No surprise: Cardio machines overestimate your calorie burn

It may not come as a huge surprise that treadmills, elliptical trainers and other fitness machines overestimate the number of calories burned. But you might raise your eyebrows at the margin of error -- especially if you're relying on a machine's calorie counter for your weight-loss strategy. According to a recent segment on Good Morning America, treadmills and stair steppers overestimate by about 12% each. That might not make a huge difference if you're only walking for a mile or so. But it starts to add up. If you're walking for an hour and the machine says you've burned 450 calories, you've really burned 400. But even that isn't too bad compared to elliptical trainers, which in the GMA study proved off by a whopping 42%.

Good Morning America's reporters enlisted the help of a human performance laboratory to measure the actual calorie burn of a test subject using a VO2 analyzer, which is the most accurate technology currently available. When asked for a response from the manufacturers of the tested cardio machines, companies told GMA that a number of factors could have affected the inflated numbers: "According to the makers, calorie counting software doesn't adjust to machine wear and tear. Also, machine resistance can change over time, which skews the calculation. And none of the machines can calculate a user's metabolism rate, health history and fitness level, all of which affect calorie counters. "

To illustrate this point, About.com's guide to running, Christine Luff explains: "For example, if a 160-pound woman with 35% body fat and a 160-pound woman with 20% body fat are both running at a 10:00/mile pace, the treadmill will display the same amount of calories burned. However, the woman with the lower body fat and more muscle mass is actually burning more calories. "

The age of the machine can also make a difference. According to a Q&A article at Prevention.com, "Exercise equipment that requires you to punch in personal info such as weight, height, and gender provides the closest approximation of calorie burn, says Gregory Florez, spokesperson for the American Council on Exercise. However, the "calories burned" readout on any exercise machine is an approximation. In fact, some older machines can overestimate calorie burn by as much as 25%."

The LA Times recently published a longer article on the topic.  The findings, based on interviews with both fitness professionals and manufacturers, echo those of Good Morning America. The upshot? Those calorie counters are estimates and nothing more -- and you can pretty much assume that the are overestimating, not underestimating. Furthermore, both studies say that elliptical trainers are the worst, mainly because there's a greater range of individual exertion and skill level for ellipticals, and because there is no standard elliptical motion like there is with treadmills  -- every model of machine is different.