Maybe we have grown impatient with teasing our bodies gently into shape on a yoga mat, or maybe we are just cheesed off with gym classes that don't deliver what they promise. Whatever the reason, a growing number of people appear to be reaching the same conclusion about fitness - namely, that you have to sweat and fight to achieve it.
Which is why the kind of vigorous martial arts that have been overlooked in the past few years are now firmly back in fashion. Leading the way is Karmaa in north London. Inside this airy, former warehouse are the prerequisites of every gym: a cardio room, holistic centre and treatment rooms - but in the main fitness studio, fight mats cover the floor, punchbags hang from the ceiling, and dozens of boxing gloves and pads are neatly piled at the side of the room.
Classes and one-to-one sessions include kick boxing, jujitsu, and Zen-do, a variation on kick boxing. "All combine aerobic activity, flexibility, kicking and punching, as well as the obvious element of learning how to protect yourself," says Rafael Nieto, a former kick-boxing champion and co-founder of Karmaa.
Considering it takes approximately nine times the energy to produce a forceful kick as it does to throw a punch, it is hardly surprising that you can burn up to 600 calories in a nonstop, hour-long kick-boxing session, and with so much kicking and lower body work it's also a great way to tone flabby legs and a sagging bottom. "Far too many gyms kid people into thinking they can get fit and lose weight without breaking into a sweat. That's rubbish. Unless you are prepared to push yourself to your own limits and get out of breath, you are never going to change your shape," says Nieto.
Despite the fact that it is unmercifully tough, Karmaa has attracted a stream of celebrities. Regulars include Sadie Frost, who brings her children along for lessons, and Jonny Lee Miller. What is perhaps most surprising is that women members outnumber men at Karmaa. "It is roughly a 60:40 ratio of women to men at the moment," says Nieto. "A lot of women come initially because they think it will help them with self-defence, which it does, but they also discover it is getting them into shape faster than anything else they have tried."
Of course, the Hollywood set realised this months ago. Tom Cruise and Demi Moore have been practising jeet kune do, which involves grappling and fighting with 28-inch bamboo canes, while Lucy Liu has taken up eskrima, a Filipino martial art with sticks and knives. Jennifer Lopez and Christian Slater prefer krav maga, derived from a brutal self-defence system - think head-butting and testicle-crunching - created by the Israeli defence forces and recently adopted by the Californian police as a fitness tool.
Despite their origins, well-taught martial arts classes are surprisingly nonviolent. "With better body awareness, what you learn is how to deal with threatening situations and how to avoid them," says Nieto. "You not only get fitter in a class like this, you become more self-empowered. It really can change you as a person."
· Karmaa (020-7485 7474; karmaa.co.uk); British National Martial Arts Association (bnma.co.uk).