There are many myths running around about exercise, fictions which are validated and perpetuated through endless repetition. And it's not just the internet and other media that are to blame - even fitness professionals get it wrong sometimes. 

Many of the misconceptions about exercise relate to how well it burns fat and calories, and how much of both get wiped away with different types of exercise. This leads some people to eat more than they should, relative to the amount of total activity they do each day, which in turn causes them to gain weight (or not lose as much as they'd like). 

We take a look at the 10 most common exercise myths, and give you the real facts.

  1. You should stretch before exercising
  2. No pain, no gain
  3. Low-intensity exercise burns more fat
  4. Exercising three times a week is enough
  5. Walking burns 300 calories an hour, so if I walk for an hour I'll burn off the 300-calorie chocolate bar I ate
  6. I'm slim and healthy, I don't need to exercise
  7. Walking one kilometre burns the same calories as running one kilometre
  8. Swimming isn't a good way to lose weight
  9. Your metabolism increases after exercise, so you burn more calories even though you've stopped exercising
  10. You burn more fat if you exercise on an empty stomach

Myth: "You should stretch before exercising"

While it's important to warm up before exercise, especially if you've just gotten out of bed or are about to undertake strenuous exercise, there's no good evidence that 'stretching' does much good.

By stretching, they mean doing static stretches to pull at particular muscle and ligament groups, like hamstrings, calf muscles, Achilles tendons and so on.

Warming up, on the other hand, is doing the basic moves of whatever it is you're about to do (running, swimming, cycling, playing soccer) at a low intensity.

Proponents of stretching say it prevents injuries and muscle soreness, and improves overall performance. However, reviews of many studies have found there's no beneficial effect on reducing the chances of injury and soreness, and there are contradictory findings on performance – some find it helps, some find it hinders, some find no effect.

Stretching is still useful to help maintain or increase flexibility, and can improve performance in the long term, so do it another time — while you're watching TV, say.

Bottom line: Stretching before exercise doesn't appear to help reduce muscle soreness or risk of injury.

Myth: "No pain, no gain"

The old 'exercise as punishment' philosophy, that it has to hurt to do you good, has just about disappeared from exercise lore – though for some people puffing, sweating or messing up their hairdo constitutes pain.

But does higher-intensity exercise have any advantages over the lower-intensity variety?

In terms of basic health benefits, such as living a longer life free from disability caused by disease, just meeting the minimum requirements of National Physical Activity Guidelines for Australians will provide many health benefits. However, vigorous exercise can provide extra protection against heart disease. It can also help with overall fitness and calorie burning.

Bottom line: Exercise shouldn't hurt, but exerting yourself a little can improve your fitness, even if it does mess up your hair.

Myth: "Low-intensity exercise is better for fat burning"

The idea is that if you exercise at low intensity, around 50 to 60% of your maximum heart rate, you burn more fat than if you exercise at a higher heart rate - like the 70 to 85% recommended for improving fitness. That would be great news for walkers: as long as you're walking fast enough that you can talk, but not sing, you're getting more out of it than the red-faced, sweaty joggers who huff and puff past you.

Sadly, it's not entirely true. Yes, you burn more fat as a percentage of the total energy you burn when exercising at low intensity, as fat accounts for almost all the fuel your body uses. But you burn more actual fat per minute, and a lot more total calories, at higher intensities.

The advantage of low-intensity exercise, especially for people who aren't terribly fit, is that it's less tiring and puts less strain on the body, so you can exercise for a longer period of time. In the end, it's the total calories burnt that count – so for maximum weight loss you can either get puffing for a shorter period or just keep walking for longer.

Bottom line: Low-intensity exercise is great for beginners, while exercising at higher intensities helps you get fitter and burn more total energy in a set period of time. 

Myth: "Exercising three times a week is enough"

There's a lot of conflicting advice about the amount of exercise you need to do to get results, ranging from 'anything is better than nothing' to 'over an hour a day, every day'. There's a bit of truth in both extremes, and everything in between, though it also depends on what you mean by 'get results'. For example:

If you go for a 30 to 40-minute brisk walk, you can temporarily reduce blood sugar, triglycerides and blood-pressure levels. So in that sense, just one session can have a benefit.

People who've lost a great deal of weight (20kg or more) seem to require one hour to 90 minutes of moderate to vigorous exercise every day to maintain that weight loss though.

The current Federal Government National Physical Activity Guidelines for Australians now recommend that for good health, adults exercise for a total of 300 minutes a week, preferably spread across all days, or equating to 60 minutes a day for five days a week. The activity should be strenuous enough to raise your heart rate and rate of breathing, but you should still be able to talk, not sing.

Alternatively, exchange 20 minutes of your moderate activity exercise for 10 minutes of vigorous exercise such as jogging, aerobics and fast cycling. For vigorous activity, your heart rate should be about 70–85% of its maximum (which is 220 beats per minute, minus your age).

Finally, for good all-round physical health, you might consider including two or three resistance or strength training (weightlifting) sessions per week, as well as a flexibility and balance program (such as yoga or Pilates, or even just some gentle stretching), in your exercise routine.

Bottom line: Anything is better than nothing, but for good health, do moderate exercise totalling at least 60 minutes a day, on five or more days a week.

Myth: "I just ate a 300-calorie chocolate bar. Walking burns 300 calories per hour, so if I walk for about an hour, I'll burn off the chocolate."

