It is highly likely, if you believe government statistics, that the most exerting thing we will each do today is pick up the TV listings tonight to choose whether we watch an FA Cup quarter final, the Commonwealth Games highlights, Channel 4's The Games or Celebrity Fit Club. When it comes to watching others exercise, we are undisputed champions.

When the Department of Health published its "physical activity action plan" for the nation last year, it said that if we are to avert the oft-foretold "health time bomb" we urgently need to get up off the couch and get ourselves fit. Only 37% of men and 24% of women, it said, are sufficiently active to gain any health benefit. Furthermore, 30% of boys and 40% of girls aged two to 15 do not meet the recommended levels of physical activity of 60 minutes of "moderate-intensity physical activity" each day. For adults, the chief medical officer recommends a total of at least 30 minutes a day of moderate-intensity physical activity on five or more days of the week. That means nothing more strenuous than brisk walking.

Those charged with improving the health of the nation should be pleased therefore that we are, somewhat paradoxically, keen gym users. Adrian Balcombe, head of the health club sector at the accountancy giant Deloitte, says that 15% of adults in the UK are members of a gym, one of the highest rates in Europe. On average, he adds, members pay about £400 a year in membership fees. However, 40-50% of members will have left the gym within a year. It is estimated that up to £200m is wasted each year in lapsed or unused membership fees.

So just how much of the £2bn the Fitness Industry Association says we spend each year going to the UK's 6,000 or so health clubs and leisure centres translates into future savings for the nation's healthcare spending? It is not possible to put exact figures to this but to put the burden of our collective slovenliness into perspective, the Department of Health says that in England alone the human cost of inactivity in terms of mortality, morbidity and quality of life is £8.2bn a year - and that doesn't include the £6.6bn-£7.4bn annual cost of obesity to the nation.

The best way to tackle this problem is not clear, although the action plan talks of "building an environment that supports people in more active lifestyles": daily routines such as walking to the shops and cycling to school, and offering the chance to participate in activities such as "aerobics, football, mountain biking, dancing and swimming".

While each choice offers obvious health benefits, it is interesting to also consider their vastly differing "carbon footprints". While cycling round the local park and swimming a few lengths may offer a similar calorific burn to the exerciser, there is a considerable difference in load on the environment. In fact, gyms and leisure centres are extremely energy-hungry places with their heated pools and saunas, extensive air conditioning systems, bright lighting, laundry demands and, of course, many exercise machines, many of which need some form of powering. Balcombe estimates that up to a third of a gym's outgoings will be its energy costs, particularly with bills rising sharply. And then there's the irony that the majority of gym users drive there. It takes an awful lot of energy, therefore, to allow us to expend energy at the gym.

There are many who pedal or row furiously on a machine in a gym and wonder why all the energy isn't being harnessed somehow and made to, say, power the television showing MTV above their heads. They are right to wonder this. A human can generate roughly 125 watts - one-sixth horsepower - of continuous power by pedalling, or about 50 watts through a hand-cranking action. This means that while you sweat away on a bike in your spin class your efforts could, broadly speaking, be powering that television mounted on the wall or a couple of ceiling fans. You would have the added incentive that if you stopped pedalling, the loud music video driving you on, or the cooling air, would cease.

A much better option exists beyond the gym, however. For the past eight years the British Trust of Conservation Volunteers (www.btcv.org/greengym, tel: 01302 572 244) has been organising "green gyms". There are now up to 70 projects across the country that encourage people to get fit and do something constructive at the same time, by puffing away for an hour or two on activities such as tree planting, hedge-laying, pruning and dry-stone walling. No more forgetting to bring a pound coin for the locker, no more being told to "give me 10 more" by a personal trainer, and no more wasted membership fees.

You say ...

It mostly depends on how you get there. I run to my local gym (five miles there and five miles back) to go circuit training. The instructor drives about 400 yards. Go figure.
Edward Collier, by email

Next week: is it ok to "offset" carbon emissions?

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