Jane Clark calls this the age of "containerized" kids.

As infants, children are plopped from car-safety seats to high chairs to baby seats to watch TV, said Clark, a movement specialist at the University of Maryland.

"It's partly for safety, of course," said Clark, professor and chairwoman of the university's department of kinesiology. But children, even infants, move too little these days, setting the stage for a sedentary, unhealthy life, she and other experts warn.

"Parents think physical activity takes care of itself in kids," said Clark. "But it doesn't."

A study published earlier this year in the medical journal The Lancet, supports Clark's point. Researchers from the University of Glasgow in Scotland tracked the movements of 78 three-year-olds by having them wear a matchbox-sized monitor clipped to their waistband for a week. The researchers found the average toddler was active for only 20 minutes a day -- far less than the hour recommended by pediatricians.

Making matters worse, children today spend much less time playing outdoors than their parents did, according to a study conducted by Rhonda Clements, a professor of education at Hofstra University in New York and president of the American Association for the Child's Right to Play, a group that promotes physical activity.

In a poll of more than 800 mothers conducted in 2002 by Clements and her colleagues, 71 percent of the mothers said they recalled playing outdoors every day as children, but only 26 percent of them said their kids play outdoors daily.

While fear of crime was an issue, the mothers also cited lack of time as well as too much time spent by their children watching television or playing computer games, Clements said.

That lack of activity is having a troubling effect on American kids. Thirteen percent of children aged 6 to 11 are overweight, as are 14 percent of teens aged 12 to 19, according to federal health statistics. And overweight children are more likely to be overweight as adults.

But a little conscious effort to get your kids moving can instill a lifelong exercise habit, Clements and other experts agree. Regular physical activity has been linked to a reduced risk of many cancers, heart disease, diabetes, high blood pressure and many other ailments.

So where to start?

Beginning when your child is just an infant, you can encourage physical movement, Clark advised. That could mean taking time to rock your baby in your arms and, when he is older, encouraging him to crawl through a living room obstacle course, to make activity fun.

When dealing with children beyond infancy, "forget the word exercise," Clark said. Instead, focus on the fun. Rather than asking, "Want to go exercise?" try something like, "Want to go outside and kick the ball around?"

Serving as role model -- and that means working out and staying active yourself -- is also a great motivator for children, Clark said. "If a mother and father both exercise, compared to those who don't, kids in that house are six times more likely to exercise," she said. "If one parent exercises, the child is three times more likely."

"Every kid starts out liking physical activity," Clark added. "By the time they are 15, the majority of kids don't like sports."

So it's crucial, she said, to keep activity fun and to plan it as a family. "Plan something on the weekend that is physically active, even if it's just walking around a museum or a fair," Clark said.

Clements tells parents to provide their children a goal or incentive to be active. "It could be as simple as, 'Let's create a backyard obstacle course,' " she said.

Give them a choice, too. "Do you want to play on the swings? What about tag with your brothers?" she suggests.

Encourage creativity, Clements said. "Encourage kids to create playthings out of objects." Remember airplanes from paper? Mud puddles?

Rae Pica, a children's movement specialist in Center Barnstead, N.H., is a big believer in the motivational power of music.

"Put on some music," Pica said. "Have a parade at home. Break out the pots and pans."

Or break out the bubbles, Pica suggested. Kids can run and jump to burst them.

When choosing a day-care program for your child, be sure to ask about the commitment to activity, Clark and Pica suggested.

"Not just recess, but movement" is what you are after, Pica said.

The National Association for Sport & Physical Education was so concerned about the sedentary lifestyles of today's children that it issued physical activity guidelines in 2002. They are aimed at helping to meet the developmental needs of infants, toddlers and preschoolers.

Clark, of the University of Maryland, chaired that task force. Among the recommendations: Encourage children to be physically active from the beginning of life.