DAVID GREENE, HOST:

As some of you may know, one of today's hottest fitness trends is CrossFit. The grueling workouts combine movements such as jumping, climbing rope and weightlifting into what they call workouts of the day. There are now more than 3,000 certified CrossFit gyms. And affiliates that used to offer only adult classes are now offering them to children as well. Some kids are starting as young as 3. But as NPR News Lauren Silverman reports, some doctors say this is not the best way to get children fit.

UNIDENTIFIED WOMAN: Five, four...

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LAUREN SILVERMAN, BYLINE: Early on a Saturday morning in what looks like a cement warehouse, kids in shorts and tennis shoes are in the middle of their workout. Right now, it's pull-ups.

UNIDENTIFIED MAN: One more. All the way up. Slow down. Slow, slow, slow, slow. One more...

SILVERMAN: This CrossFit Kids class, in Rockville, Maryland, opened two years ago and was the first of its kind. Now, you'll see kids doing hand stands and bicep curls in hundreds of these classes all over the country. Parent John Belamaric is standing off to the side, outfitted in his own CrossFit gear.

JOHN BELAMARIC: I brought my daughter Audrey, who's 5, and my son Owen, who's 8.

SILVERMAN: Belamaric's been doing CrossFit for over a year.

BELAMARIC: There's something addictive about it. It hurts, but then afterwards you feel good, so.

SILVERMAN: He liked it so much he wanted his kids to try it. This weekend it's a circuit of push-ups, pull-ups and squats. A few months ago the focus was on learning dead lifts and an Olympic lift known as the clean and jerk.

Belamaric's daughter loves the routines. His son Owen is not so sure...

Do you like CrossFit?

OWEN BELAMARIC: Not really. But I like it a bit.

SILVERMAN: What don't you like about it?

BELAMARIC: Well, the workout, it's always really hard.

DR. JORDAN METZL: So when people think about strength training for kids, the initial thought is are you crazy, kids should not be lifting weights.

SILVERMAN: That's Dr Jordan Metzl. He's a sports medicine physician at the Hospital for Special Surgery in New York City.

METZL: But it seems pretty convincing that strength training is terrific for all kids.

SILVERMAN: In 2008, the American Academy of Pediatrics revised its policy statement on weight training for children and adolescents. They used to recommend against weight lifting, but after considering new research they determined it's safe for kids to start a light weight-lifting routine after the age of 8.

Dr. Metzl explains kids who strength-train won't look like mini-Schwarzeneggers - that type of bulking up doesn't happen until after puberty.

METZL: The muscles don't look better, they just act actually a lot stronger and it is very helpful for them.

SILVERMAN: The problem is some kids push themselves more than they should. A few months ago, 10-year-old Sean Cooper set a personal record.

SEAN COOPER: I lifted 85 pounds. Ten-year-old lifter 85 pounds on dead lift.

SILVERMAN: That worries Dr. Tim Hewett. He's a former power lifter and director of the Sports Medicine Biodynamics Center at the University of Cincinnati. He says there is no evidence that CrossFit is good for adults, let alone for kids.

DR. TIM HEWETT: The people who use most this type of training are people like military people, Navy SEALS, and we don't really even have any data that's even highly effective in highly conditioned people like that.

SILVERMAN: Dr. Hewett says some CrossFit trainers aren't teaching appropriate techniques for weight-lifting. And that could be dangerous.

HEWETT: What kids are attempting to do is Olympic lifts like a snatch or a clean and jerk and they don't have the power to properly perform the exercise, to bring it up to their shoulders and then bring it over their head. So they're grabbing this thing at their waist and they're trying to twist and turn their torso which is putting their spine at significant risk with weights that are greater than they can handle.

SILVERMAN: The best way to train kids, according to both doctors, is with high repetition light weight training. Kids should be able to do eight to 15 reps of whatever weight with no problem. Of course, Dr. Hewett says kids should be incorporating activity into their daily lives and that will keep them in shape.

HEWETT: Doing things they like to do, getting out into the park, into the playground, playing relatively low impact sports and doing that throughout their childhood and hopefully throughout their lifetime.

SILVERMAN: So if your kid wants to do CrossFit and the class has good trainers and they're lifting light weights, that's just fine. The point is to make exercise fun and a part of everyday life early on.

Lauren Silverman, NPR News.

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