Joyce Kwon grinds and struts to the pulsating beat of rapper Bobby Brown, classic funk and R&B, surrounded by perspiring bodies doing the Roger Rabbit and the RoboCop. With her elbows askew;! la Janet Jackson and her legs rebounding like M.C. Hammer, she twists and turns and breaks into the Cabbage Patch. The walls throb. The heat generated by 60 sweat-soaked bodies threatens to set off the overhead sprinklers. Joyce Kwon is happy. “If it wasn’t for this I wouldn’t be making it in school,” says the 22-year-old UCLA student, who’s tried running, weight lifting and traditional aerobics. “Nothing relieves the stress like Street Jam.”

Street Jam is the collective name for a new movement in movement. A blend of inner-city street dances (hip-hop) and low-impact aerobics, the craze began in-where else?–Los Angeles and promises to create a new rhythm nation. Jane Fonda’s Beverly Hills Workout center, her flagship studio, closed last month, partly because her fellow fatburners tired of the same old regimen. And the competition was too stiff–or too funky. The new sweat central is Voight Fitness and Dance Center in West Hollywood, home of Cardio Funk, Cardio Salsa, Low Impact Funk and now Street Jam. Every week, 1,500 students pack the center’s maze of studios to hop off the pounds. Says Voight cofounder Henry Siegel, as the 5 o’clock shift piles on its sweats and prepares for an hour of fast moves: “It’s really taking off. It’s going through the ceiling.”

Anyone who’s ever ogled the tightly muscled dancers on an MTV video knows that today’s moves are tomorrow’s life insurance. It only made sense that the high-energy activity on the dance floor would translate well to the gym. Siegel’s wife, Karen Voight, initially put the idea to work with the more jazz-oriented Cardio Funk in 1984. Ten months ago, Christophe Toledo, a former Club Med sports instructor/choreographer, brought the hard-driving Street Jam to the Voight studio. Now it’s the most popular class, says Toledo, because “there are many variations and no rules. You can’t street dance wrong.”

The music starts, and with a swing of the hips and a twist of the knees, the class has begun to raise its heartbeat. The look is angular, fast paced and close to the ground. The leader layers on instruction, so that one step follows another, until, to everyone’s surprise, they’re doing The Troop. Dory Kaiman, owner of The Studio in Chicago, says older women “are surprised when I tell them they’re dancing funk.”

It’s the kids, however, who have caught the craze big time. “I don’t like to exercise,” says professional dancer Lisa Kouchak, 21, of L.A. “This is kind of disguised in dance.” “It beats being on a Stairmaster, standing in one place, going up and down, nowhere,” says actor Darryl Bell of L.A., 24, a regular on “A Different World.” Bell compares it to dancing in a nightclub with one exception: “Here, if you’re out of step, nobody cares. In a club, they’ll laugh at you.” It took Joyce Kwon just three days to learn Street Jam. Now she finds herself doing the moves unconsciously throughout the day. “I do it when I’m cooking,” she says. And she probably doesn’t have to watch what she eats.