Jen Singer, a mother of two young children in Kinnelon, N.J., finds another factor contributing to sedentary lives: parental concern for children's safety. "My mother would tell us to go outside and play until dinnertime," she recalls. "But I can't send my kids outside unless I go with them. If I'm making dinner, they have to stay inside and watch a video."

At a time when childhood obesity is the subject of daily headlines, experts point to obvious culprits: too much junk food and too many hours of television. Yet those represent only part of the problem. From school buses and supervised play to more homework and less recess, a host of other factors in families, schools, and suburbs have created a sea change in children's lives, conspiring against physical activity and contributing to their expanding waistlines.

In the past 30 years, childhood obesity has doubled for children between the ages of 2 and 5 and tripled for 6- to-11-year-olds. More than 15 percent of children between 6 and 19 are considered obese. Countering that trend, child advocates say, will require nothing less than a multipronged national effort.

"There's been a great shift in cultural values," explains Rhonda Clements, president of the American Association for the Child's Right to Play. "In schools, we see a much greater emphasis on a need for academic skills at the earliest possible age. Unfortunately, because of the emphasis on schoolwork and productivity, some of the basic childhood activities that were part of every other generation have been eliminated."

The changing lunch hour

Today, less than 6 percent of high schools require juniors and seniors to take physical education. Dr. Clements also sees an "enormous decrease" in the number of school playgrounds. And recess has disappeared in some elementary schools where principals, anxious about preparing students for high-stakes standardized tests, have deemed it "nonproductive."

Lunch hour is another culprit in some schools. Although cafeteria menus are coming under fire, the problem goes beyond what children eat to include when they eat. Crowded schools must extend lunch hours to serve everyone.

"If lunch hour is too early and the kids aren't hungry, they may not eat a healthy meal, and then they'll snack later," says Chris Economos, an assistant professor of nutrition at Tufts University. "If lunch hour is too late, they'll snack first and not be hungry for a good lunch."

Students also have limited time to eat, Professor Economos says. "We're trying to get them to eat healthier food, but that takes time to chew."

The demise of neighborhood schools also encourages sedentary lives. Many school districts have built "mega-schools" on huge sites at the edge of town, which few students can reach by walking.

Suburban sprawl also means that parents may drive children miles to play with friends or participate in extracurricular activities, sports, music lessons, and tutoring sessions. Mothers average an hour a day driving their children around, according to the Surface Transportation Policy Project.

For more than 14 million children - one-quarter of students between kindergarten and 12th grade - no parent is home after school. They must take care of themselves. Many receive strict instructions from parents: Lock the door and don't go outside. It's a recipe for inactivity and an opportunity to snack. Only 11 percent of students - 6.5 million - attend after-school programs, where they are likely to get a nutritious snack and take part in fitness activities.

"Changes in family life mean that even many young kids are feeding themselves," says Steven Mintz, co-chair of the Council on Contemporary Families. "It's not just after-school snacks. It's dinner."

Mr. Mintz sees food as a response to stress for some children. "We as a society underestimate the kinds of stress that kids feel. Food is one of the ways we deal with this."

Why so much youthful anxiety? "School is not such a pleasurable place as it once was," Mintz says. He attributes part of that to the disappearance of recess and gym, and to the greater emphasis on testing. Even play is institutionalized, taking the form of play dates or organized activities, such as Little League.

"Those activities often are very stressful," Mintz says. "They're competitive but not fun."

His own son, he notes, responded to stress by "going home and eating more than he should have."

Even at home, the growing emphasis on academic excellence is changing children's lives. "You should see the homework my first-grader has," Ms. Singer says.

One of the biggest factors contributing to children's less active lives, she feels, is a culture of parental fear. Although her 7-year-old son wants to ride his bicycle down their street and back, she won't let him go alone.

"I just can't be sure that he'd be safe from predators," she says, echoing the concerns of many parents. "When I was about his age, my cousin and I rode our bikes to another town, bought candy, and came back. But times have changed."

Other changes in children's lives can be found in their toys. "The toy vehicles kids are riding in are battery-powered, rather than kid-powered," Clements says. "Computer games simulate the sport instead of having children actually play the activity. Playing golf in your living room by computer cannot replace the values we acquire from the actual social interaction and physical participation in sports."

More time at the mall

A consumer culture is also changing children's activities. Families that might once have enjoyed weekend activities outdoors are now likely to say, "Wanna go to the mall?" Children do far more shopping than in the past, according to Juliet Schor, author of "Born to Buy." In 1997, she says, the average child between the ages of 6 and 12 spent more than 2-1/2 hours a week shopping, a full hour more than in 1981. Children spent five times as much time shopping as playing outdoors.

The distance from one end of the mall to the other encourages some parents to buckle children into strollers, which are now available in larger models to accommodate bigger children. And what is a trip to the mall without a stop at the food court?

Yet encouraging news exists. Within the past two years, three states - Virginia, Michigan, and Connecticut - have passed laws mandating recess. Clements is "extremely hopeful" that all state education departments will follow suit.

Efforts are also under way to reinstate physical education, Economos says. Recommendations include a minimum of 150 minutes a week for elementary school students and 225 minutes for high school students.

A national Walk to School Day in October is also gaining recognition. By one estimate, 65 percent of students walked to school 30 years ago. Today only 10 percent do.

In June, the Council of Educational Facility Planners International dropped its recommendations calling for vast acreage for large school sites. "That's a big deal, because it will give school districts more flexibility in locating schools on smaller sites in places accessible by walking and biking," says Constance Beaumont of the Oregon Transportation and Growth Management Program.

Advocates also hope to increase the number of after-school programs, giving more students activities.

"As parents and educators, if we don't expose them to a physically active lifestyle as a young child, they will not acquire the values that are needed for lifelong participation in sports and physical activity," Clements says. "The first step toward curbing this obesity issue is making sure children are active on a daily basis. It's cost-effective."

As evidence that change is possible, Economos points to the widespread shifts in public attitudes toward tobacco, recycling, and seat belts.

"It's all about a societal shift," she says. "It's a long-term process. If we're all going to chip away at this for the next 20 years, we will see a change."

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