The expert: Holly Perkins, certified strength and conditioning specialist in Los Angeles
The answer: The treadmill pulls ahead of the elliptical -- but just barely. Of course, that's assuming you're working at the same level of exertion. "If we were to compare apples to apples--same heart rate on both machines -- you would burn a few more calories running on the treadmill," she says.
What's the advantage of hitting the belt? Simple: Your feet don't stay planted the entire time. "Any time your body leaves the ground, as with running, that requires more energy," says Perkins.
The caveat, of course, is that how hard you push yourself determines how many calories you burn on either piece of equipment. Although most people find it easier to elevate their heart rate on the treadmill, says Perkins, those who consider running to be absolute torture may prefer to fire up their calorie-burning engines on the elliptical. "If you're on the elliptical and you crank it up to level-20 resistance, you're going to be burning more calories than if you jog along on the treadmill at four miles an hour," she says.
If you're one of the treadmill-hating exercisers, you can make your elliptical workout tougher by going hands-free. "If you're holding on, you're stabilizing yourself," says Perkins. "That means all of the little baby stabilizer muscles don't have to work." By letting go, you force these balancing muscles to do their job -- and that can help you torch a few extra calories, even if you're swinging your arms to help keep your body upright. (Similarly, if you hold on to the treadmill bar, you'll compromise your burn.)
You can also try switching the direction your legs are pumping. "A lot of people forget to go backward on the elliptical," says Perkins. "That's an awesome way to increase the intensity and get a new kind of workout."