Mysterious Treadmill Pace?

You may have noticed that running on a treadmill feels differently when compared to running outdoors. Understanding the root cause of this difference, it turns out, may actually help improve our run training.

We've tested Stryd on multiple runners and across multiple treadmills, and after all our data collection and analysis we think we know why. The root cause is that your treadmill's belt speed isn't actually constant. More specifically, when your foot strikes the belt, the motor is loaded and the belt slows temporarily. Conversely, when your body is in the air, the motor applies an extra speed to the belt to recover from the previous loading. This extra speed is recorded by the treadmill, but it isn't applied to you as the runner.


Notice from the figure above, that

  • the belt speed (in blue) reduces at each foot strike, and to compensate,
  • the treadmill motor temporarily over speeds the belt when the runner is in the air.

The extra distance traveled by the belt (shaded in red) while freely moving underneath the runner does not cause any extra effort or metabolic cost to the runner. Our study shows that, on commercial grade treadmills with good calibration, the "free distance" recorded by the treadmill accounts for about 2% of the overall. (Note: some laboratory grade treadmills with specially designed flywheels are more resilient to such effects, while older and less powerful treadmills are more susceptible).

Please see this article for additional explanation and actions you can take: