NEW YORK (CNNMoney.com) -- On my first day of Core Fusion and after 20 squats at the barre (but who's counting?) my legs were shaking, and I was gasping for breath. The others around me seemed to move with strength and ease - obviously they were die-hards who had jumped on this craze long ago.

Fusion classes, which mix body sculpting with flexibility, strength training and balance work, have become the latest trend in fitness, and boutique gyms offering them are popping up from New York to Los Angeles.

Core Fusion at Exhale Spa

Physique 57

The low-impact workout first became popular more than 30 years ago, with a fitness regime called the Lotte Berk Method that combines pilates, yoga, barre work and core conditioning. After working each muscle group, a corresponding stretch ensures that the muscles formed are long and lean, not short and bulky.

Although it's been three decades since its origination, the famed Lotte Berk Method has recently enjoyed a rebirth in popularity and spawned numerous other fusion-type classes.

Exhale Spa in New York began offering Core Fusion, its take on the Lotte Berk Method, developed by by husband-and-wife team Fred Devito and Elisabeth Halfpapp in September 2003. Since then, Exhale has opened locations in Los Angeles, Chicago, Bridgehampton and Boston.

Despite the cost, which is about $30 a session, there is a cult-like following, mostly thanks to word of mouth and accolades in New York magazine (voted Best Fusion Class 2006) and Boston magazine (voted Best of Boston 2006). Exhale suggests that clients even call ahead to ensure a spot.

And although Core Fusion is offered in a modern space with a Zen-like appeal, don't expect to hear any Sanskrit chants in the background. Instructors keep the energy up with help from Madonna, Britney Spears and U2.

In addition to Exhale's Core Fusion classes, plenty of other fusion workouts have been popping up all over the country. In the last few years, the Venice, Calif.-based Sacred Movement gym added Core Fusion classes to its offerings, while a gym called Core Conditioning opened in Studio City, Calif., offering ballet, pilates, gyrotonics and physical therapy. Physique 57 recently opened in New York promising 57 minutes of action - from pulsating bends at the ballet barre to muscle-strengthening floor exercises using a strap and a ball.

Jennifer Vaughan Maanavi, the owner of Physique 57, said the popularity of the class has been well beyond her expectations. "We get about 200 people a day," she said, referring to her New York City location, which opened in February, "and there is a lot of demand to expand."

Fitness trainers have also become breakout stars thanks to fusion.

Tracy Effinger made a name for herself training celebrities in Los Angeles with her own version called The Effinger Technique, which she describes as a combination of athletics, yoga and dance work used to create long, lean bodies.

Rupa Mehta designed her own combination of pilates, yoga, Lotte Berk and resistance training called the Nalini Method, named after her mother. Her hour-long class in New York City uses mats, resistance bands, weights, ballet barres and exercise balls to create a strong, toned and graceful body.

So is fusion here to stay? It may be too early to tell, according to Philip Haberstro, executive director of the National Association of Health and Fitness. "There will be things that come and go, but if it helps people to get moving or to get with a lifestyle that encourages regular physical activity then it's positive."

I can't wait to go back - as soon as I can walk again.

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