Walk, run or swim

Myth: "Walking one kilometre burns the same kilojoules as running one kilometre."

What's amazing about this myth is that it has persisted for so long. Scientists have known for years that it's not true, but you'll still hear it touted as fact by any number of health and fitness industry professionals.

A version of it even features on lists of exercise myths - except that the 'myth' is that 'running a mile burns more kilojoules than walking a mile' - which is actually true.

Walking is an extremely efficient way for humans to move around. With straight-ish legs and a reasonably level centre of gravity, you're bit like a bunch of swinging pendulums with good momentum, especially when you include your arms.

Running requires a lot more energy than walking. You're effectively jumping from one foot to the other, raising and lowering your centre of gravity far more than walking - more like a bouncing ball than pendulums.

In fact running consumes around 40 to 50 percent more gross kilojoules, or twice as many net kilojoules, per kilometre than walking - at least up to a certain walking speed. (See the 'walking-off-a-chocolate-bar' myth for the difference between net and gross kilojoules.)

At speeds over 8km per hour, walking burns more kilojoules than running at the same speed because it's more inefficient and difficult.

If speed's not your thing, the good news is that it doesn't matter how fast you run: it's the distance that matters. So running a kilometre slowly burns as many kilojoules as running a kilometre quickly.

However, as running burns twice as many net kilojoules per kilometre as walking, if you run twice as fast as you walk, you'll get twice as far and therefore burn four times as many net kilojoules in the same amount of time.

Bottom line:

Running burns twice as many net kilojoules per kilometre as walking.

Myth: "Swimming isn't a good way to lose weight."

Media reports in recent years have suggested that people who swim for exercise don't lose weight, or even that they gain weight.

There are several possible explanations for these findings:

• One study found that swimmers gained weight (though not a significant amount) while walkers and cyclists lost weight. Kilojoules weren't restricted (so the swimmers may have eaten more) and exercise intensity wasn't monitored or controlled. This one study seems to be cited over and over again, giving the impression that the evidence is based on lots of different sources.
  
  
• Another study found that people who exercised in cold (20°C) water ate more afterwards than people who exercised in neutral (33°C) water. So swimming in cold water may stimulate the appetite, possibly resulting in weight gain if kilojoules aren't restricted.
  
  
• If you swim pretty slowly - essentially floating, with minimal propulsion - you're likely to expend less effort than if walking or cycling.
  
  
• Swimming without changing energy intake might mean that you're building muscle and losing fat, so your weight may not change much - though your clothes may become looser.
  
  
• Finally, you might get tired out from swimming and laze around the rest of the day. Improving your 'efficiency', through stroke correction lessons, for example, can help prevent strains, injuries and fatigue caused by poor technique.

If you're very heavy swimming can be a great way to start exercising because your weight is supported and it works all parts of the body (especially if you include some work with a kickboard and flippers).

It's possible to lose as much weight swimming as walking or cycling - you just have to do it fast enough (you might need to take your pulse regularly to help you determine how hard you're working).

Bottom line:

Swimming can help you lose weight, as long as you go fast enough and long enough. Just watch your diet as well.

Kilojoule and burn cardio machines

While it's tempting to believe the kilojoule-burn figures clocked up on the stair climber, treadmill or exercise bike, you could be fooling yourself.

Some machines don't ask for your weight, and base their calculations on a weight of 70kg. If you are heavier or lighter than this, you'll be burning more or fewer kilojoules than what is claimed on the display.

All machines assume you're using the correct technique for the exercise, but it's easy to (perhaps unwittingly) expend less effort than assumed. Common energy-saving errors in technique include:

• Holding onto the bars of a treadmill or step machine to support your weight, instead of swinging your arms
• Leaning forward and/or taking baby steps on the step machine
• Not using the swinging handles to exercise your upper body on the elliptical cross-trainer.

The other problem is that some machines give you gross kilojoule burn rather than net kilojoule burn figures. (See the 'walking-off-a-chocolate-bar' myth for more about net and gross kilojoule burn.)