Let the group fitness gadflies flit from belly dancing to body sculpting to circus stunts. Pilates people opt to take long, steady aim at the core.

And they say the payoff is sweet: strength without bulk, slender thighs, flat-as-a-board abdomen.

"With Pilates, the focus is core strength," said Jessica Matthews of the American Council on Exercise. "It concentrates on centering and encouraging improved posture and strength throughout the core."

The core, or powerhouse, refers to the muscles that gird the torso from the lower rib cage to below the beltline. Pilates is a system of over 500 exercises that promises to condition the total body by centering on that center.

The mat exercises comprise several series of leg lifts, chest curls, and roll ups - each one said to be the equivalent of six sit-ups - and the signature "hundred," which entails much flapping of the arms and legs.

And while they might look like sophisticated sit-ups, the moves are performed with precision, concentration, breath control and flow. In fact, Pilates was originally called Controlology.

"Because you're so aware of where the exercise is coming from, you're really focused on where you're working," Michele Bastos, Pilates instructor at the Crunch national chain of health clubs, said of the regimen now practiced by an estimated 10 million people worldwide.

Unlike the 5,000-year-old practice of yoga to which it is often compared, Pilates is the 20th century creation of one man.

German-born boxer, gymnast, and physical therapist Joseph Pilates originated his system as a rehabilitation tool in the 1920s. Some of the first people he treated were soldiers returning from World War I.

When he moved to New York City in 1926, his studio was located near the New York City Ballet. Dancers, George Balanchine and Martha Graham among them, spoke of 'going to Joe's' to strengthen their bodies and ease their aches and pains.

The dance connection stuck. As did the notion that Pilates produces a dancer's long, lean body.

"I believe it's true," Bastos said.

"Pilates strengthens and lengthens muscles at the same time, so it's different from weight training, which only works concentric muscles. In Pilates the muscles get really long," she explained.

"Even in breathing we try to work the abs. We inhale through the nose and exhale through the mouth to get all air out of stomach," she said "We do not allow ribs to pop out."

Mat classes remain the most popular, but Bastos sees interest in the Pilates equipment growing.

"You start on the mat and move to the machines." Bastos said of the Reformer, the Cadillac and the Chair.

The founder himself created the forerunners of these wood-and-leather contraptions of levers, springs and pulleys when, working in an infirmary, he rigged hospital bedsprings to offer light resistance to bedridden patients.

"The mat works against gravity only. On machines you have spring tension," said Bastos.

Of course, similarities between Pilates and yoga have spawned the inevitable hybrid: Piyo or Yolaties, by any other name.

Other so-called fusion classes provide the cardio workout missing from a Pilates. Something called Rock Star Pilates adds spinning. "It's perfect if clients only have one hour," Bastos said.

Or, if they need to do a little flitting.