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).
Let’s ALL Be More Accurate!
Further, when performing our study, we found many treadmills which had poor calibration. This means that the treadmill displayed a speed which was not accurate compared to the measured average belt speed (notice the red line in the plot above is ~6.9, while the treadmill was set to 7.0). The good news is that all treadmills can be calibrated to an accurate speed without a professional’s help, including to compensate for the overestimated 2%. Here’s how you can do it by yourself, without any specialized tools, and in only a few minutes time.
See the entire process step-by-step, in only 50 seconds of video
Measure true belt length
(Using a roll of sticky tape and a tape measure) Manually rotate the belt and place three pieces of tape at points evenly distributed across the belt. Measure the distance between the three and add them together to get true belt length.
Measure true belt speed
(Using a friend with a stopwatch app) Keep one piece of tape on the treadmill belt, and set the speed you wish to calibrate to. Have your friend watch you run, counting tape (belt) revolutions as they measure time. Stop the timer when you reach 100 revolutions.
True Belt Speed = Revolutions * True Belt Length / Time