Does an activity have to get you out of breath to count as exercise? Do you really have to do half-an-hour a day? These are just some of the dilemmas many of us face when working out the best way to get fit. The good news is scientists do broadly agree on the best ways to get fit, they just have not been very good at telling us what they have discovered. "We haven't done a great job of distilling down a large number of studies and say what this means for the average person who is trying to get in shape," acknowledges Simon Marshall, a specialist in exercise and sports psychology at San Diego State University in California.
So how should you go about getting fit? We have set out the latest evidence and exploded some myths along the way. Read on, and you can decide for yourself. What counts as exercise? The standard advice is that we should aim for 150 minutes a week of moderate-intensity exercise. The tricky question here is what "moderate" means. Gauging the intensity of an activity by measuring how fast it makes your heart beat is old hat. These days, metabolic rate is the preferred measure. It is usually represented in units known as the metabolic equivalent, or MET. This is the metabolic rate during the activity in question divided by the rate when sitting doing nothing. Moderate exercise is defined as anything that clocks up between three and six METs. This even answers the hoary old question about golf: yes, it does count as exercise, notching up a respectable 4.5 METs if you walk round the course, or one MET less if you ride round in a golf buggy. Musicians may be dismayed to learn that playing the flute gets a mere two METs, though drummers earn a more respectable four. Walking gets anywhere from two to 12 METs, depending mainly on speed and terrain.
Is pumping iron necessary? Several studies have suggested a link between muscle strength and living longer, but for a long time it was unclear whether other factors were confusing the picture. In the past few years, however, some large, well-designed studies have settled the question. One study, published in 2008, measured the muscle strength of almost 9,000 American men and followed their health for 20 years. The death rates among those whose muscle strength was in the bottom third for their age group was around 30 per cent higher than for the other two thirds.
How much and how often? Half an hour of moderate-intensity exercise at least five days a week used to be the required regime to keep fit. Now the consensus is that exercise does not have to be portioned out in daily doses. If you aim for 150 minutes per week you can divide it up however you like. Jogging can kill you "Look at Jim Fixx!" cry the couch potatoes, citing the celebrity runner credited with kick-starting the jogging craze in the 1970s. At the age of 52, Fixx famously dropped dead from a heart attack midway through a run. Could exercise be a killer lying in wait for the unwary?
The risk of a heart attack does rise during vigorous exercise. But the extent of the rise depends heavily on how accustomed you are to that exercise. For someone who is completely unfit, the risk can rise by as much as 100-fold, relative to when they are resting. For someone who regularly runs five times a week, their risk while exercising roughly doubles. The lesson, says David Stensel, an exercise physiologist at Loughborough University in Britain, is to be careful when you take up exercise
How do I know I am fit? "Fitness" can refer to a number of attributes, including muscle strength and flexibility, but is usually used to refer to aerobic fitness, also known as cardiorespiratory or cardiovascular fitness. This boils down to how effective the body is at delivering oxygen to muscle cells. The best way of assessing someone's aerobic fitness is to measure their VO2max - the maximum consumption of oxygen they can achieve during a session of exercise that gradually increases in intensity.
There are ways to estimate VO2max that do not require a sports science lab. The only equipment needed for the Rockport Fitness Walking Test, for example, is a watch. Time how long it takes you to walk a mile as quickly as possible, then measure your heart rate. Plug the time and heart rate, along with your age, gender and weight into the appropriate equation or find a website that will do it for you - try the Brian Mac Sports Coach site at www.brianmac.co.uk - and you will get a ballpark value for your VO2max.
You need to monitor fluids Everyone knows the importance of keeping hydrated. Common advice is to deliberately drink beyond what thirst dictates, or "push fluids", to combat dehydration and keep performance up to scratch. Usually that is a waste of time, and just occasionally it can be fatal. Exercise-associated hyponatraemia is a dangerous condition that occurs when people have drunk so much that the concentration of sodium in their blood falls too low. This leads to excess water moving into the tissues of the brain, causing brain swelling. In rare cases - 12 have been recorded worldwide - the victim has died.
Ron Maughan, a physiologist and sports nutrition specialist at Loughborough University, says blanket guidelines are flawed because people vary in how much they sweat. He recommends that people weigh themselves before and after their exercise to find out how much they sweat, and drink enough to maintain their body weight. Others say drinking to satisfy your thirst is all that is needed. Focus on fitness, not fatness
Whether being overweight is an absolute bar to fitness has become one of the most hotly debated questions in exercise science. Steven Blair at the University of South Carolina is one of those who doesn't accept what might at first sight seem plain common sense - that being fat means you must be unfit. In a study published in 2007, Dr Blair recruited 2,600 people of varying weight and timed how long they could run on a treadmill before becoming exhausted. Among those who were mildly obese, only a third met a common definition of being physically unfit, and only half of those who were moderately obese were unfit. Dr Blair points out that measures of aerobic fitness - the body's ability to deliver oxygen to the muscles - have nothing to do with the amount of fat tissue present.
In the 12 years during which the subjects were followed, Dr Blair's study found that the risk of dying was more closely linked to fitness than fatness. People who were fit but obese had a lower risk of dying than people who were unfit but of normal weight. That is important, says Dr Blair, because while many overweight people find it hard to get slim, they could still become healthier with more exercise. It is a point he would like doctors to bear in mind when advising overweight patients.
What if I get injured? Pulled muscles and twisted ankles are the downside of sports and exercise. Sometimes it is hard to know whether to rest an injury, see a doctor or even push on through the pain. If you are starting a new activity, do not be put off by some aches and stiffness during the first couple of days. "There's discomfort which you get just from using parts you're not used to using," says John Tanner, a musculoskeletal physician at the Bupa Wellness Clinic in London. "If it becomes a pain and intrusive, either stop and give it a break, or get some advice on technique."