will be given two-pound dumbbells.

Even though it may not seem like much weight, it benefits kids in several important ways, Dr. Behm says.

"It's giving them co-ordination and balance because they have to learn how to lift it properly, so that will help them in terms of performance in sports and activities of daily living. It's providing them with more strength and endurance," he says.

Perhaps most importantly, weight training helps to boost bone mineral density – the main reason the new fitness guidelines recommend kids engage in muscle- and bone-strengthening activities.

Bone density peaks and begins to decline at about age 20, which is why CSEP recommended these strengthening activities. They want kids to build up the density so they'll have more to lose when they are older, Dr. Janssen says.

But, he says, kids don't need to begin lifting weights until about age 10. Prior to that, he says, playing on the jungle gym, jumping around, skipping, running, push-ups and tumbling around on the playground are good enough muscle- and bone-strengthening activities.By age 10, kids are becoming competitive in the sports they play. "That's when you might see that introduced because they want to develop the strength and the power that's going to be beneficial for the sport that they're engaged in," he says.

Yet while kids may be showing an interest in weight training, it's important they not overdo it, Dr. Behm says.

Kids should only lift between 50 to 60 per cent of their one repetition maximum for safety reasons, he says. Lift more than that and an injury is more likely, if only because a kid drops a weight on her foot.

And, of course, supervision is essential. As Ms. Theberge has discovered with the kids in her program, youngsters often want to go too hard.

"It's all a maturity thing," Dr. Behm says. "Typically, what's going to happen is that kids are going to want to have a weightlifting contest. Johnny lifts 50 pounds and so Tommy wants to lift 55 pounds and they want to lift it over their heads and they drop it on their heads. You've got have somebody there to make sure it's a weight-training program and not a weightlifting program."

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