We've all heard of high-intensity interval training, but how does it actually work?
Regular exercisers will be familiar with the term 'high-intensity interval training' (working out at 80-90pc of your maximum heart rate), which has been a buzz phrase among athletes since the 1970s.
It isn't just athletes than can benefit from the method; anyone looking to burn fat and build their cardiovascular fitness can employ the principles of HIIT, if they know how to use it. But, for the non-experienced, the details are confusing. Some HIIT programmes advocate as little as three minutes a day, others 10 to 45 minutes.
Can you really do all your cardio training in three minutes? Yes, says expert trainer John O'Shea, of London's prestigious Grayshott Spa - "provided you're working at your absolute maximum endurance".
If you're a woman, you can work out what this is, approximately, by multiplying your age by 0.9 and subtracting that total from 209. If you're a man, multiply your age by 0.8 and subtract the total from 208.
This gives a rough guide to your maximum heart-rate capacity: a 45-year-old woman will have a maximum heart-rate capacity of 168 beats per minute; 90pc of that takes you to 151.2 bpm. "But learn to go by how you feel," suggests O'Shea.
I don't know whether you've trained at your maximum heart rate, but it's what O'Shea somewhat understatedly calls "a psychological challenge".
Your heart crashes through your ribs and cardiac arrest seems imminent. If that floats your boat and you can keep it up for 180 seconds, the three-minute cardio workout is for you.
Otherwise, break it down into short bursts. A typical HIIT programme could be 20 seconds flat-out, with 30-second "rest" periods - when you're working at 70-80pc capacity, until you've accumulated three minutes total at your maximum, which in this instance would take seven-and-a-half minutes.
You can get your HIIT in all kinds of ways - static bikes, elliptical trainers (which have the advantage of giving the arms a workout), walking the dog uphill and down dale.
Plunge into the YouTube world of HIIT and you arrive at a woman and a baby in a baby-bouncer performing an intense dance-mountain climber routine to the three minutes and 47 seconds of Taylor's Swift's 'Shake it Off', which is an ideal HIIT length, but you'd really need to be at full blast.
Still, it shows that even harassed mums can work HIIT into their schedules.
Jillian Michaels, who has a big following on US TV, devised what she called the 3:2:1 HIIT for a chain of Canadian gyms.
The 3:2:1 consists of three minutes of strength exercises, (compound squats with bicep curls, lunges with tricep kickbacks and free weights), followed by two minutes of cardio (jumping jacks, spotty dogs, mountain climbers or skipping) and one minute of abs and tricep dips.
"Always warm up first," says O'Shea, "to avoid tearing cold muscles. After exercise of any kind, you must stretch, otherwise you'll end up foreshortening your muscles.
"Hold each stretch for at least 15 seconds, after which, you should be able to go into the stretch more deeply for another 15." That brings the workout to 11 minutes.
Michaels recommends four sessions of 3:2:1 a week. If you're doing all your HIIT on a bike, treadmill or power walking - ie pure cardio - you'll still need two sessions of resistance/weights for 20-30 minutes (a demanding Pilates class counts. It's excellent for bone strength, posture and can be done anaerobically).
It's important to vary exercise, to give muscles time to recover. You could do pure HIIT one day, followed by resistance training the next, or combine them in one session, à la Michaels and follow with a day of what O'Shea calls "active rest", ie walking.
Time efficiency, plus evidence that HIIT burns calories for hours after the session is over, make it appealing.
"Not that we should define our health via calories any more. They're just a tiny part of the picture, as is your weight. Go by how your clothes fit," says Stephanie Moore, a qualified nutritionist, psychotherapist and fitness trainer.
Bottom line: to burn fat you don't need masses of cardio.