Biceps can be nice. Yoga gives you poise. And a flatter stomach will help you get into that Christmas party outfit again. But aerobic exercise - the kind that really works out your heart, lungs and circulation - is unbeatable in the good it does your body.

"The benefits are almost endless," says Helen Luery, clinical exercise physiologist at University College London. "Regular aerobic exercise reduces the chances of having a heart attack or a stroke by as much as 30 per cent. It reduces the risk of developing some cancers. It gives you a greater feeling of well-being and helps you deal with stress. And if you are ill, it gives you greater longevity in disease and an increased ability to survive major surgery."

The trouble is that most of us, if we do aerobic exercise at all, do it wrongly, either dragging ourselves reluctantly to another 20 minutes on an exercise bike or jumping straight into a 10-mile run and wondering why we feel so bad the next day. No wonder all those new year's resolutions to get fitter seem to evaporate by the middle of February.

"My own experience of aerobics is a bit of a boom and bust cycle," says Ann Shuttleworth, an "intermittent" gym-goer from south London, "getting on the treadmill and pushing myself to run further and faster than ever before while hating every minute of it. Then not going back for weeks because I just can't face all that misery."

There is however a better way. And all it involves is finding a kind of aerobic g-spot called the "anaerobic threshold".

When our muscles do any significant work, they have to convert glucose and fat in our body into energy they can use and for that they need oxygen. That's why, when you start to exercise, your breathing increases and your heart pumps faster, distributing the oxygen-rich blood to where it's needed.

There comes a point though where, however hard we breathe, the heart and lungs can't provide enough oxygen for the level of effort we are making. Everyone will recognise this moment: you get short of breath; your heart is pounding against your rib cage; and your legs are turning to jelly. Short of oxygen, the muscles are turning to a kind of physiological Plan B: using an anaerobic process to turn the glucose and fat into usable energy without using oxygen. Anaerobic respiration works but it's much less efficient and the body can only sustain it for a couple of minutes or so. Pretty quickly you have to slow down or rest so your heart and lungs can catch up and you can return to aerobic respiration, which is much more sustainable.

As you'll have guessed by now, this border between aerobic and anaerobic respiration is the magic anaerobic threshold and it is an excellent marker for how hard (or not) we should be working when we do aerobic exercise.

The anaerobic threshold is usually defined - by fitness types and manufacturers of heart-rate monitors - as a percentage of your maximum heart rate. (You subtract your age from 220 and aim for your heart to beat at about 45 to 55 per cent of the result while you exercise). However, there's an easier way to find it and that's to try and have a chat with the person next to you while you are exercising. If you can talk easily, you're under the threshold and need to push yourself a little harder. If you can't talk at all, you're over it and you can ease off a little. If your words . . . come out in . . . small bursts . . . in between gasps for breath, you're in the right place. It's as simple as that.

The advantage of exercising at this level is that you are pushing your heart and lungs just enough for them to grow stronger without giving up and leaving you panting for breath at the side of the road.

"Training raises the anaerobic threshold," says Luery, "because it increases the strength of your heart, your lung capacity and the amount of haemoglobin in your blood, all of which delivers more oxygen to your muscles.

"Exercising right on the threshold gives the heart and lungs the maximum sustainable work-out. Even short periods of pushing the threshold are much more beneficial than 20 minutes in your comfort zone below it, where you're not really pushing your heart and lungs at all.

"Of course, all regular exercise has health benefits. However, to improve your fitness you must challenge your body and one of the most effective ways of improving your threshold is to do short periods of work above it and then drop back for an 'active recovery'."

Dax Moy, an ex-Royal Marine and now a personal trainer based in north London, goes even further.

"My view is that most people are doing aerobic exercise the wrong way. 'Extensive' exercise, say 20 minutes or more on a treadmill, releases hormones that cannibalise your lean muscle as well as using fat as an energy source. Yes, you lose weight but you lose it just as much from your muscle as from your fat, so the proportion of fat on your body stays the same. If you want proof, it's well known that the fattest people in the fitness industry are women aerobics instructors and they are doing 20 classes a week."

Instead Moy recommends doing short bursts of very intensive activity - less than two minutes - high above your anaerobic threshold ("the hardest you can do it"), interspersed by periods of gentler exercise or rest.

"Intensive exercise uses lots of calories and promotes muscle strength," he says, "and it takes just a fraction of the time people usually spend on cardiovascular activity. And it's a more natural way of exercising. People who work in fields don't do step classes all day but they have very healthy hearts because they are doing intensive exercise for short periods and then resting in between."

Whether you decide to exercise right on your anaerobic threshold or for short bursts high above it, both Luery and Moy agree that many people quickly become discouraged.

"Men tend to set their goals too high," says Luery, "going straight into that 10km run. Women tend to take on too many things at once: going on a diet, doing more exercise, stopping smoking . . . People need to realise that jogging or the gym aren't the only options available to them. There are plenty of ways of doing aerobic exercise, from a brisk walk to cleaning the house. You just need to find one you enjoy and that you can keep going with - and you need to start at a low intensity and build up."

So is this the end for the hour-long aerobics class? Not necessarily. As one sweat-soaked gym-goer told me last week on the way out of 60 minutes of movement and disco music, "I am much too old to go clubbing any more, but this way I can have a good dance and still be tucked up in bed by 10.30 with my book and a nice mug of Horlicks."

LITTLE AND OFTEN BEATS THE FIXED TIME ROUTINE

And for those of us who have barely done any exercise at all? “Start just by getting off the bus a stop early and walking the last bit of your journey,” says Dax Moy. “Then, when that’s not making you short of breath any more, you can get off two stops early. You want a small increment every day. Can you beat yesterday’s time by just one second? Most people tend to set the increments too high and then they fail, or else they stay at the level they’re comfortable with and by the end of the year they’re still in the same place. And try to do a little bit of exercise every day, rather than the government’s 30 minutes, three-times-a-week recommendation. Little and often is much better.”

WANTED: FITNESS GUINEA PIGS

■ Helen Luery and her team at University College London are looking for volunteers of all levels of fitness to take part in a countrywide research programme into how the body absorbs oxygen. Participants will undergo a cardiopulmonary exercise test, which takes about 30 minutes including 10 minutes on an exercise bike, and be given feedback on their underlying level of fitness. The research aims to lead in particular to a greater understanding of the treatment of patients in intensive care. Tel: +44 (0)207-323 9911; www.xtreme-everest.co.uk

■ Dax Moy, tel: +44 (0)207-354 3550; www.daxmoy-pts.co.uk