Sarah Siebach isn't much different from most members of the Lifestyle Family Fitness Center in Cary, trying to squeeze an intense workout into her busy schedule. Last Friday afternoon she was hoping to do the treadmill, maybe pump some weights, before heading home to shower and meet friends for dinner at Bahama Breeze.

She's a lot like the gym's typical member, except for one thing: She's a junior in high school.

"I love it here," she says, surrounded by rows of high-tech workout machines and ubiquitous TV monitors playing up-tempo music videos. "It makes my mind and body feel better when I'm done."

Time was kids got a workout which weren't called workouts by riding bikes, going to the neighborhood pool, playing pickup baseball. Kid stuff. Today, increasingly, they're hitting the treadmill or the elliptical trainer, doing a session on the weight machines, taking a group exercise class.

"What we're seeing now is kids taking on what normally would be considered adult exercises," says Dr. Bob Duggan, a foot and ankle surgeon in Orlando, Florida, who specializes in sports medicine.

According to the International Health, Racquet and Sportsclub Association, which represents about 9,100 health clubs worldwide, 4.1 million kids (ages 6 to 17) belonged to health clubs in 2006, the latest year for which statistics are available. That's up 28% since 2000 and a more than twofold increase over 1987, when the association began tracking kid memberships.

The group reports that health clubs are responding to the teen trend as they did a couple of decades ago when there was a new demand for women-oriented clubs.

"Whether health clubs for teens ever rack up the numbers or achieve the success of single-sex express workout clubs remains to be seen," the Boston-based association reports in its online newsletter.

But, it adds, with the nation's teens having an estimated $200 billion in spending power a number "expected to more than double within the next five years" the clubs are doing their best to find out.

What the Kids Like

Geoff Dyer still remembers the sting of being an overweight teen growing up in Melbourne, Australia.

"It's an intimidating experience to go through high school when you're obese," says the founder of the Lifestyle Family Fitness Centers, which has three facilities in the Triangle.

So three years ago he began offering free summer memberships to kids ages 12 to 17. About 2,400 teens signed up the first year, 6,000 last year. This summer, Dyer says the health club chain expects 10,000 teens to take them up on their offer.

The most popular offerings?

"The girls like the cardio," says Dyer, meaning treadmills, elliptical trainers and such. "The boys like weightlifting over cardio." Group exercise classes are also popular, says Dyer, including the music-driven Body Jam and Body Pump, the latter of which he describes as "hip-hop exercise."

The Triangle's YMCAs still offer traditional kids programs soccer, dodge ball, basketball. But they've also begun courting kids with traditional adult exercises. Especially popular at the Banks D. Kerr YMCA in North Raleigh has been an orientation class that middle-schoolers must attend before using the facility's weight machines.

"We were completely booked through the last week of May," wellness director Jaime Kivett said of the class, which was offered three times a week but is now offered five times.

Even younger kids are emulating the exercise of their parents.

"We have a kids yoga class for ages five to nine that is packed," said Kivett. "There's 40 or more kids in that class." This fall, Kivett says they plan to start offering a Kid Fit class that will involve "balance, body movement, push-ups things like that."