If fitness is the prescription, there are better, more beautiful, greener ways of achieving it than in a mechanical gym. An Oxfordshire doctor has come up with a whole new milieu, ideal for those who can see the wood for the trees. Caroline Green reports from outdoors.

Dr William Bird, a GP in the Sonning Common medical practice in Oxfordshire, had been wondering for some time how he could persuade more people in the village to take regular exercise. One evening, after a particularly stressful day at work, he went for a walk round the village and, as he appreciated the scenic countryside around him, he had an idea. Why not get people to use the environment as a resource for getting fit, instead of trying to force them to join gyms or take up new sports?

He duly set up a programme of organised local walks and three years on, the so-called Health Walks are so successful that eight other practices around the county are to start similar schemes.

Bird began to see promising results, and found he was prescribing less medication generally. Now he's about to take things a stage further. In conjunction with the British Trust for Conservation Volunteers (BTCV) he has set up an innovative scheme in which people will get fit by doing environmental conservation work.

The Green Gym project will involve monitoring the physical fitness of a group of volunteers as they carry out a range of BTCV-organised events such as tree planting, hedge maintenance and cleaning up rivers. Although there has been a general perception for many years that the countryside may have a therapeutic effect both mentally and physically, hard evidence to date is sketchy to say the least, particularly where there's an altruistic, volunteering element. Bird aims to get the facts on whether this form of exercise really does have beneficial effects on fitness and mental well-being.

"The philosophy behind the Health Walks and the Green Gym is that we can use the countryside as a resource for health," he says. "It's the first time anything like this has been done. Lack of exercise is our biggest epidemic in this country, but the NHS can't afford to tackle the problem on its own. We have to find a way of making physical activity attractive, and it has to be cost-effective for the National Health Service."

Many surgeries around the country already have a system in place for "prescribing" physical activity under the Exercise Referral Scheme, where patients with a range of health problems, from heart disease to high blood pressure, high cholesterol to arthritis are given eight free sessions at a local gym. The surgery pays for the sessions, but they're heavily subsidised by the gyms themselves, which are relying on the fact that people may sign up at the end of the programme. But according to research commissioned by William Bird, there is a drop-out rate of up to 80 per cent with the Patient Referral Scheme. People just don't stick with it. "There are lots of reasons why it often doesn't work," he says. "It's intimidating for people who may be old and unfit to go to gyms and be faced with all these gorgeous, lycra-clad bodies. They may have to travel some distance to them, and going to a gym is not a very sociable activity, which also puts people off."

By contrast, the Health Walks have had a drop-out rate of only 12 per cent and Bird is hoping that the Green Gym project will offer similar results in the long term. Dorothy Rose, an ex-nurse, found, like a lot of retired people, that giving up work deprived her of her main source of exercise. She retired in 1986 and by 1994 she was overweight, had high blood pressure and her cholesterol levels fell into the "very high risk" category for developing heart disease. "I'd let myself go," says Dorothy, who is now 65. "It's very easy to slip into the habit of just sitting about when you stop working. I did go to a gym with my son once and it frightened me. It was full of strange equipment and no-one spoke to anyone else. I thought `this isn't for me.'"

Dorothy was involved in the walks from the beginning. She says: "Within six months, my cholesterol was down, my blood pressure had dropped, and I'd lost three stone in weight." Sixty volunteers, all unfit, will be picked by Dr Bird in January: 30 people will be involved in conservation projects, while the other half will carry on their normal inactivity and act as a control group. At the end of six months, they will be examined and have their weight, blood pressure and cholesterol measured. They will also fill out a standard questionnaire used by psychologists to assess a subject's state of mental well- being.

At the same time, the Department of Health Care Studies at Oxford Brookes University has been commissioned by Bird, with funding from the Countryside Commission and Shell UK, to test healthy subjects' responses to various conservation tasks. Twenty people will be hooked up to heart monitors while they carry out a range of activities from simply identifying wild flowers to digging ponds. The idea is to get a precise picture of the musculoskeletal, calorific and metabolic demands of each task.

Using this information, the Green Gym activities can eventually be graded according to how strenuous they are, and what demands they make on the body.

Yvonne Trchalik, age 23, is the Green Gym Project Officer. She says: "I've never been into sports or going to a gym and for me, conservation work is the way I keep fit - I've been doing it since I was 13. I can feel a difference in myself when I've been on a week's conservation work: I feel fitter and stronger.

"Conservation work can be a form of aerobic exercise, because you get a bit out of breath doing it. One of the more gentle tasks is what's known as plug planting, where you plant wild flowers using a special tool. You don't even have to bend over to do that. Coppicing involves chopping down trees to promote regrowth and increase the diversity of species in a wood, and that's among the most strenuous activities.

"I like to work with a group of people on something like making a kissing gate (a gate now used instead of a stile) and then being able to see it in use afterwards. I'm not a very strong, burly sort of person but doing this sort of work has definitely made me stronger and fitter." It's the first time the BTCV has been involved in a health project. Trchalik says: "Research in the past has suggested a link between the environment and a feeling of well-being, but the Green Gym is the first time conservation work has been linked to health."

Because she's now fit and well, Dorothy Rose won't be involved in the research project. But once the Green Gym is up and running, other local people can get involved too and Dorothy fully intends to try her hand at a spot of digging, planting and dredging. "We do quite a bit of that kind of work anyway," she says. "As we walk around we hammer down nails on stiles and cut our own paths.

"I find it very interesting. It's beautiful to watch the changing seasons and it's like a social event as well as a way of exercising. I've discovered now that getting older doesn't necessarily mean you can't get fit."