If you look up most exercise and calorie tables, they'll tell you that a 68kg person walking for one hour at a moderate speed of 5 to 6km/h burns between 250 and 300 calories. They don't usually remind you that by sitting around doing nothing or pottering about, you'd also burn some calories — maybe up to 140. In other words, they give 'gross' calorie expenditure per unit of time, not 'net' calories (the amount above and beyond what you'd normally expend). 

Now assuming the chocolate is surplus to your daily energy needs, and you want to burn off an extra 300 calories, you'll need to do 300 'net' calories worth of exercise. Researchers have worked out that the net calories burned walking in this situation would be about 180 calories per hour. This means more than 1.5 hours of walking to burn off the chocolate. And eating it took you all of... how long?

Bottom line: Calculating energy expenditure based on gross, rather than net calorie burn overestimates the contribution of exercise to total daily expenditure.

Myth: "I'm slim and healthy, I don't need to exercise"

Even if you're slim and seem to be healthy, if you're inactive you're not as healthy as you could be. And just because you can't see the fat, doesn't mean it's not there. Researchers have found that people who maintain their weight through diet rather than exercise were likely to have major deposits of internal fat, making them "thin outside, fat inside", or TOFI's,  in terms of health risks.

Furthermore, studies have found that people who are 'fat and fit' – that is, who are overweight but do regular exercise – are healthier than people who don't exercise, and therefore likely to live longer.

Meeting the minimum exercise recommendations can go a long way to achieving good health. In addition to preventing unhealthy weight gain, exercise benefits include:

  • Normal blood pressure.
  • Healthy cholesterol levels.
  • Lower levels of anxiety and depression.
  • Stronger bones and better balance (particularly important for older people otherwise at risk of osteoporosis).
  • Reducing your risk of (or helping to manage) type-2 diabetes.
  • Good cardio-respiratory fitness, muscular strength and flexibility.
  • General self-esteem and psychological wellbeing.

Bottom line: Exercise contributes to good overall physical and mental health – it's not just about body weight.

Myth: "Walking one kilometre burns the same calories 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.

Walking is an extremely efficient way for humans to move around. With straight-ish legs and a reasonably level centre of gravity, we're a bit like a bunch of swinging pendulums with good momentum, especially when you get your arms in on the action. But running requires a lot more energy than walking: you're effectively jumping from one foot to the other, and raising and lowering your centre of gravity far more than you do when walking. Think more like a bouncing ball than pendulums.

In fact, running consumes around 40 to 50% more gross calories, or twice as many net calories (see the 'walking-off-a-chocolate-bar' myth for the difference between net and gross calories) per kilometre than walking – at least up to a certain walking speed. For example, at speeds over 8km per hour, walking burns more calories than running at the same speed because it's more difficult as your gait is more inefficient.

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 fast burns as many calories as running a kilometre slowly. However, given that running burns two times as many net calories per kilometre as walking in a given amount of time, that means if you run twice as fast as you walk you'll get twice as far and therefore burn four times as many net calories. The Straight Dope presents a fun discussion of running versus walking.

Bottom line: Running burns twice as many net calories per kilometre as walking.

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

Some media reports have turned the received wisdom – that swimming is great for heavy people who want to lose weight – on its head. We're now told studies have found that people who swim for exercise don't lose weight or even (shudder) end up gaining weight.

There are several possible explanations for these findings. A 1987 study found that swimmers gained weight (though not a significant amount) while walkers and cyclists lost weight. Calories 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 indeed stimulate the appetite, which may result in weight gain if calories 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. And don't forget, body fat helps you float!

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.

Myth: "Your metabolism increases after exercise, so you burn more calories even though you've stopped exercising"

Bonus! Not only have you just burned off 400 calories doing aerobics, but you'll keep burning calories for hours afterwards. Better still, the less fit you are, the slower you recover and therefore the more you burn.

Experts call it excess post-exercise oxygen consumption (EPOC), whereby increased metabolism means consuming more oxygen, which in turn burns calories. Most people who are aware of this post-exercise metabolic increase probably overestimate the degree of extra energy burned though. While it's not entirely bunkum, one extra biscuit could wipe out that calorie debt – and then some.

For any sort of meaningful EPOC effect, you have to exercise long and hard. Low or moderate-intensity exercise – walking or slow jogging, say – would have little to no effect. Even at higher intensities you need to exercise for almost an hour for any appreciable increase in metabolism. Experts argue the level required would be intolerable for the average person, and only serious athletes would be capable of sustaining the required intensity for long enough to make a difference.

Bottom line: Any increase in metabolic rate after exercising is likely to be fairly insignificant for the average athlete.

Myth: "You burn more fat if you exercise on an empty stomach"

The theory goes that if you don't eat before exercising, your body has to use fat as fuel, because there's hardly any sugar to burn. And if you want to lose weight, it stands to reason that burning off fat is a good thing. There's nothing wrong with the basic theory – evidence shows that more fat is burnt after longer fasting conditions than in the few hours after eating, when carbohydrate is the preferred fuel.

But in fact it's the total amount of energy burned that's the issue, not whether it's fat or carbohydrate. You may find you get tired and don't exercise as hard as you would if you'd eaten beforehand – and therefore you'll burn less total energy.

Depending on how hard and how long you intend to exercise, and whether exhaustion might affect your speed and endurance, it may be better to have some fruit, yoghurt or toast at least half an hour before exercising, as well as a drink to help prevent dehydration, and have the rest of your breakfast after.

Bottom line: Eating before exercise may help by giving you more energy for a better workout, and it's total energy burn that counts.