Whizzing past the window at regular intervals are the legs of an impossibly slim woman dangling from a trapeze

In my week

Rob used to play Rugby. He dropped out for a year because of injuries and was taken to Circus Space by a friend. Ordinary people, when they've got injuries, take up low-impact sports like sitting around, or moaning about getting old. Not Rob. He took up tumbling, then acrobalance and then the flying trapeze. Now he works there as well. This is how modern youth runs away to join the circus.

We're not very good at circus in this country. Indeed, "circus skills" has become a shorthand catch-all insult, like "trainspotting", "anorak" and "chalet girl". The phrase conjures images of drunks in freezing caravans supplementing their income through unlocked windows. That, or blokes with beards and ponytails cross-legged on the floor, running foam-rubber balls up their arms while giving expositions on advanced spliff-rolling.

Circus Space aims to change this: built in the shell of Shoreditch Power Station, they run BTECs, practice for pros and classes in everything from knife throwing and whip cracking to clowning. So deadly is their aim, they're acting as one of the venues for the upcoming London International Mime Festival.

Waiting in the reception area (parquet floor, round tables, No Smoking signs) for Rob to show me what's what, I peer through a picture window. Six people in loose garb - mostly sweats, but there's a T-shirt and clown pants in there - bounce off a trampette and starfish in the air. Whizzing past the window at regular intervals are the legs of an impossibly slim woman dangling from a trapeze. I'm reflecting on the rule learned and forgotten at school - that only girls with straight hair are good at gym - when a man speaks behind me. "I just had a cup of coffee," he says. "I haven't had one in ages. It's made me go really hyper. Really stressed." Ah.

Rob comes back from the shop and leads me up a corridor. At the foot of the stairs hang bits of A4 paper. They say things like "male partner required for longterm partnership with young girl performing adagio and aerial work. Directed by top circus director" and "for sale: Japanese Unicycle, 24ins wheel, very little used". Rob's a nice guy, even though he's on a mission, like fit people always are, to stress how easy their craft is. "You don't have to be hyper-fit at all," he says. "It's not a long-capacity sport. And all of them tone you. You find your muscles get stronger quite quickly." The thought of my keyboard-shot arms trying to hold me onto a bar for more than two seconds turns me green.

On the petit volante, impossibly slim woman swings forward, pointing her toes, back, lifting her bum to avoid the platform ("when people start," says Rob, "you see them hitting their legs all the time"), flips over and hangs by her legs. A bearded man hooks his feet into the cradle, 20ft in the air, drops to vertical from his knees, and proceeds to pull up to horizontal using only his stomach muscles. Your tummy needs to be more washboard than washbag to do something like that.

The tumbling class continues. It's led by Adrian, once a British Youth Gymnast. His left forearm is in plaster. "He did it at his gym," says Rob. "He was on the high bars, doing spins. They wear these gloves to protect their hands. His glove snagged." Adrian kept spinning while his hand didn't. He broke his arm in two places. "Clean breaks, though," says Rob.

The tumblers queue for the trampette; they've graduated to mid-air somersaults, trying to hit the crash-mat - doubled up now so it's 4ft thick - upright. Most land on their backs, or pitch forward and finish the manoeuvre prone on the plastic. One girl - she's got the ponytail bit right, but the rest isn't following naturally - has little luck even hitting the trampette. She takes a run-up, goes "oops, sorry", runs back again, runs up again, goes "oops, sorry" again and gets the giggles. A man in trainers has been walking forwards and backwards on the tight-wire - it's about 2ft off the ground - solidly for about an hour and a half. One would have thought boredom would have set in, but tight-wire obviously has equivalent properties to computer games.

The people on the trapeze - what do you call them? swingers? trapezoids? - take it in turns to strap on a safety harness called a Lunge, though whether this name was derived from lungeing horses or lungeing towards the ground is shrouded in mystery. Secured to the ceiling by a series of pulleys, it prevents the athlete from plummeting too quickly. "You need it," Rob had said earlier. "When I first started, I was concentrating so hard on my trick that I forgot to hold on to the trapeze." It's fun watching people let go and swoop down in slomo: it reminds me of Lulu as Peter Pan.

They fling themselves from the bar and catch the hands of the man on the cradle. The tumblers come off the floor and prepare to go out into the night. Everyone, it seems, has come by bike. London's bike riders are a serious lot. The sub-group no longer consists of people who've lost their driving licences; nowadays it's a way of defining yourself. As they strap on anti-taxi protection, they make plans. "You here Monday?" "Sure am. It's part of my New Year's Resolution to get my arse over here more often." "Yeah. It's part of everybody's. That's why the classes are so full. Give 'em a couple of weeks, though, and they'll have gone off the idea."