Just four months ago, Alex Rummel could barely keep up in gym class. Before he was able to complete half a lap, he would see other kids finish and sit down, said the seventh-grader at Heritage Middle School in Westerville. Even warming up for football practice often left him gasping for air.

"It was anything. If I was hanging out with friends walking down the street, I would be behind," said the 12-year-old who lives with his parents in Columbus and Minerva Park. With their help, he found a kids' boot camp for eight to 13-year-olds at Metro Fitness in Worthington. He also works with a personal trainer.

Alex isn't alone in his desire to be fit. The younger-than-18 set is the second-fastest-growing demographic for gym membership, said Rosemary Lavery of the Boston-based International Health, Racquet and Sportsclub Association, which tracks industry data. The number of fitness-club members ages six to 17 more than tripled between 1987 and 2006.

"I think kids aren't always interested in a sports program, and no one plays in the street anymore," said Michael Dyer, founder of Lifestyle Family Fitness, based in Lakeland, Florida. "So where does a teenager come for exercise these days? The fitness center."

Several central Ohio gyms and recreation centers offer a range of options - from weightlifting to aerobics classes and personal training - for youths.

Lifestyle Family Fitness, which has 10 area branches, allows 12- to 17-year-olds to exercise free until 5:00 PM daily during a summer program that began in mid-May and continues through July 15. Lifestyle expects 10,000 participants nationwide this year, a spokesman said, up from 2,400 in 2006.

In the fall, Ohio State University will again offer a strength-training class for youths seven to 18 years of age through the Age Groups and Camps program, said director Jilaine Anderson.

Frequent changes in activities help keep interest high. "Kids don't get that interested if it's a matter of getting on the treadmill or pumping iron," Anderson said. "The OSU trainers keep things moving, moving, moving."

At the Gahanna YMCA branch, more parents are requesting that adult activities be adapted for younger age groups, director Chris Angellatta said. In 2007, he said, Columbus YMCAs had about 3,600 teenagers participating in structured exercise programs - almost triple the number in 2003.

One reason for the increase is the addition of two new Y branches, where 1,100 teens are members. Another is parents' concern about their children's fitness, he said. "Childhood obesity is in people's minds now, where it wasn't before. There was a time a few months ago where a big story would run on it every week. People come in and ask what we have for children."

Ron Rummel, Alex's father, saw an opportunity to teach his son to begin early to live a healthful lifestyle. Rummel described himself as a "professional couch potato" - until he began working out a year ago. "I didn't want him to go the same route," he said. "Plus, I've been doing this for a year, and I thought this could be something we do together."

Editor's Note: News modified from original release.