The fitness buffs crowding into the Orchid Pagoda studio in the Merrifield area on a recent Saturday morning could have been the typical crowd in any Fairfax County fitness studio on a busy winter weekend. Dressed in sweats and leotards, toting athletic bags and water bottles, they crowded in the door for another workout.

The atmosphere was anything but the usual. Instead of a pounding beat, a serene flute melody greeted visitors to Orchid Pagoda. Instead of sweating their way through the barbells, exercisers took to something called a Pilates "machine." The black leather, blond wood and stainless steel contraption took them through a series of flowing movements.

Instead of shouting instructors in high-volume aerobics classes, there was only the squeak of floorboards as the studio's Pilates (puh-lah-teez) instructors moved quietly from machine to machine in the sunlit room, moving an arm here or pressing a hamstring there to correct a movement. The only sound from the students was their slow breathing as they moved smoothly through the stretching and strengthening techniques.

The scene at Orchid Pagoda reflects the sudden explosion in the popularity of Pilates, an 80-year-old method of gentle body conditioning. Several Pilates studios recently have opened in the county, and the often-pricey classes are selling out. Health clubs such as Washington Sports Clubs and Life Time Fitness in Fairfax City have added Pilates equipment and classes.

Just last month, the Fairfax County Park Authority opened its first Pilates studio in a county-owned facility, buying $18,000 in Pilates equipment for the Providence RECenter, where more than a dozen classes have opened.

The trend is part of what's called "active relaxation" -- practices that include Pilates, yoga, tai chi (a slow-moving martial-arts-based exercise) and Nia (a kind of New Age aerobics that combines dance, martial arts and yoga). The practices are catching on among aging fitness buffs who want to stay in shape but with fewer of those joint-jarring aerobics classes and foot-pounding jogs.

Retiree William Bentley, 69, took up yoga just over a year ago when his wife gave him a gift of yoga classes for Christmas. Now he's a regular at the Health Advantage Yoga Center in Herndon, where he does sun salutations and warrior poses, he said, "mostly to fight off the ravages of aging."

Until recently, Pilates was relatively obscure. Developed in the early 1900s by German fitness trainer Joseph Pilates, it proved popular with dancers and athletes who liked the focus on improving flexibility and strength without building bulk.

Pilates, a boxer, circus performer and self-defense trainer who studied yoga and meditation, developed his techniques while interned as an "enemy alien" in England during World War I. Using springs from old-fashioned hospital beds, Pilates devised special machines to provide progressive resistance to heal and strengthen muscles.

His program is based on the concept of strengthening the body's "powerhouse," the corset of muscles around the pelvis and lower abdomen. When these muscles are under strain, so goes the theory, other joints and muscles also will be stressed.

From that modest beginning, an industry has sprung. The Pilates regimen is especially popular with dancers. Beyond that, celebrity practitioners include actresses Sharon Stone and Julia Roberts, singer Madonna as well as football players with the San Francisco 49ers and the Oakland Raiders, who have an endorsement deal with the manufacturers of some Pilates equipment.

Carmen Fermin, fitness director at the Providence RECenter, said Pilates "demands you make a big effort. But when you are finished with the movement, you feel light."

Since its origins, the practice of Pilates has splintered into several different schools. Purists stick strictly to the original movements choreographed by Joseph Pilates, which are taught by Pilates Guild-trained instructors in one-on-one training sessions on the equipment. The guild's Web site is www.pilatesguild.com.

That form is practiced by Pure Joe Studios in Reston, one of the few guild-certified studios in Fairfax County. It offers group mat classes (Pilates exercises that can be done without the equipment) and individual instruction on its nine pieces of Pilates equipment. Pure Joe's four full-time instructors travel to New York City once a month for more training at the Pilates Guild, co-owner Andrea Chastant said.

Unlike other Pilates studios, which also offer classes in Nia, yoga, tai chi and other alternative fitness pursuits, Pure Joe sticks with Joseph Pilates's original program.

"We teach only authentic Pilates," Chastant said.

Though Chastant's studio is only three years old, it now has 360 clients who come as often as three times a week, a demand so high that Chastant said she plans to move to a bigger facility later in the year.

At Orchid Pagoda, started by Sarah Cohn Christensen two years ago after she sold her Annapolis laser-manufacturing business, the approach is more eclectic but still based on the traditional Pilates machines.

Among the Pilates equipment at the Orchid Pagoda is a Reformer, based on Joseph Pilates's original bedlike platform that has a sliding carriage attached to a number of springs offering gentle resistance. Another piece is the Cadillac, an upright contraption to which springs, bars and straps are attached and suspended.

The Providence RECenter and Orchid Pagoda break with the usual one-on-one Pilates training by offering small group classes, which helps keep costs down. The Park Authority charges $190 for 12 55-minute classes, or about $15.80 a class. An eight-week series of classes runs $240 at Orchid Pagoda, or $30 a 55-minute class, compared with as much $65 an hour for individual Pilates equipment instruction.

"It opens up the affordability of using the equipment to people who either cannot afford a private lesson or who, instead of just coming once a week, can come two and three times a week," Christensen said.

One recent popular offering has been Pilates for Golf, a conditioning program using Pilates equipment developed recently for golf buffs.

On one recent Saturday, Pilates instructors Kathy Boisvert and Leila Conklin paced the floor of one of Orchid Pagoda's studios, issuing instructions incomprehensible to anyone but Pilates devotees:

"Let's work our towers."

"Maintain that plank as you pull in and out."

"Find a neutral spine release."

"Float the shoulders down the back."

"Let the leg release away toward the nose."

Burke nurse-practitioner Carol Gibson, 36, started Pilates classes in June after getting tired of the pounding of step aerobics. Her endorsement is ringing.

"It's the most wonderful thing I've ever done," she said.

Providence RECenter's Fermin got into the Pilates business when she applied for and won an $18,000 grant from the Park Authority last year to purchase Pilates equipment. With that, Fermind bought six pieces of a type of Pilates apparatus that has been slightly modified for fitness training.

"It was a very important need for the people in the community who I'm working with," said Fermin, a former dancer who learned Pilates in her native Venezuela. "In order to achieve their goals for any type of sport or recreation . . . they need to have body awareness. So I started Pilates."

Demand for the classes, which started last month, is high, Fermin said. Her biggest problem now is finding enough Pilates instructors.

Vienna resident Kathy Reynolds, 36, took Pilates mat classes for a year at the Providence center until the equipment classes started up in January, when she signed up for the group equipment class. It has "helped tremendously," said Reynolds, who added that the program is challenging.

"I still can't do a roll-up correctly," she said with a laugh.

Julie Zagars works out on Pilates equipment in a class at Orchid Pagoda, a fitness studio on Lee Highway that also offers yoga and Nia instruction.Bonnie Bowers leads a Pilates mat class at Orchid Pagoda, one of several studios and health clubs in Fairfax County that offer the popular method of gentle body conditioning.Dalla Black, standing in foreground above, helps Michele Bell stretch during a class taught by Olga Rasmussen, right, at Health Advantage Yoga Center in Herndon. Ursula Bass, below, exercises in Bowers's class at the Orchid Pagoda. Yoga and Pilates are part of a trend called "active relaxation."