At a growing number of health clubs, gyms and YMCAs across the country, the treadmills and barbells aren't just for adults anymore.

While experts debate whether it's a good idea, more teens and "tweens," or preteens, are hitting the workout circuit to get in shape.

The trend also is driven by parents worried about childhood obesity while schools cut physical education classes. Not to mention health clubs — long considered adult-only zones that often barred youngsters — which see the potential for a new moneymaking market.

Clubs belonging to the International Health, Racquet & Sportsclub Association reported 4.5 million members under the age of 18 in 2002, a 25 percent increase over the 3.6 million in 1998. Of those 4.5 million members, nearly three-fourths were ages 12-17, said Brooke MacInnis, a spokeswoman for the association.

"The overarching problem is that kids are not getting enough exercise, and the health club industry is trying to respond," MacInnis said. "It is becoming very popular for health clubs to open up to children."

MacInnis said about a fifth of the association's members who answered a survey in 2002 were offering junior programs, and she expects that to grow.

"Historically, teens and tweens have been doing things like swimming or playing basketball," MacInnis said. "You are going to see more and more specialized workouts for kids."

Whether youngsters, particularly preteens and early teens, should be doing individual training is still a matter of debate. While the National Strength and Conditioning Association, the American College of Sports Medicine and American Academy of Pediatrics all say supervised, moderate workouts are fine, some people worry that developing bodies can be damaged by weight or cardio workouts.

"It's a myth that started 20 or 30 years ago and it's absolutely false," said Avery Faigenbaum, a professor of exercise science at the College of New Jersey, in Ewing, N.J.

"Kids really need places to become physically active," he said. "In this age of budget cuts in schools, it's not happening there anymore. Rec centers tend to be more sports-oriented, so the kids who aren't good at sports don't go. There is a niche that health clubs could fill."

Still, Faigenbaum stresses that young people must be supervised, especially with weights.

"I am in no way saying that an 8-year-old can come home from school and go into his basement and do weight training," he said. "They absolutely have to be supervised. Health clubs should either offer supervised classes or nothing at all."

Some health clubs still resist the idea.

"Their bodies are just not ready for it," said Mary van Thullenar, a manager at Dale's Athletic Club in Overland Park, Kan. "It would do more damage to have them do those exercises than to do things like swimming, floor drills, conditioning, aerobic classes."

Dr. John Acquaviva, an associate professor of health and human performance at Roanoke College in Salem, Va., said young people generally don't train hard enough to damage their bodies. But he still believes people under 16 shouldn't do serious training because of the risk of injury and because they quickly become bored.

"I'm concerned about burnout," Acquaviva said. "If they start to believe that working out is regimented, boring and a punishment of some kind, that lessens the chances of them continuing to work out in their adult years."

Kelsie J. Bayless, an 11-year-old from Grandview, Mo., seems anything but bored. She took a YMCA "Tweens on Weights" class that teaches kids how to properly use weights and cardiovascular machines. But she mostly spends her time on tae kwan do, a home-school physical education class, swimming and running. She, her three siblings and their mom work out at the Y about three or four times a week.

"There's a lot of people you can be with, and there are a lot of cool things to do," she said. "It makes me feel good when I work out."

Kelsie's parents, Wendi and Robert, struggle with their own weight and want their children to develop healthy habits. They admit they wouldn't go as often if their children weren't going.

"For them it's fun, for us it's more of a chore," Wendi Bayless said. "Taking them makes us have to go, too."

Despite the national trend, Robert Bayless noted that not every fitness club is kid-friendly, calling some "almost anti-family."

MacInnis, of the health club association, acknowledged that some people don't want children in their health clubs.

"As the infusion of family fitness continues, an adult will have to decide if it's going to bother him to have a 14-year-old working out beside him," she said. "You might have to ask, 'Will there be kids in my class?' And if you don't want that, look elsewhere."