Some seasons it’s cardio, other moments glorify the upper body, a few years ago it was all about the yoga butt. But now, like trans fats or Marc Jacobs handbags, core strength is taking up space in the zeitgeist. I’ve been inundated with the same message in yoga studios, at my chiropractor’s office, and even at my local knitting café: pull in the belly, line up the spine. They say it improves posture, eliminates pain, and, of course, makes you––whatever your gender––look smokingly hot in a sweater.

If you want to see what the fuss is about, check out Core Fusion. My alterna-fitness-loving friend recently told me that if she had to choose one exercise class, that’d be it. “It’s the most challenging, lowest impact exercise I’ve ever done,” she told me. It’s also one of the most pricey, though, ranging from $30 to $20 a shot.

A kind of pilates/yoga/dance mutt, Core Fusion is taught only at Exhale, an upscale spa and gym chain with outposts in Los Angeles, Boston, Chicago, New York City, Bridgehampton, and a new Santa Monica location opening in December. The sweaty, heavily sound-tracked, one-hour classes promise to lift your tush, crunch your abs, and align and tone you throughout that dastardly, stubbornly mushy core––plus deeply relax you with a yoga-like savasana (corpse) pose at the end. In perfect hardcore fashion, all Core Fusion instructors are personally trained by one the method’s creators, Fred DeVito and Elisabeth Halfpapp, who both taught the 30-year-old Lotte Berk method for many years.

Classes are available at most locations every day, but those just starting might want to try a more in-depth workshop, like Alicia Sauer’s Introduction to Core Fusion course on December 3 at the Boston location.