ABC News' Paula Faris and Cathy Becker report:
CrossFit is a military style workout program that's popular among fans of extreme exercise.
It is now available for children, but some people question whether the intense cross-training system, which uses dead lifts and bar bells, is safe for kids.
Eric Von Frohlich, who runs EVF CrossFit in Manhattan, said the children's CrossFit is not as hardcore as the adult program.
"It's scalable. We teach the movement and then allow that individual's ability to come through," Von Frohlich told ABC News. "As long as there's proper coaching, proper guidelines, and we're teaching them proper movement, there's no chance of them having any kind of problems."
Not everyone agrees. Dr. Paul Stricker, a pediatric sports medicine specialist, says cross training can be too intense for some children's bones and tendons.
"Strength training has benefit, also risks," Stricker of the Scripps Clinic in San Diego added. "If you have the wrong child at the wrong time with the wrong amount of experience and the wrong supervision, it can be quite risky."
In a statement, CrossFit Kids said: "Our intention is to make children safer in their daily lives, such as when lifting a backpack or on the field of play when they have to run, cut, jump, etc."
Parent David Bernie of New York City said his elementary school-aged sons have been more focused and energized since they started the program about six weeks ago.
"They just love this. It's incredible," said Bernie, who also attends CrossFit classes himself.
Von Frohlich's own daughter is in the CrossFit class that he teaches for kids 8 to 12.
"To know that she'll have a lifetime of great habits," the CrossFit coach said, "and she enjoys it is every parent's dream."