Teens Must Shape Up or Blimp Out, Experts Warn : * A U.S. study finds they’re exercising less, making them more inclined to become overweight, among other problems. Some P.E. instructors are trying to do something about it.

Some say the Mario Brothers are to blame. Others say it’s because of Gilligan (the Skipper, too). Maybe it’s caused by MTV, working parents or school budget cuts.

Whatever the reasons, teen-agers nationwide simply aren’t getting as much exercise as they used to even a few years ago. Instead of spending their after-school hours riding bikes or climbing trees the way their parents’ generation did, they’re more likely to be found at home in front of a television or computer, flexing only the muscles necessary to push the buttons on the remote control or manipulate a joy stick.

Even when they’re enrolled in school physical education classes--and fewer than half are, according to a study published last month by the U.S. Centers for Disease Control--most kids hardly even work up a sweat.

The CDC reported that only 48% of the 11,631 ninth- through 12th-graders surveyed were enrolled in physical education classes last year, down from 65% in 1984. Of those, only a third said they exercised as much as 20 minutes three times a week, which is widely considered the minimum necessary for cardiovascular benefit. One in four of those students said they never exercised as long as 20 minutes at a time in any physical education class.

It’s a problem even in body-conscious, outdoor-oriented Orange County, say local physical education instructors and other experts who are trying to find innovative ways to counteract it. But here, the national trend manifests itself a little differently. With beaches and parks and all-year mild weather, kids here have more exercise opportunities than in other parts of the country. But they aren’t always able to take advantage of them.

Because most families need two incomes to survive here, children are often left alone and told to stay inside with the door locked after school.

“Parents are reluctant to let their kids go out and play, so they use TV and Nintendo as a baby-sitter,” says Dick Peter, director of physical education at Woodbridge High School in Irvine.

“The word play doesn’t mean the same thing it used to,” says Carol Strausburg, chairwoman of the physical education department at Fountain Valley High School, who has seen a slow deterioration since she began teaching in 1966. “A lot of kids just don’t know to go out and just enjoy moving around.”

Kids who have spent their growing years as couch potatoes are more likely to end up overweight and out of shape by the time they get to high school, says Stephen Weiss, an orthopedic surgeon who practices in Irvine and also volunteers as the team physician for Laguna Beach High School.

“I see kids, even athletes, who are overweight, out of condition; they get short of breath, their muscles cramp up,” Weiss says.

Sports injuries are a focus of his practice, and Weiss says those are also more likely for those who are in bad condition. “If you’re in good shape, you’re more flexible and less likely to be injured, and you’ll recover faster if you are” injured, he says.

Out-of-shape kids who’ve never had much success with sports become so discouraged that after they’ve fulfilled the mandatory two years of physical education required by the state, they stop trying.

“The result is that some of the people who are in most need of exercise are least likely to get it,” Peter says.

Weiss was so concerned about the problem that he and his family donated $125,000 worth of fitness equipment, including both aerobic and weight machines, to the high school when it expanded its fitness center earlier this year.

The center is now so popular that strength coach Bret Fleming, who also coaches the basketball team, now oversees six fitness classes a day, all of them electives available in addition to traditional physical education classes.

The center is open both before and after school, and Fleming estimates that about a third of the school’s 680 students make use of it in some capacity.

“In the class, one of the goals is to teach kids how to set up their own training program, with fitness as a lifetime activity,” Fleming says. “Most 17- and 18-year-old kids have a hard time picturing themselves getting flabby at 35. But if they don’t establish a healthy lifestyle at this age, it’s going to catch up to them later on. And it’s much more difficult if you have to start in your 30s.”

Says junior Dan McMillan, 16, “Most people around here are really into fitness; especially in the beach communities, people want to look good and feel good.”

McMillan, an amateur surfer who spends two to three hours a day in the water, admits that the center’s flashy new equipment was what lured him into signing up for an elective fitness class.

But even after two weeks, he says he’s feeling the benefits. “I’m already getting stronger,” he says.

Senior Katie Grow, 17, says she’s always been athletic. “I was doing volleyball, basketball and track, but now I’m just doing track. I do aerobics after school, too.

“I plan on working out the rest of my life.”

Monica Gehling, also 17 and a senior, says she took the class “because I wanted to tone up. I’m not really an athlete.”

Senior Jason Abraham, 17, plays football, basketball and baseball, but he also signed up for fitness.

“I don’t just do this for athletics,” he says. “I do it for myself. But it does help me in athletics, too.”

At Buena Park High School, Principal Christine Hoffman says physical education teachers are trying to find ways to make exercise more fun. “We’ve added some mini-units, such as weightlifting, badminton, a dance program, even a class called Ultimate Frisbee. That’s been very popular.”

The school is also instituting a lunchtime intramural sports program. Hoffman says 77% of the students there participate in some sort of physical education.

Strausburg says she isn’t surprised by the CDC finding’s that teen-agers enrolled in P.E. don’t get much aerobic exercise.

“We lose time taking roll and doing other administrative things, and then even when you get them into an activity, they probably won’t be moving every minute,” she says. “Even if it’s volleyball, if you don’t have the skill, you end up standing in one spot all the time.”