You walked to the station this morning, marched up the escalators and you'll be taking the dog out when you get home tonight. So that's your day's exercise taken care of, right? Well, not necessarily.

In the 1980s, exercise was all about feeling the burn. But in recent years, we've been persuaded that exercise doesn't have to hurt. In fact, it doesn't even need to be exercise, with activities such as housework and walking up the stairs counting towards the daily total.

But now the goalposts have moved again. The American College of Sports Medicine (ACSM), one of the major international organisations responsible for issuing health and fitness guidelines, became so concerned that its advice to "accumulate 30 minutes of moderate activity on most days of the week" was being misinterpreted (letting us believe that light activities were sufficient), that it went back to the drawing board earlier this year. The British Association of Sport and Exercise Sciences (Bases), the UK's equivalent body, is doing the same.

"The promotion of 'lifestyle activities' since 1995 hasn't increased physical activity levels in the UK or the United States," says Dr Gary O'Donovan, a lecturer in sport and exercise medicine at the University of Exeter, who is currently leading the panel writing the new Bases guidelines. "Many people give up on moderate activity because it is too time-consuming, and brings little improvement."

A clear demonstration that lifestyle activity doesn't replace "proper" exercise comes from a study at Bristol University. The research, published in the Journal of Epidemiology and Community Health, found that women who did more than eight hours a week of heavy housework were actually slightly more likely to be overweight than those who did none - and no amount of vacuuming, scrubbing and cleaning lowered resting heart rate, which is an indicator of enhanced aerobic fitness.

There are three main changes to the new ACSM guidelines. The "30 minutes of moderate activity" bit remains, but "most days" has become a more specific "five days of each week" (presumably to fox those of us who are kidding ourselves that three days counts as "most"). More importantly, the paper stipulates that this 30 minutes is in addition to "routine activities". In other words, walking around the office, pulling weeds in the garden and eschewing the lift - while worthwhile - no longer count. The report points out that even those lifestyle activities that are energetic enough to count as moderate exercise aren't performed for long enough to have any meaningful impact.

But perhaps the most significant revision concerns the issue of intensity. For the first time, "vigorous" exercise is offered as an alternative to moderate activity. Instead of labouring through five 30-minute sessions a week, you can zip through three 20-minute high- intensity workouts - or mix and match the two. At least part of the reason for this amendment is to dispel the myth that moderate intensity exercise is better than more vigorous activities - the "exercise doesn't have to hurt" message.

A study from Wales recorded the activity levels of 2,000 men aged 40-64 over a 10-year period. There was no evidence that either light or moderate physical activity (walking at a leisurely pace, bowling) reduced the risk of dying from heart disease, while more vigorous activity - jogging, swimming, hiking or walking at a brisk pace for one hour at a time - did.

"The simple truth is that the harder you work, the fitter and healthier you'll become," says O'Donovan. In other words, the new guidelines offer not a formula for optimal fitness, but a minimum recommendation for safeguarding your health, and your heart.

Assuming that you do manage to meet the weekly target, you can rest assured that you'll be significantly reducing your risk of cardiovascular disease, diabetes and some cancers. But what constitutes moderate activity?

The issue of how exercise intensity is measured is complex. Scientists use a range of different measurements. "How best to describe exercise intensity to the general public so that it gets the right message is difficult," concedes David Swain, a professor of exercise science at Old Dominion University and a fellow of the ACSM. "One of the simplest ways to rate intensity is the 'talk test'. If one is exercising hard enough to notice that one is breathing harder, but is still able to speak in complete sentences comfortably, then that is 'moderate' intensity. It should be a level that can be easily maintained for at least 30 minutes. A 'vigorous' intensity is one that makes it difficult to speak in complete sentences, but that can still be maintained continuously for several minutes."

Since we all have different levels of fitness, it's clearly impossible to prescribe any single given activity or target heart rate as suitable for all. That's why O'Donovan takes issue with the universal promotion of brisk walking. "There is compelling evidence that brisk walking protects women from diabetes, heart disease and breast cancer, but for most middle-aged men, walking simply isn't challenging enough," he says. "Men who want to reduce their risk of diabetes and heart disease should consider jogging or circuit training, or, better still, running or competitive sports."

Hmmm, that sounds like hard work. Given that it's widely accepted that there are proven health benefits from doing five 30-minute sessions of moderate exercise a week, is there really any need to work harder?

Yes, says O'Donovan. "Vigorous exercise offers benefits over and above those gained through moderate-intensity exercise. There's a further reduction in risk of obesity, diabetes and heart disease and a bett er quality of life. There is also emerging evidence that vigorous activity is essential in the prevention of some cancers, including colorectal cancer. Besides, shorter, sharper sessions are less time-consuming - and lack of time is often cited as a barrier to regular exercise."

Swain agrees. "If a person does a moderate-intensity workout for a given amount of time or a vigorous-intensity one for a shorter amount of time, and both cases burn the same number of calories, the vigorous exercise will be better."

Which raises the question, why didn't the ACSM simply replace the "moderate" exercise message with a "vigorous" one? Well, figures from 2005 indicate that less than half of US adults even met the previous ACSM target, so announcing that the stakes have now been raised further would not, perhaps, be a wise move. "Many people are too out of condition to attempt high-intensity exercise," adds professor Klaas Westerterp, professor of human energetics at Maastricht University. Westerterp's headline-grabbing research in 2001 revealed that gym-goers burned fewer calories overall than generally active non-gym-goers, because they compensated for their efforts by being less active for the rest of the day.

The bottom line is that we need to do both. The new Bases guidelines will recommend that no one goes more than two days without some moderate to vigorous physical activity. "Incorporate more activity into daily routines that you do automatically," advises Westerterp. "And," adds O'Donovan, "endeavour to include some vigorous activity in your week".