Treadmill running is an inherently different strain on the human body than outdoor running. Being confined to a small belt that is moving below you and in a stationary location alters the effort and efficiency in a few ways that can make a treadmill run feel easier or harder than comparable power on an outdoor run. These changes in sensation do not indicate that the Stryd footpod is malfunctioning or reporting incorrectly. Instead, treadmill workouts should be built differently from outdoor runs. Below are a couple of things that might contribute to a change in perceived effort for a treadmill run and how to adjust your indoor workouts to account for these factors. If more than one applies, the effects of each one are not additive (do not decrease your power targets by 2x if gait changes and heart rate both seem applicable).
NOTE: Stryd offers a collection of Treadmill Workouts standard with the Stryd Mobile App. These workouts were made with the treadmill in mind, and won't need modification under the effects described below.
Running on a treadmill belt changes your running mechanics. The treadmill belt limits where you can step and it is moving below you, and it also simulates much more constant running conditions that you experience on an outdoor run. Recent studies have shown that there are substantial changes to the way humans run on a treadmill with comparison to outdoors. The changes the treadmill creates in your natural stride can make it feel like you are running at a higher power than you are. If you notice this trend, reduce the power target for treadmill workouts so your perceived effort is similar to what it would be outside.
Runners will sometimes note that their heart rate is higher for a treadmill run than an outdoor run even when their running power is the same. When running in a stationary location, it is common for heat to build up around the body versus be lost to the environment as it is in an outdoor run. The decreased ability for the body to cool itself can increase your heart rate at the same power output.
It is also worth noting that heart rate is a poor metrics for work in general. It fluctuates from day to day and is affected by factors beyond running strain. It is often inconsistent and can vary even without the heating effects described here.
For the purposes of keeping the effort feeling the way you think it should, decrease your power targets for treadmill workouts compared to what they would be for an outdoor run.
Ventilation and air flow
Ventilation and airflow are limited in treadmill running with comparison to outdoor running. This can lead to the same heating effects described under Heart Rate above, but can increase perceived effort at a given power number without increasing heart rate. If your efforts is higher due to stagnant air or overheating, reduce your power targets for treadmill workouts with respect to the outdoor workout equivalent.
Treadmills are stationary. It is difficult for them to provide the same kind of mental distraction and engagement with your surroundings that you will get running outside. The reduced distraction can cause a runner to become more focused and aware of their effort, and can consequently feel they are working harder at the same power number.
Mental distraction and fatigue has little impact on the work of your body. Any change in RPE associated with mental fatigue does not require a change in power targets for your treadmill workouts, but you may choose to lower power targets if you believe it may help improve the run experience.
There is no air power contribution when a runner is stationary as they are on a treadmill. It is normal for air power to contribute 0-10 watts to an outdoor run, but there is no air power in treadmill workouts. This means pace or incline must be adjusted to reach a similar power target.