It's lunchtime in a central London gym, and the atmosphere is infernal. The sweating hordes are cleansing themselves of fat and guilt (it's only five weeks since Christmas) in a spectacularly self-flagellating fashion. One girl looks so penitent I could cry for her: she lunges forward, arms trembling under the weight of kettlebells, her movements distorted by exhaustion.
But what's that? From behind a door comes the sound of music and laughter. Inside, a young woman in fisherman's pants, her braided hair tucked under a patterned bandana, shimmies and wiggles to pumping Latin music. The eight women in her class are shaking their booties as though they're Jenny from the Block instead of Clemmie from Kensington.
This, ladies and gentlemen, is Zumba. It is the world's biggest branded fitness programme and it is probably happening at a gym or community centre near you. Founded in 2001 in Miami by three Albertos - Perez, Perlman and Aghion - it has 14 million weekly participants in more than 150 countries.
"It would be quicker to tell you the countries we're not in," says Alberto Perlman, the CEO of Zumba Fitness (pronounced "zoomba"). He takes a sip of mineral water and settles into an armchair in a London hotel. "We're not in North Korea, Iran or Cuba. We're in Antarctica, though. We had a class there with four people, with some penguins in the back." He laughs the easy laugh of a 35-year-old multi-millionaire.
Perlman is here to open Zumba's first office outside of America. The UK is the company's second-biggest market, with 1.2 million people practising its mix of aerobics and dance steps. He hopes to quadruple that number and there's every reason to expect he'll do it. In American culture, Zumba is so ingrained it was a plot point in a recent Desperate Housewives episode. Michelle Obama has held a class on the White House lawn. They even have Zumba classes at the Pentagon. (They probably don't wear the tasselled Zumba cargo pants and neon string vests at that one. The branded clothing, incidentally, is a pounds 60 million business in itself).
Six months ago, the fifth annual Zumba conference took place in Orlando, Florida. Up to 8,000 Zumba instructors paid $399 (pounds 250) each to attend five days of dancing and rousing speeches. When Alberto "Beto" Perez tried to lead a class, the people swarmed around, reaching out to touch him and offer gifts, as if he were a modern-day messiah. They queued to tell reporters: "Zumba is not just a dance, it is a life choice."
I stumbled on a class in London a couple of years ago, and its quasi-spiritual elements were striking - we sat in a circle breathing together - as was the strong sense of community among the students. "I think the people who take Zumba classes understand friendship better," Perlman tells me. When Hurricane Sandy hit America, he says, "the instructors who had lost their homes stayed in other instructors' houses. They would take in their family, that's how strong the Zumba connection is."
So what is Zumba's secret? "It's inclusive and it's fun," says 25-year-old Laura, a trainee solicitor. "It doesn't matter if you can dance or not. Nobody is judging you."
I've been to a lot of dance classes over the years and it's true that Zumba is uniquely unintimidating. Street dancing is brilliant but the moves are hard. The Latin dances, in which you're aiming for heat and sensuality, can be excruciatingly embarrassing to mess up. Anyone, regardless of age, fitness or natural rhythm, can handle a Zumba class. "Even my mum does it," says Laura. "She loves it."
Perlman has his own mother to thank for his present success. After graduating from Boston's Babson College in 1998 he got caught up in the dotcom bubble. "It was crazy. I started an incubator for internet companies for Latin America. And in 2001 it crashed."
Perlman found himself back home in Miami hunting for a new career. "All my mum talked about was her dance fitness class with Beto. She told me: 'Go and meet him. Maybe you can start a gym together.'" Many 23 year-olds would have replied: "Yeah, right, Mum," but Perlman gave it a shot. Which just goes to show you should listen to your mother.
Beto already had a big fan base. One day, in his native Colombia, he had forgotten the music for his aerobics class so he played his own rumba and samba tapes instead. He threw a few dance moves into the mix and found that his clients loved it. He moved to Miami and they loved it there, too.
Perlman saw potential in this high-energy style, which not only burned up to 1,000 calories per hour but, miraculously, people seemed to really enjoy. The third Alberto signed up - Alberto Aghion, a childhood friend of Perlman's - and business grew steadily.
But it was the recession that made them. "We were bracing for tough times [in 2008]. It was the opposite. People who were out of a job wanted to become instructors." People reducing their expenses signed up for Zumba, a cheap, stress-releasing hour of fun in an anxious week. The business has grown 800 per cent since 2008 and is now estimated to be worth pounds 318 million.
Zumba Fitness actually makes little money from the classes. The profit comes instead from the video games, DVDs and clothing - and the instructors. They pay around pounds 190 for training and pounds 20 a month to Zumba Fitness thereafter. In exchange, they get music, choreography - from Zumbatomic for children to Aqua Zumba, "the pool party" - and access to what Perlman calls "an internal Facebook where instructors mentor each other".
Most Zumba practitioners are female, more markedly in Britain (98 per cent) than anywhere else. (In Latin America, where men grow up dancing, it is more like 75%). Perlman hopes to redress the balance, possibly by creating a toning-focused variation for men.
But he has more ambitious projects, which include a Zumba record label. He was recently approached by the hip-hop artist Pitbull. "He said to us, 'You have 14 million people taking classes every week. You guys are like a radio station.'" Pitbull and Wyclef Jean, among other artists, have since debuted singles via the Zumba network, prompting the music industry magazine, Billboard, to call Zumba "the next major music platform".
Maybe the Zumba fanatics are right: it isn't just a dance.