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The latest personal fitness trackers and exercise machines offer a bewildering array of metrics to track and quantify your progress. But the simplest and most broadly applicable remains the humble calorie, which reflects both the intensity and duration of your workout.

The problem is that the estimates you get from machines and online calculators reflect population averages and neglect individual differences. To get a more personalized take on your calorie use, and to accurately compare the results of different kinds of workouts, keep the following three factors in mind:

Body shape

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Researchers produce calorie estimates by asking volunteers to perform various tasks while measuring their oxygen consumption, which allows them to calculate energy use. To account for the fact that volunteers come in all shapes and sizes, the average result is expressed in calories per kilogram of body weight – which is why exercise machines ask you to input your weight.

This approach produces decent estimates, but it overlooks the fact that two people with the same weight can have dramatically different height and body composition. People with more body fat than average burn fewer calories per kilogram of overall weight during weight-bearing exercises such as walking and jogging, which means that cardio machines overestimate the number of calories they burn.

Height also makes a difference, according to a new study just published in the Journal of Applied Physiology by researchers at Southern Methodist University in Texas. Studying subjects ranging from 3-foot-6 to 6-foot-11, they came up with a formula based on both height and weight that predicted the energy cost of walking more accurately than the traditional weight-only formula.

So how much difference does this make in practice? For someone who is 5-foot-2 with a BMI of 25, the standard formula underestimates calorie burn while walking briskly by about 5 per cent; for someone 6-foot-2 with the same BMI of 25, the standard formula overestimates it by about 5 per cent.

Fitness

The fitter you get, the easier your workout will feel if you don't increase the intensity. That's reflected in calorie burn too: As you get fitter, you body gets more efficient and needs fewer calories to maintain the same pace, even if your weight stays the same.

One way to correct for fitness is to measure heart rate while you exercise. Research going back to the 1980s has found that there's a direct relationship between heart rate and calories during "moderate" exercise, when heart rate is between about 90 and 150 beats per minute. The problem is that the exact relationship is different for each person and requires careful measurement of maximum heart rate, so it's not very practical.

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The best use for heart-rate monitors is to compare your personal rate of calorie burning while doing different activities. A South African study in 2005 found that the relationship between heart rate and calorie consumption is essentially the same for running and cycling. That means if you're used to jogging at 140 beats per minute, you can monitor your heart rate to figure out how fast you need to cycle to reach the same level of energy consumption.

The same should apply to most continuous aerobic activities that you perform upright, such as stair-climbing or cross-country skiing. Different rules apply to intermittent activities like lifting weights; heart rate is also lower if you're horizontal or underwater, so swimming heart rates aren't directly comparable.

Heat

Exercise feels harder in hot conditions, so it's no surprise that maintaining the same pace on a hot day burns more calories. But even if you slow down enough to keep your effort level the same, exercise in the heat still places greater demands on your body and burns more calories – in particular, it burns through your carbohydrate stores more quickly without changing your rate of fat-burning.

This may sound like a bad outcome, since most people who count calories are really trying to shed fat. But in most cases, what really matters is how many calories you burn, not whether those calories came from fat or carbohydrate stores. Researchers at Australia's Garvan Institute of Medical Research published a study in 2010 in which mice were genetically altered to burn more fat instead of carbohydrate, but the change had no impact on their body weight or body composition.

In the end, knowing the exact number of calories you burned during a workout isn't all that useful for most people. But understanding how to compare the calorie-burning effects of different kinds of workouts and how to adjust for changing conditions can help you plan an exercise program that progresses steadily and consistently. If you're trying to lose weight, there's no magic calorie-burning number other than "a little more than last week."

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Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com.

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