Is Running Outside Really Better Than Running on the Treadmill?

Studies showed that running on a flat treadmill burned about 4 percent less energy, but that difference could be eliminated by setting the treadmill incline to 1 percent.

Air resistance depends on how fast you’re going. If you’re running really fast, air resistance takes a bigger toll, which means that a really fast treadmill running is artificially easy by a larger amount. Conversely, when you’re running really slowly, air resistance is almost irrelevant.

The biggest differences between treadmill and outdoor running aren’t in how you run, but rather in how you respond to the experience.

People probably run slightly differently on treadmills: when you look in detail at the biomechanics, you find subtle differences in things like knee angles and peak ground forces, but the overall pattern is that the two movements are close enough that you don’t have to worry about the differences, as long as the belt is properly calibrated and not too soft.