Get retroactive: Want a fitness alternative to prancing about in leotards? Emma Cook walks back to happiness on Hampstead Heath

It has a kind of logic to it,' says Oscar, a prim-looking businessman in his late forties, explaining the virtues of striding across Hampstead Heath every Saturday morning - backwards. He is one of about 25 people to have been swept off his feet by a new north London fitness fad. The group is led by Terry Clarke, fitness guru and creator of retro-walking, who has been 'walking back to happiness' for the last three years.

Terry was eager to discover ways of developing the 'opposing muscle groups, in particular, the quads'. These are the muscles on the front of our thighs which are sometimes under- stretched in normal exercise. Walking a 17-minute mile (at 3.5 mph) on flat ground will burn off about 300 calories - on a steep hill it can rise to 500 calories. In designing her 'group power walking regime' she was keen to attract a different type of health-conscious client. One who preferred to stay wrapped up in the open air rather than contend with 'mirrors and slim bodies dancing around in thong-type leotards'. Targeted at the Hampstead nature-lover, her brochure outlines a sedate programme of 'stretches at the gazebo', 'dips on the benches', 'crunches on the lawn' and 'grunting on the gravel' (something I never witnessed during my retro experience).

By nine o'clock, twenty or so men and women are stretching their calf muscles, hamstrings and triceps outside the Kenwood gazebo. A woman with a pram anxiously swerves to avoid the energetic leg movements and arm swing exercises. Walkers are aware that their activities attract some attention. 'They think we're crazy,' says Iradj, an Iranian publisher in his late forties. 'But because they're so English round here they're more embarrassed than we are.'

'Right, walkers, time to move backwards,' instructs Terry as we reach the bottom of Parliament Hill. A slight scurry ensues as people link up arms, facing backwards and forwards, to form about four straight lines. Each line takes off up the hill resembling a bizarre Morris dance formation. 'Linking arms is where the bonding bit comes in. It's social as well as practical,' explains Terry. 'If you walk backwards on your own how would you avoid the dog poo?' Trust seems to be an essential element. 'I would never let a stranger guide me,' says one man emphatically.

Feeling rather vulnerable, I am guided uphill, arm in arm with two women. The temptation to turn round and check one's path is overpowering.

Towards the top of the hill, a dull pain spreads across my upper thighs which I presume is a good sign. 'Can you feel anything yet,' Terry shouts enthusiastically. My pulsemeter (a black rubber belt strapped tightly around my chest) races towards 164, which is, I am told, the upper limit for a first timer.

It's nearly 10 o'clock and the women are in high spirits. The walk is as much about appreciating the surroundings as improving aerobic fitness. 'Ladies, look over there at the wonderful colours of those red berries,' orders Terry. The ladies coo in appreciation. They are more enthusiastic than the men who seem a little reticent about the classes in general. Back in the car park the men stand around meekly. 'One good thing about it is they sell rather a good cup of tea at Kenwood House which is something to look forward to,' says Iradj who, under his wife's duress, attends every Saturday. 'I have to tell you it's really not desperately interesting,' he confesses. Another man nods his head in sympathy. 'I only do it because I'm told to.'

Terry Clarke, Healthshape Studio, Edgware, London (081-905 4993). Meet at 8.30am and 3pm every Saturday, Kenwood car park. pounds 5 per session. Private sessions pounds 30 each or pounds 28 each for 10.

(Photograph omitted)