How Accurate Are Cardio Machines' Calorie Burn Displays?

What you don't know could be hurting your workout and your waistline

BY ASHLEY OERMAN

The question: "I just exercised on a cardio machine. Did I really burn as many calories as the machine says I did?"

The expert: Greg Justice, an exercise physiologist and author of Mind Over Fatter

The answer: In a word, no, says Justice. Most cardio machines use your weight and age to calculate the calorie burn for the average person meeting those two conditions. What it's not always accounting for: your gender, height, body fat percentage, and fitness level. All of those things determine how many calories you're actually burning. So if two women both weigh 135 pounds but one has 20 percent body fat and the other has 35 percent body fat, for example, they're not going to burn the same number of calories when they do the same exact workout, says Justice.

A machine’s calorie count could also be impacted by the kind of machine it is. In an experiment done by the University of California, San Francisco's Human Performance Center for Good Morning America in 2010, treadmills overestimated calorie burn by 13 percent, stair climbers by 12 percent, and stationary bikes by seven percent. The worst offender? The elliptical, which overestimated calorie burn by 42 percent. Yikes!

Bummer, right? But the problem here is not only the misdirected ego boost you feel as that number climbs higher—the erroneous blinking screen could also wreak havoc on your waistline and your endurance. "Problem with taking that number literally," says Justice, "is that someone thinks they burned 1,000 calories, then thinks they can eat 1,000 calories' worth of food." Plus, says Justice, a person might be tempted to do a slow and steady workout instead of an intense, fast sweat session—which, he says, helps you burn more calories post-workout—since the calorie counter looks higher after 40 minutes rather than 15. The point is, even if the number on the screen is higher after a long, moderate workout, your metabolism isn't as revved up as it would be after a speedy, hard workout.

But before you swear off indoor equipment forever, Justice says there is a very valuable purpose for those calorie counters: Instead of taking the counts literally, you can use them to gauge how hard you're working. Try this method: Write down what your screen says after a cardio machine workout. Then, try and top it each gym visit in the same amount of time.

And if you still want to keep track of the calories you're torching, Justice recommends using this formula from a 2005 study in the Journal of Sports Sciences: Calories Burned = [(Age x 0.074) - (Weight x 0.05741) + (Heart Rate during exercise x 0.4472) - 20.4022] x Time / 4.184. (Easy, right? Hah!) Though, he says, there's no perfect way to measure calorie burn because there are so many variables involved.

Comments:
The most accurate way to measure calories burned is the room calorimeter where the subject remains for a few days. The walls of the room absorb heat produced by the subject. There are calculations to take account for heat produced by machinery (eg gym machines) and hot food. (This is quite expensive).

I always am looking at gym machines and thinking how come I have not burned a single calorie yet - I have moved a great deal already. We really are very efficient.