AT 33, IT'S STILL a singular sensation.

"A Chorus Line," one of the longest-running musicals in Broadway history, remains as relevant now as it was when it opened more than three decades ago. A new production of the Pulitzer Prize- and Tony Award-winning story of 17 dancers auditioning for a chorus line is being staged at the Forrest Theatre through Jan. 4.

Last week, several cast members kicked up their heels and taught a free dance-aerobics class to a packed house at Center City's 12th Street Gym. The dancers demonstrated classic moves from the production, ending with that famous line kick that closes the show.

Class participants were thrilled as they got a chance to experience firsthand the full-body workout required of professional dancers. After the class, I caught up with two cast members to get the 4-1-1 on their secrets to staying fit, healthy and in shape.

First, I chatted with 26-year-old Alex Ringler, a 2005 University of the Arts graduate who was thrilled to be in Philadelphia again.

When I saw the production, Ringler was a pure delight as the flamboyantly gay character, Greg. No child prodigy, Ringler earned his dancing stripes the hard way. Surely his alma mater is proud.

Ringler began dance training at the tender age of 5, and, except for a brief break at age 10, he has trained vigorously to this day.

Ringler said that he has worked consistently since his 2005 graduation, appearing in touring productions of hits such as "West Side Story," "Cats" and "Seussical."

In case you were in doubt, Ringler made it clear that "yes, the show is physically demanding and we must stay in dancer-shape."

And staying in "dancer-shape" is no easy task. The choreography is full of jumps, turns and kicks that require the dancers to combine all of their fundamental skills from ballet, modern and jazz dance. They need to be able to belt out a song and act, too.

Ringler's personal routine consists of five- to seven-day-a-week workouts of 45 to 90 minutes. He varies it with circuit training, resistance training and cross training.

"Your body even adjusts during the show," he said. "So you really have to mix it up and make an effort if you want to stay on top."

When not on tour, he adds up to three dance classes a week, such as ballet, contemporary modern and jazz, to his routine.

He also maintains a clean diet. That means no pigging out on pizza and no Philly cheesesteaks till after the show closes.

Performing at this elite level is no joke. The strength and stamina required of these performers is synonymous with any elite athlete.

Not convinced? Ask 26-year-old Chi-town native Ian Liberto, who plays the "strange" Bobby in "Chorus Line." Liberto is a former competitive middle-distance swimmer who entered the dance world at the ripe old age of 17.

I was fascinated, because most professional dancers begin formal training around age 5. But Liberto said that he found that his four hours a day of training as a competitive swimmer was great preparation for a professional dance career.

"I was already flexible, and the breast stroke and butterfly require lots of body awareness and coordination, which are transferable skills to dance," he said.

Indeed, both professions require similar skills. I just never thought about it that way.

But surely it must have been difficult to change from an athletic career to an artistic one?

"You really have to want it," Liberto responded. "It's not an easy career and it's not an easy lifestyle. So many dancers, so few roles, and everyone is just as talented as you are."

Along with rehearsals, dancers must get in regular workouts - and watch their calories, too. And if that weren't enough, they must make their modest earnings stretch to accommodate dance classes, voice lessons, acting lessons, personal fitness, nutritious foods and commissions to agents and managers.

A dancer's body is his or her instrument, and the triple-threat performers in "A Chorus Line," much like the characters they portray, dance not for fame or fortune, but for love.

Make some time in your holiday schedule to see "A Chorus Line." Rife with unexpected challenges and victories, it will entertain, as well as inspire you. *

Kimberly Garrison is a certified personal trainer and owner of One on One Ultimate Fitness in Philadelphia (www.1on1ultimatefitness.com). E-mail her at kimberly@1on1ultimate fitness.com. Her column normally appears each Thursday in Yo!