School-aged children need at least one hour of physical play a day, an international panel of experts said Monday.

Parents have been asking how much activity their children need to maintain health. To come up with the one hour answer, a panel convened by the U.S. Centers for Disease Control and Prevention reviewed more than 850 studies.

Panelists focused on the role of physical activity on measures such as building strong bones and a healthy heart, along with less tangible benefits including self-esteem and academic success.

"The young developing skeleton is much more trainable or amenable to the benefits of physical activity than older and more mature skeletons," said Cameron Blimkie, a professor of kinesiology at Hamilton's McMaster University, who looked at the role of exercise in building bones for the report.

Although Canadian guidelines already suggest children get up to 90 minutes per day, the overall fitness of Canadian kids just earned a D from the national group Active Healthy Kids Canada.

"The interpretation is often that this is the target not the minimum ... and once we've achieved that we can kick back, rent some videos and chill out," said Mark Trembley, the group's chair. "That's not the case."

The report recommends no more than two hours a day of sitting around watching TV, or playing video games.

Instead, sedentary activities should be replaced with age-appropriate activities for children to learn skills, said the report, published in the June issue of The Journal of Pediatrics.

The payoff from physical activity is not only improved health, but better self-esteem and, to a lesser extent, even higher marks.

"There is something about attaining something in phys-ed," said Mark Edwards, a teacher at Bisset Elementary School in Edmonton, where daily physical education and teaching skills are part of the curriculum. "Something about learning a physical activity that has tremendous impact on children with their self-esteem."

Paveena Nijjar, 12, said it's fun to be active, and not difficult to stay in shape. "No, unless you're just lazy and you just don't want to do it," said the sixth grader at Bisset who has a black belt in karate, plays soccer and basketball and earns straight As.

"You only have one body. Look after it."