Why go to the gym when you can fidget in your chair to get fit? What's the point in power walking when tidying your desk can fight flab? Such were the questions raised last week by scientists in America, who claimed that minimal everyday movements make the difference between being fat and thin. Those who sit still in front of the television, for instance, could do themselves a favour by tapping their feet to burn calories, said researchers at the prestigious Mayo Clinic in Minnesota.

For anyone who pulls on a pair of trainers too infrequently, it will undoubtedly come as music to the ears. But is it really what we need to hear? Britain is a nation in colossal calorie credit: we already consume far more energy from food than we expend through any sort of exercise.

Two-thirds of men and three-quarters of women barely manage to meet even the minimum recommendations set by the government of 30 minutes of cumulative activity a day. And many experts believe that findings such as those by the Mayo researchers, along with the trend for fashionable, but hardly gut-busting activities such as yoga, encourage a softly-softly approach to exercise that is fuelling our complacency.

We are, they claim, being lulled into a false sense of security that a minimal amount of exertion is all that is required to stay in shape. In what is being dubbed "the yoga effect", a growing number of people cling to the false belief that sweaty effort is unnecessary in the pursuit of a healthy body. "The painful truth is that often people either don't do enough exercise or don't put enough efforts into the workouts they do attempt," says Richard Godfrey, a researcher in exercise physiology at Brunel University. "We would all like slimness and fitness handed to us on a plate, but you don't get to lose weight and look more toned by taking things too easily."

In what was considered a landmark report last year, the chief medical officer, Sir Liam Donaldson, said half an hour of everyday tasks such as housework, gardening or collecting the newspaper with a little gusto were enough to burn some calories and that it was not necessary to do anything more strenuous such as a gym workout or swimming.

Yet, abysmally, we fail to meet even that paltry target. With eight million Britons paying monthly subscriptions to private gyms and health clubs, the nation is still getting fatter.

Louise Sutton, head of health and exercise science at Leeds Metropolitan University, says that conflicting advice about activity levels means we have lost sight of what we really need to be doing to stay slim. "Thirty minutes a day is a bare minimum set to ward off obesity and reduce the risk of heart disease but it is not enough for most people, especially if they are trying to lose weight," she says.

"The fact is that we have become complacent. We don't want to face the facts that exercising to get fit entails hard work. And then we wonder why the bulges on our thighs and stomachs aren't shifting."

Celebrity role models don't help matters much. Despite their public protestations that exercise beyond stretching in the occasional yoga class is their recipe for a perfectly honed physique, don't be fooled. "Very often these people spend hours under the watchful eye of a personal trainer - they just don't like to admit it," Sutton says. "Spiritual, mind-body classes are hugely popular but they rarely push the body into what is known as the training zone when your heart rate increases significantly and you are working your aerobic system. Madonna may be well known for her devotion to yoga, but what is less often publicised is that she runs and cycles as well to maintain her fitness. Yoga in isolation won't work."

So, if you harbour great expectations from your body, say Sutton and others, then you must be prepared to step up the workload. But by just how much? Ironically, the claims that fidgeting helps to fight fat come just two weeks after the US raised the activity marker for people who want to lose weight.

In its latest official guidelines, the US Department of Agriculture, which sets the standards, urged seriously overweight adults to get 90 minutes of moderate-intensity exercise a day. Others looking to prevent weight gain should aim for an hour of daily activity. It is likely to have left the average American sobbing into their cheese burger. But, say fitness experts here, these are more realistic levels towards which we too should be aiming if we want tangible results.

"Research by the American College of Sports Medicine (ACSM) sets what are widely regarded as blueprint targets of exercising three to five times a week for 20 to 60 minutes at 55-90% of your body's maximum capacity, calculated according to your heart rate, if you want to improve fitness which constitutes a fairly vigorous workout," says Dr Greg Whyte, director of science and health for the English Institute of Sport. "But that doesn't mean we can rest on our laurels for the rest of the time." Whyte believes too many people have "reached a point where they think going to the gym three times a week is enough". But, he says, while working out will contribute considerably towards overall fitness, "there are 23 and a bit hours remaining in the day and we should try to be active at least during some of them".

Aiming to cover 10,000 steps a day, as recommended by University of Tennessee research last year which showed it lowered body fat, body mass and led to a reduction in waist size, is a good starting point, says Kate Owen, a research physiologist at the Olympic Medical Institute in Northwick Park Hospital, Harrow. Research by the British Heart Foundation shows the average Briton walks less than half that amount per day, achieving a distance that would classify them as sedentary in the States. "Ten thousand steps equates to about five miles and will burn around 500 calories," Owen says. "But once you have reached that daily amount ideally you should aim for more if your fitness is to improve."

Godfrey suggests that people starting from scratch should build up aerobic fitness over six months by walking, but beyond that you should progress to any activity that vigorously works the large body muscles - running, cycling, swimming, tennis, squash and rowing are all good choices - aiming for at least three hard 30-minute sessions a week on top of other daily activities. "It is this aerobic exercise that improves the mechanics of the cardiovascular system, enabling the body to use oxygen more efficiently and boosting your metabolic rate," Godfrey says. "You will gradually change shape as your body relies on its energy reserves to fuel the extra work."

The ACSM also suggests one weekly resistance or strength training session as you get fitter. "Muscle tissue actively burns calories, so the more muscle you develop, the more energy you will use sitting still," says Sutton. "An American study showed that the metabolic rate of middle-aged men who did light weight training three days a week for 12 weeks rose by 15%, enabling them to shed four pounds of fat each without dieting."

And just when you begin to congratulate yourself for reaching a new level of fitness, it is usually time to shift the goalposts. "It is important to build an element of progression into an exercise or activity plan," Sutton says. "If you find you are not getting fitter or progressing closer to your target on the scales in spite of the fact that you aren't doing any less exercise, then you have hit what is called an exercise plateau. The body and its muscles respond to the overload principle and there comes a time when you will have to extend the duration or intensity of your workouts further. The reality is that the only way forward is to work harder."