Open the door on this class, however, and Ms Fonda's influence vanishes. There are no blue and red step platforms, no ranks of exhausted looking women and no one is wearing scrunch bands, Princess Di-inspired "fitness outfits" or bobble socks. Instead, 40 men and two women in no-nonsense T-shirts and shorts are doing intense press-ups beneath rows of swaying punch-bags. A bell rings every three minutes. In the middle of the hall is a full-size, fully furnished boxing ring. Welcome to the KO Circuit.
The KO Circuit, where you spar, shadow box and pummel punch bags rather than people, is the brainchild of Isola Akay. And it is a superb example of how answering a market need can enable you to get your own way. Mr Akay, a startlingly young-looking 60-year-old, arrived in Britain from Ghana 32 years ago. As well as setting up a sports shop in the Queens Park area of west London, he founded the All Stars youth club for boys and gave his young members free lessons in boxing and football. Mr Akay soon became a local legend. Boys from the club began to win British boxing titles; his own son boxed for Ghana in the 1984 Olympics. In this sometimes dangerous area of London, the club gained respect, a permanent following and alumni that included several professionals. It expanded, taking over an old church hall; it received pounds 30,000 a year from the borough of Westminster, and it thrived.
Last year, however, the grant was axed. Mr Akay, who had put 30 years of effort into building up All Stars, was suddenly looking at a club with no funding and no future. Then, while on holiday in New York, he was invited to see a fitness class at Gleason's Gym in Brooklyn. "To my amazement I saw all these white-collar workers, professional men, doing boxing training," he says. "It was absolutely the same regime as you would use for a professional boxer, except with no contact. I couldn't wait to get back to London and get it off the ground. I knew this was our life-saver."
Training at the KO Circuit is the same as that for conventional boxing, with the emphasis on fitness and speed - but without the bruises. "The only difference between this lot and real boxers," says Mr Akay, waving over the rows of heaving men, "is that they don't get their fists on each other. I honestly believe there is no better way to achieve all-round fitness. It has nothing to do with Boxercise, either." Mr Akay shudders at the mere mention of step aerobics' close cousin. "Boxercise is taught by women who don't know a thing about boxing."
Indeed, the KO Circuit is purpose-built for fitness enthusiasts who shun the rather girly atmosphere of the step class and who certainly don't mind paying for their daily burn. "Most of these guys are doctors or lawyers," says Mr Akay. The authenticity of the All Stars is ideal for serious professionals who want to know they are working out. Instead of the traditional corporate club set-up, all white fluffy towels and polished floors, All Stars' bare brick walls are furnished with faded posters advertising Joe Frazier/ Mohammad Ali bouts and the ceiling is hung with hard leather punch-bags that mean business.
The no frills, what-you-see-is-what-you-get atmosphere is a key to the club's success. It's rather like shopping at Gap. You work out for two hours. You sweat a lot and punch things and then you go home feeling manly. "There's no fancy side to it," says 51-year-old David Lieber, who runs a cosmetics firm and has been coming to the KO Circuit for over a year. "Gyms these days have got so superficial. This place is honest and it has a purpose." At a cost of pounds 5 per person per class, the combination of tough authenticity for people who are willing to pay for it has saved the club's future. Mr Akay continues training boxers to hit people and everyone else pays to get fit and feel tough.
"I've tried everything," says Amir Kasraie, an optometrist, attacking a punch bag with sweaty vigour. "Martial arts. Step. Weights. That sort of stuff just didn't motivate me. But this is all-out, no fancy trainers, no million-dollar buildings like the Harbour Club. Uuuugggh!" he shouts suddenly, raining blows at the bag. At which point I feel forced to admit that the mass exercise routine of the class does seem rather flat by comparison. "Aerobics is terribly dull," agrees Jason Gared, a 27- year-old surveyor, between sessions with a skipping rope. "And it doesn't give you all-round fitness. Not like this. After two hours of this, you can feel the fat falling off you." Mr Gared is breathing rather hard. "I haven't been to class for about three weeks. I'm exhausted," he says, waving the rope by way of explanation.
It's not just an issue of fit versus fat. According to regulars, a certain pinnacle of masculinity can be attained by a two-hour session with Mr Akay and his two pro-coaches cheering you on as you hit the punch bag. "Aerobics is, well, it's just too feminine," says 30-year-old Bertie Miller, the managing director of a commercials production company. Mr Miller lives in west Hampstead and, when he gets his licence back, drives a BMW. "Aerobics gives you co-ordination. But what I need is co-ordination in a macho-centric form," says Mr Miller firmly. He wipes his face with a towel. "This gives you the physical exhaustion you lack in a modern-day job. In my job, yeah, sure I can manipulate people and I can gain fulfilment from the daily challenges. But this gives you a sense of physical achievement. And you can run away quicker if you're ever in trouble." The three-minute bell rings. "Gotta get into the ring," says Mr Miller, leaping up under the ropes.
Everyone in the class has a go in the boxing ring with the coaches. The coaches wear square "Focus"' pads on their hands. The participants, wearing proper boxing gloves, hit the pads. It's the nearest you get to hitting someone. "Right to the head! Right to the body!" shouts the coach, putting his hand up to the appropriate area and warding off the blow.
Although Mr Akay claims that non-contact boxing is as exciting as the real thing, it's clear some would like to go all the way. "Yeah, I would like to hit someone," says James Philips, a rather mild looking 42-year- old marketing consultant. Mr Philips looks about him and fastens the tapes around his wrists in a contemplative manner. "Just as a tester, you see. To test the skill side of it. I'd find out where I was in the pecking order."
According to Lisa Maclaren, a law student and one of the few women in the class, it's precisely the concept of the pecking order that keeps the attendance up. "Guys are very keen to show their prowess in competition. Particularly when I'm around," she says. She clambers into the ring and punches the coach's Focus pads with considerable force. The coach grins.
Inspired by his success in west London, Mr Akay has big ambitions for the KO Circuit. "I'm hoping to open another club, but in the City this time," he says. "I need to cater for some more of these yuppies." He looks around at the picture of diligence and effort going on around him. "Get more of them out of their wine bars," he says, and laughs loudly.
All Stars Gym, 576 Harrow Road, London W10. Tel: 0181-960 7724.