Fitness trends come and go, but millions of American women swear by Curves (search), a bare-bones gym that caters to those who are out of shape or don't feel comfortable in traditional health club settings.

"It’s not intimidating," said Loraine Strombeck, a 56-year-old registered nurse from Saugerties, N.Y. "And people don’t wear the jet-set exercise clothing that you see at other gyms."

Curves attracts women of the 76 million-strong baby boomer generation —  the oldest of which are in their mid-to-late 50s —   who want to try to prevent heart disease, diabetes and cancer, but might be intimidated by the hard bodies and complicated machinery found elsewhere.

The success of Curves, named the fastest growing franchise by Entrepreneur Magazine (search) in January, comes at a time when the number of obese adults has soared to 44 million, according to the Centers for Disease Control and Prevention, and while sales in exercise equipment and traditional health club memberships are reportedly lagging.

But Curves, with about 6,000 locations in the U.S., is different from other gyms, which thrive on tough classes like boot camp and cardio strip tease, aimed at young exercisers.

"We definitely spend our advertising money on the 30- to 60-year-old woman," said Gary Heavin, CEO and founder of Curves International. "We used real women in our ads. The average woman was a size 14."

Jennifer Gilbert, 34, said the gym has transformed her body.

"Since January, I've lost 24 inches, dropped from a size 22 to a size 18 and lost 26 pounds," said the hairdresser from Kingston, N.Y. "My butt is smaller and flatter and that saddle-bag area is gone."

Gilbert, who has tried step aerobics classes and nautilus, said she's never stuck with a gym routine this long.

The first Curves gyms were opened in small Texas towns in the 1990s and gained momentum in the suburbs. Major cities were the last frontier.

"We had 50 locations within 10 miles of Houston, but not one inside Houston," Heavin said, adding now there are 50 Curves in Houston.

Until 2003, there was no national ad campaign for the women-only gyms, but members spread the Curves story like gospel; some even became franchise owners.

"We've created a culture of women helping women," said Heavin, whose new book, "Curves: Permanent Results Without Permanent Dieting," includes eating plans. "About 90 percent of our franchise owners are women."

The gyms promise a complete workout in 30 minutes. Members move between strength-training “hydraulic resistance” machines that are set in a circle and maintain a target heart rate by jogging in place between circuits.

Group fitness classes, treadmills and showers -- mainstays of traditional gyms -- are not offered at Curves.

Dr. Cedric Bryant (search), chief exercise physiologist for the American Council on Exercise, said the use of the specialized equipment "removes some of the intimidation factor" of strength training.

"The machines are smaller in size," said Bryant. "They appeal to the female population."

But Bryant said the machines only provide one of two phases of resistance training, which he illustrates as the raising and lowering of a bicep curl. "Because you don't have the negative portion of the exercise, you are going to limit skeletal muscle and bone development. I would say that Curves is giving the minimum effective dosage."

The Curves Web site includes testimonials of extremely overweight women who have lost more than 100 pounds. But the gyms seem to measure success more by inches lost than weight.

"I don't know about a change in my body, but I have more energy," said Strombeck, who added that the energy boost enables her to do other activities like bicycling.

Bryant recommends supplementing a Curves workout with cardio activity such as a 30-minute walk.

Strombeck, who joined Curves six months ago, inspired her daughter to join one in New York City.

“The best thing about it is I know the amount of time I’m going to spend,” said Patricia Strombeck, a 29-year-old teacher from Staten Island, N.Y. "And I did lose a half-inch from my waist."

But Patricia, who enjoys running and yoga, is already fit and said she hasn't met her goals of flatter abs and stronger arms with Curves' regimen.

“I don’t like the fact that you can’t stay on one machine for a longer time,” she said.

Bryant said women like Patricia probably need more of a challenge.

But perhaps it's the Curves atmosphere that's lost on younger people.

“The music is pretty bad,” Patricia said. “The songs are cheesy and they play them pretty loud.”

Her mother, however, said the tunes keep her moving.

“I like the music,” said Loraine. “Sometimes they’ll play oldies, sometimes they’ll play country. It’s very upbeat and gets you going.”