Elliptical illusion: How accurate are those calorie counters?

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A little over a year ago, I was killing time on an elliptical machine at a local gym, using a wristwatch heart-rate monitor to track the workout. The monitor had settings for my age, weight, sex and height, and kept a running count of the estimated number of calories used.

The machine's onboard electronics also provided a calorie estimate. After an hour, the discrepancy between the two was striking: According to the watch, I had used about 600 calories. The machine had me cracking 800.

Since then, I have intermittently repeated the comparison on different types of machines from different manufacturers and using heart-rate monitors from different companies. The result is typically the same: The machines come out with a higher calorie count than the heart-rate monitors, and the difference is often substantial.

Which to believe? Does it matter?

It is easy to get hung up in the numbers, but ultimately any of the available methods for calculating calories should be considered estimates. Short of going to a lab or clinic, the best you'll get is a sense of magnitude, whether you are using a high-end sports watch, a machine at the local gym or a manual formula (see related story).

A calorie is a measure of energy: When you read a food label, the number of calories reflects the amount of energy that is packed in there. Once that food is in your body, the potential energy gets unleashed through a few processes, but the dominant mechanism involves the use of oxygen to break down carbohydrates, fats and (sometimes) even proteins into other chemicals. Though different fuel sources require different amounts of oxygen and produce different amounts of energy, the boiled-down version is that each liter of oxygen reflects the body's use of about five calories.

So to really count calories it's necessary to measure the amount of oxygen used — hence the crazy mask they hook you up to during exercise and stress tests. That's hardly practical at the gym.

But there is a good proxy for measuring oxygen use: your heart. As the body demands more oxygen during exercise, the heart beats faster to increase the supply.

How heart rate relates to oxygen consumption varies from person to person. Age, weight, sex, body composition, fitness level and other factors all play a role. Drawing from population models and their own research, the companies that manufacture heart-rate monitors have developed formulas that couple heart rate with those different variables and massage it all into an estimate of calorie usage.

The onboard calculators found on treadmills, elliptical trainers and other devices use basically the same approach. Depending on the machine, however, they typically don't allow you to enter as much information about yourself as a heart monitor. The machine might ask for your weight and age, for example, but not your sex or an estimate of your fitness level. Fewer variables mean a rougher guess.

In addition, the most important parameter of all, your heart rate, may well be missing from the equation.

Though exercise equipment typically has the capacity to measure heart rate, that requires the user to either keep a steady grip on a pair of electrodes — impractical, particularly for running on a treadmill — or wear a chest strap that sends a signal to a wireless sensor.

Equipment makers can't assume that will happen, said Bob Quast, vice president of brand management for Life Fitness, one of the major exercise equipment companies. So, at least in Life Fitness's case, the company builds formulas for estimating calories that are independent of the user's heart rate. The company does in-house studies using lab equipment to measure the calories burned in different types and intensities of exercise among people of different ages, weights and sexes.

That information is built into the formulas used on the machines.

"It is accurate for the general population between 5 and 10 percent," Quast said. "The key is to keep going back to the same product and use it over time" to compare different workouts.

A number of studies have concluded that exercise machines typically err on the high side. One paper presented at an American College of Sports Medicine conference found the calorie counts on one brand of elliptical machine to be about 26 percent higher than lab-based estimates of the calories used. In their textbook "Exercise Physiology," physiologists Sharon Plowman and Denise L. Smith cited several similar studies, including one that found error ranges of between 39 percent and 79 percent.

"It is best to interpret the console values for caloric expenditure as an approximation rather than an absolute," the two wrote, "and to accept that the approximation is probably high."

That doesn't mean the gadgetry isn't without value. Hop on the same model of treadmill each day, for example, and if it says you used 500 calories one afternoon and 550 on the next, that's progress.

If a more precise estimate is important to you, then a heart-rate monitor might be a good investment. These are particularly helpful if you are just starting out and learning to gauge your effort level, and there are models to help with more-advanced training, as well.

But keep in mind: It is still an estimate. Your own sense of how hard you are working will be, in many ways, just as helpful.

In other words, listen to your heart.

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