Classes are out for children across the country, but the fight to boost fitness and curb fat among America's youth doesn't end with the school year, researchers and health advocates say. That's why they're encouraging parents to turn off the television and video games and find ways to get kids moving.

Groups ranging from the American Heart Association to the National Association for Sport & Physical Education succeeded in efforts in Florida and Oklahoma this year to increase the time children spend in physical education classes. But the fourth consecutive defeat last spring of a similar measure in the Maryland legislature highlights the difficulties such campaigns face, even when concern about childhood obesity is high.

About 9 million children ages 6 to 19 (or 16 percent) are overweight under the standard used by the Centers for Disease Control and Prevention — three times the percentage in 1980. (CDC does not use the term "obese" in describing weight problems in children.)

Many of the same factors that have hampered efforts to limit junk food sales on school campuses — including tight budgets and political turf battles — have also blocked efforts to expand physical education. Daily PE classes, once routine, are now offered by fewer than 10 percent of public schools, even though regular exercise is known to fight fat. Most elementary students get far fewer than the 150 minutes of PE per week recommended for that age; in many Maryland schools, elementary students get only about 30 minutes.

But experts say schools don't deserve all the blame for children's growing waistlines. A study of 5,380 kindergartners and first-graders published last year in the American Journal of Public Health found that students gained more weight during the summer than they did the rest of the year. Once classes resumed, the children's body mass indexes fell, researchers found.

Health experts say this study underscores the need for parents to share responsibility for keeping kids fit. During the summer, when kids may have less structure to their day, that primarily means getting them outdoors.

"Studies have shown that the more time kids spend outside, the more active they are," said Charlene Burgeson, executive director of the NASPE, which publishes a summer fitness calendar for students, available at http://iweb.aahperd.org/naspe/template.cfm?template=teacher_toolbox_jun08.html.

"It's not a complex equation; you don't have to have a lot of equipment and space," Burgeson said. "For families, it's just thinking about how they spend their discretionary time. Do you rent a movie or take a walk?"

An added incentive: Health experts tout a growing body of research that links physical activity with improved academic performance. Paul Houston, executive director of the American Association of School Administrators, said shrinking school budgets and pressure to boost reading and math scores have reduced the class time available for PE programs.

Meanwhile, children's activity outside school has also dropped: According to NASPE's 2006 "Shape of the Nation" report, more than a third of young people in grades 9 through 12 do not participate in "vigorous physical activity" outside school.

Four years ago, health advocates were dismayed to discover that in many Maryland public schools, elementary students were getting only about 30 minutes of physical education a week. So when a coalition of groups, including the American Heart Association and the American Diabetes Association, sought to boost that, they thought convincing others would be an easy sell.

They are still fighting.

When the proposal was first defeated, "we were absolutely in shock," said Michaeline Fedder, Maryland director of advocacy for the American Heart Association. "But you get used to it, and you understand where it's coming from."

Houston, of the administrators association, said the key to winning the battle will be getting parents mobilized.

"It's much better to have awareness programs for schools and states that discuss the lack of physical education," he said. "If you do that, parents are going to demand more of it and schools will respond."

In the meantime, experts advise parents to take advantage of the warm days to encourage their children to "disconnect from their computers and go outside and play," Burgeson said. "Even though obesity is really driving the awareness and the conversation, what folks are finding out is that the benefits go way beyond weight control and apply to overall physical and mental health energy and the ability to focus."

HERE'S SOME WAYS TO GET KIDS MOVING

To get kids moving this summer, here are some tips from the National Association for Sport & Physical Education, www.naspeinfo.org.