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  health > heart > story page AIDSAlternative MedicineCancerDiet & FitnessHeartMenSeniorsWomen

Burning calories is not an exact science

September 6, 1999
Web posted at: 2:09 PM EDT (1809 GMT)

In this story:

A not-so-straightforward answer

The body composition factor

Speed is important too


By Miriam Nelson, Ph.D.

(WebMD) -- You're not alone if you've been wondering about the accuracy of calorie counters that are built into treadmills, stationary bikes and other cardiovascular machines.

Many people who sweat and agonize through the cardiovascular, or "cardio," component of a workout understandably want to pin a magic number to their accomplishment -- often in the form of "calories burned." Of course, the designers and manufacturers of cardiovascular exercise equipment make this very easy, providing you with more details about your workout than you ever dreamed: calories burned, distance traveled, number of flights climbed, just to name a few. But are all of those details reliable?

A not-so-straightforward answer

For fitness-center personnel who regularly confront this question, the answer is somewhere between "Yes" and "No." Within each piece of equipment is a computer that uses standard formulas to calculate the number of calories you're expending. The accuracy of the formulas depends on the type and brand of equipment, but all are far from perfect. Weight-bearing equipment, such as a treadmill or elliptical trainer, for example, is probably a little more accurate than non-weight-bearing equipment, such as a stationary bike.

That's because weight-bearing equipment takes your body weight into account when estimating the calories you've burned. The more you weigh, the more calories you burn during a given activity. Regardless, the number of calories the machine indicates that you have burned is not exact, indicating a range of calories you've actually burned. More often than not, the machines will overestimate by as much as 30 percent, though an overestimation of 10 percent to 15 percent is more likely.

The body composition factor

However, what this formula -- and all of the others -- do not account for is body composition. People who have more body fat and less muscle mass per pound will burn considerably fewer calories than those who have more muscle and less fat. For instance, a 130-pound woman with 20 percent body fat will burn calories more effectively than a 130-pound woman with 30 percent body fat.

Speed is important too

Calculations of non-weight-bearing machines are even less precise because they not only overlook body weight and composition, but frequently do not account for your speed. These machines, which include rowing ergometers, only include distance and the time of your workout in calculating calories.

The formula that non-weight-bearing machines use depends on an "average rpm cycling speed," usually 70 rpm (revolutions per minute). So the only variables in the equation are the level of intensity -- meaning the resistance you are pedaling against -- and the time you spent working out. With most stationary bikes you will find that if you ride for 10 minutes at 50 rpm, 70 rpm and then 90 rpm, the number of calories you burned at each of the different speeds are all the same or very close.

To correct for the inaccuracies, try the following tips:

  • Enter a body weight that is lower than your actual weight.
  • Try switching machines so your muscles avoid becoming conditioned to one. When you switch between different machines, the change in motion and the muscles you use will force you to work harder, helping you burn more calories.
  • Add an extra five minutes to your exercise session, but without the assumption that you'll burn more calories.
  • When using weight-bearing equipment, don't let the handles support your body weight for you -- they are intended to assist with balance only.
  • Try to concentrate on how hard you're working and sweating rather than on how many calories the machine shows you're burning. When you focus on being consistent and increasing your intensity, you'll be more successful at maintaining a regular exercise routine. This will maximize any cardiovascular and weight-loss benefits you'll achieve.
  • Miriam E. Nelson, Ph.D., is director of the Center for Physical Fitness at Tufts University. She is author of the international best-sellers "Strong Women Stay Young" and "Strong Women Stay Slim" (Bantam).Copyright 1999 WebMD, Inc. All rights reserved.

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