The shimmy travels to Nathalie's upper body with a mesmerisingly rhythmic side-to-side shuffle. "Shake the boobs!" she whoops, arms outstretched. "Yes, that's right. Really shake them." She grasps women by the shoulders to demonstrate this unfamiliar motion that is key to the belly dancing class she is teaching.
Given the emphasis on trim perfection that is the objective of so many aerobics classes, it is hard to believe that these words are being spoken in an American gym.
In a different part of town, two drummers are furiously banging out African rhythms. The room is packed and steamy, full of arching, stamping bodies gyrating and shaking with a dynamic energy. Atito Gohi who teaches the class as well as drumming, beats faster and faster until, defeated, the dancers dissolve into raucous, endorphine-charged laughter.
Perhaps it is a reflection of the current thirst in the US for things ethnic. Perhaps it is merely representative of the diverse range of nationalities that populate American cities. Or maybe it is sheer boredom at the mundane idea of yet another step-aerobics or spin class. Whatever, Stateside gyms are increasingly supplementing their aerobics schedules with ethnic dance- based work-out alternatives.
The interesting - revolutionary even - thing about classes like African and belly dancing is their cultural roots, which bring with them new ideas about fitness. These dances focus first and foremost on celebrating the body. The idea is for people to get in touch with their natural rhythm; to liberate the body rather than shrink and sculpt it into a size 8.
But given the energetic nature of both classes - especially African dancing - a sculpted body is often a speedy side effect. Atito Gohi's class is a prime example. It was last year voted the most popular class at New York Sports Club.
"A lot of people appreciate the fact that I break up this look of being slim with small boobs, flat abs, and that I just use the beauty of my body," says Nathalie. As you can imagine, she is pleasingly voluptuous. "People feel comfortable to explore the belly in my class," she says, caressing her rounded stomach. "Women love it, they love themselves."
She attributes the enormous success of her class to two things: "People are bored of normal classes. And they are sick of looking at skinny people teaching them all the time." African dance seems to have the same liberating effect. "There are no aesthetic guidelines - it's come one, come all," says Sophia Clerk, the trainer. "It's actually more welcoming if you have more of a derriere."
Other US gyms are following suit. Classes like Latino groove, (combining aerobics with Cuban dance), Brazilian/African dance, and capoeira rhythm (based on a Brazilian martial art) are all taking off. "We work all the muscle groups," says Dwayne Earle, a veteran aerobics instructor who teaches African dance. He estimates that one of his African dance classes burns 800 calories, compared to about 600 for step aerobics. But more than calorie- counting, the class is all about uplifting the spirit, he says: "It brings about a joy - a lot of people have compared it to a religious experience. They become more in touch with their bodies, freer with their bodies, more flexible, stronger and toned."
Belly dancing bodies, too, become more lithe. "It uses everything," says Nathalie, 31, who has been a personal trainer and fitness instructor since leaving the Israeli army, where she was a sports teacher. Since the body moves in a circular motion during belly dancing, rather than the linear movements involved in step classes or kick boxing, many more muscles are engaged. And in addition, the dancer becomes much more flexible.
Ninety-five per cent of men and women who try it return, she says. And by the end of the class I attended, even the most inhibited women have loosened up. Timid, awkward little shakes have turned into proud, majestic sways and swoops. Shoulders are shimmying, hands are twirling, and hips are provocatively flicking. "Yes, yes" yelps Nathalie, clutching her hips. "This is the part that we hate, so shake them; that's the shimmy - do you feel it jiggling?"
For information on similar classes in the UK, contact Pineapple Studios (tel: 0171 836 4004).