How Accurate Are Exercise Machines?

Feb. 21, 2013— -- They crunch your calories, calculate your distance and tabulate your heart rate, but how accurate are those exercise machines on the floor of every gym?

Sixteen percent of Americans belong to a health club, and many rely on the machines to keep track of calories burned, distance traveled and heart rate reached. Despite a still-faltering economy, health club memberships have increased by more than 10 percent over the past three years -- 50.2 million Americans belong to a health club, according to the International Health, Racquet and Sports Club Association.

So it's become more important than ever to learn more about what these machines can do. ABC News' Linsey Davis headed over to New York Sports Club in New York City to investigate.

After running for one mile on a treadmill, the machine logged 94 calories burned. But what was her body really burning? To get an answer, Davis was fitted with an oxygen analyzer, which can count calories burned down to the decimal point, at the Hospital for Special Surgery in New York,

After doing the exact same warmup that she did for her first treadmill run, Davis ran another mile at the same pace. The oxygen analyzer counted only 75 calories -- about 20 percent less than the gym's treadmill.

Experts said the discrepancy in calorie counts occurs because treadmills take only limited factors into account, such as pace, weight and age. They generally do not factor in running form, metabolism or specific body type.

"The best it can do is give you an estimate based on generalities," exercise physiologist Polly de Mille told ABC News.

Manufacturers acknowledged that the machines produce rough estimates, but said they were getting more accurate as the technology improved, and that in any event, they provided useful guidance to exercisers.

What about the heart-rate monitor?

Davis hopped on an elliptical machine to compare its monitor to the specialized one she'd strapped to her chest. According to the machine, her heart rate stood at 136 beats per minute, while the specialized monitor logged 135 beats per minute. Even when Davis sped up her pace to test her own limits, the elliptical monitor showed 175 beats per minute compared with the monitor's 173 beats per minute -- it was pretty accurate.

And what about distance?

Experts said treadmill belts could wear out or stretch over time, but that with regular maintenance, a mile on the treadmill would be the same as a mile run outdoors.

But if a mile on a treadmill is truly a mile, why does it feel as if you're working so much harder when running outdoors?

"There are many things in that mile that might dramatically affect how you're maintaining a certain pace," said de Mille.

Treadmills are housed in climate-controlled gyms or homes. When running outdoors, the temperature can vary tremendously, which can have a notable impact on the difficulty of that mile.

When running on a treadmill, your legs are pulled back, whereas outdoor runners have to pull their body weight over their feet with each stride, meaning it takes more effort to cover the same distance.

But there is a way to make sure your treadmill workouts are as tough as your outdoor workouts.

"If you put it up to between 1 and 2 percent in elevation," said de Mille. "It'll re-create the demand of some wind resistance."

Some believe this is sweating the small stuff, but those calories do add up, and next time, you may have to run just a little bit faster.