Want a more challenging treadmill? The Woodway Curve treadmill is like a giant hamster wheel that can help you burn up to 30% more calories compared to traditional treadmills and encourage good running form. This top-of-the-line treadmill is expensive but at least you'll save a little bit on the electricity bill. It's on the Biggest Loser and in some high-end gyms. We've seen other self-propelled treadmills but only the curve seem to run effortlessly and smoothly – no need to hold on the bars and push the running surface to get it going. Just run.