It wasn't long ago that health clubs were an adult domain, a place for grown-ups to stay in shape now that school sports, gym class and active play were no longer part of their daily routine.

But with schools across the country scaling back physical education programs, and America's obesity epidemic (search) spreading to a generation of children more likely to spend their play time surfing the Internet than jumping rope, America's health clubs have recently been besieged by an adolescent invasion.

"I don't come to the gym to look at guys or anything, I just come to get my body in shape," said 17-year-old Nicole Demaine, who works out seven days a week at a Chicago health club. Demaine is not an athlete, and like many children who either do not enjoy or are not very good at traditional sports, fitness options outside of a gym are limited.

"Teens who work out have a better stress level, they sleep better," Demaine said.

The trend has been embraced by the health club industry, which has found a promising new source of business in a population it once barred from membership. According to the International Health, Racquet & Sportsclub Association (search), the ranks of members under 18 swelled 25 percent between 1998 and 2002; in 2002, IHRSA clubs reported 4.5 million minor members.

Many clubs, in fact, are offering "junior" fitness programs designed specifically for the needs of still-growing bones and muscles.

At X-2 Youth Fitness (search) in suburban Chicago, teens and pre-teens can choose between such arcade-style fitness games such as a video game that is powered by a stationery bicycle or a machine that makes you dance. It's enough to keep 11-year-old Jessica Sikorski coming back to work out four times a week.

"I hope I'll lose some weight and get stronger," Sikorski said.

Many health experts concerned with the sedentary lifestyle of today's children are very encouraged to see teens and pre-teens embracing fitness. In fact, a recent study found that children, if given the choice, elect physical activities over sedentary play, suggesting that the weight and fitness problems among children may be the result of their parents' lifestyle habits.

When researchers at Vanderbilt University designed an eight-week fitness program for third-graders, they found that at the end of the eight weeks, the children had voluntarily moved onto more intense physical activities. They also found that the children had changed their preference of leisure time activities from sedentary ones to physical ones.

However, while doctors agree that children need exercise, some doctors are concerned that an adult gym is not the appropriate or safest place for children to get their exercise. Whether or not it is healthy for young teens to weight train remains a point of debate within the fitness community, and the machines in health clubs are designed for adults. The risks of injury and muscle overuse, as well as lack of proper supervision and instruction, are considerable, they say.

"If a child is not through puberty, you have the risk of problems with bone development and overuse of the muscles," said Dr. Rebecca Unger.

Nonetheless, if done safely and in proper moderation, odds are that a fitness habit that starts early in life will become a life-long lifestyle.

The Associated Press contributed to this report.

Steve Brown is an author, radio broadcaster and seminary professor at Reformed Theological Seminary in Orlando, Florida.