Is your treadmill lying to you about the calories you burned?

Today's guest posting comes courtesy of author, columnist, blogger, athlete and even physicist, Alex Hutchinson. Last week I emailed him to ask him his opinion regarding the validity of gym equipment calorie counts, and following a brief exchange he kindly agreed to provide me with a guest posting on why treadmills are liars. His blog Sweat Science is a must read for me and if it's not on your blogroll, it should be.

So here's Alex to answer the question, "Can your treadmill really count calories?"

Lately I’ve been having a lot of fun with the new 2011 Compendium of Physical Activities, which has a snazzy new website that allows you to compare the calorie-burning powers of pretty much any activity you can think of, from coal mining to water aerobics to butchering small animals. It’s pretty simple: each activity is assigned a number that tells you how many calories you burn per kilogram of body mass per hour.

As a way of comparing the demands of different activities, it’s great. But there’s a problem when you try to apply those numbers to yourself. Personally, I’m pretty sure that if I tried to butcher a small animal, I’d end up burning a huge number of calories because my heart would be racing, and I’d do it all wrong and end up chasing a headless chicken around and around the yard. But the Compendium doesn’t know that about me: it only knows my weight.

That’s the same problem faced by the cardio machines at the gym. They count calories in basically the same way the Compendium does (and in some cases probably rely on exactly the same data): researchers study a few “average” volunteers and figure out how many calories per kilogram of body weight they burn at different intensities. But none of us is “average.”

If you have more body fat than average, you’ll burn fewer calories per kilogram of total body weight. That means the number on the treadmill (or elliptical or exercise bike) is an overestimate. If you’re less aerobically fit than average, on the other hand, you’ll burn more calories than the treadmill thinks. Other factors like height, age and sex also skew the results – not to mention more obvious things like using the handrails to support some of your bodyweight, which is a common cheat on the elliptical and treadmill that the machine doesn’t take into account.

But all of these factors are relatively minor compared to the most misleading part of exercise machine calorie counts: the difference between gross and net calorie burns. For example, an 80-kg woman walking for an hour at 2.5 mph will burn 240 calories, according to the treadmill. But if she had spent that hour lying on the couch, she would have burned 80 calories just to stay alive – so she really only burned an extra 160 calories by exercising. That’s an overestimate of 50 percent!

The lower the intensity of the exercise, the bigger the difference between gross and net calorie numbers. If you want to make a back-of-the-envelope correction, you can subtract 1 calorie for every kilogram of bodyweight per hour of exercise from the cardio machine’s number. Still, that’s just going to give you a very rough estimate. For meaningful feedback, it’s probably better to focus on things like how far you went, how fast, how hard – and how it made you feel.

Alex Hutchinson is a columnist with the Globe and Mail and a senior editor at Canadian Running magazine. His new book, “Which Comes First, Cardio or Weights? Fitness Myths, Training Truths, and Other Surprising Discoveries from the Science of Exercise,” was published in May. He blogs about the science of fitness at www.sweatscience.com.