A common question I receive from many of my athletes regards treadmill running. This is a question that I’ve had myself and you have possibly had as well. Most runners use a Garmin or other GPS watch to track their runs – including for indoor running. The Run Indoor setting on many GPS watches allows you to track and record your treadmill run, just as you would an outdoor run. However, several of the runners I coach ask me about the accuracy of the Garmin Run Indoor mode. More often than not, the Garmin treadmill sensor records a different distance – and therefore a different average pace – than the treadmill.
So which is more accurate – the Garmin or the treadmill? Why is there a discrepancy in the distance between the two? Why is the discrepancy even more prominent on interval workouts than easy runs? (Do note this post refers to a motorized treadmill – non-motorized treadmills may render different metrics).
Is Your Watch or Treadmill More Accurate?
Unless you are using an old treadmill, the treadmill will be calibrated for the appropriate distance. This is especially true of newer treadmills, especially home models. The advanced technology on these machines yields accurate data.
If the treadmill feels harder or easier than outdoors, that does not mean the treadmill is slower or faster. A 2019 meta-analysis published in Sports Medicine concluded that while VO2max and heart rate tend to be similar on the treadmill compared to outdoor running, endurance is poorer on the treadmill. This could be due to the lack of air circulation, monotony of the treadmill, or other factors. Even if eight miles “feels” easier on the treadmill, you still ran eight miles if your treadmill says so.
How Does Garmin Indoor Mode Work?
Unless you utilize a foot pod, a GPS watch tracks distance indoors using an accelerometer. Accelerometers calculate the distance covered based upon your stride length, stride frequency, and arm swing. The accelerometer calibrates based on your form and cadence on outdoor runs. It then counts your steps on a treadmill run and estimates your distance in the recorded time frame.
Running on the treadmill is relatively the same as outdoors in terms of heart rate and energy output. However, many runners do slightly alter their stride on the treadmill. As a 2017 study published in the International Journal of Exercise Science notes, stride length and stride frequency tend to differ when running on the treadmill than outdoors.
The difference in gait on the treadmill versus outdoors makes sense. Even if your overall form is similar (as this study in The Journal of Sport Rehabilitation determined), many runners slightly alter their form based on the different environmental cues. The confined space of the treadmill can also cause many runners to change their form. You may do this if you are worried about flying off the back of the treadmill and subconsciously alter your stride. If you are used to running on rolling hills and set the treadmill to 0% incline the whole way, your form will be different than outdoors.
If the Garmin treadmill mode is calculating the distance run based on your stride and arm swing, and you change your stride and arm swing on the treadmill, it therefore makes sense that the distance calculated is different than the treadmill reads. The treadmill calculates distance based on the number of revolutions of the belt – unless the treadmill is very old or miscalibrated, it should be relatively accurate.
You will know this is happening to you if your cadence tends to be lower on treadmill runs compared to outdoor runs. If your cadence is relatively the same yet numbers still differ, know that it is likely the shortcoming of the technology. Your cadence may only vary 15 steps per minute between easy and hard paces; that is certainly not precise enough to accurately capture the difference between a 7:45/mile and 8:15/mile, for example!
Many newer models of Garmin, Coros, and other GPS watches allow you to calibrate the distance after you complete the run. Some (such as Coros) even use an algorithm to learn from each of your indoor runs to provide a more accurate read in the future.
But most of us runners are analytical numbers geeks – we want accurate data and we want to be able to easily track our workouts. These few tips will help you minimize the discrepancy between your watch and the treadmill – and help you get the most out of your indoor run.
How to Optimize Garmin Indoor Mode Accuracy
Focus on Your Form
If the indoor mode of your GPS device and your treadmill read statistically significant differences (say, more than 15% of the distance of the run), your form may drastically different on the treadmill than outdoors.
For example, most runners will have a similar cadence on the treadmill than outdoors. However, there are outliers in both directions. If you notice that your Garmin treadmill mode reads slower than outdoors, you may have a lower cadence on the treadmill than indoors. If that is the case, be mindful to maintain quick turnover on the treadmill.
Whether or not you care about the GPS data, pay appropriate attention to your form on the treadmill. Avoid holding onto the front bars or running at the very front of the belt. Running at the front of the belt can shorten your stride and inhibit your arm swing. You want to run in the middle of the belt, giving yourself room for your stride and arm swing to open up more naturally. Run tall, avoiding hunched shoulders. Cue yourself to “run light,” which encourages shorter ground contact time and a quicker cadence. Don’t obsessively check your watch, as bringing your wrist forward and craning your head down throws off your form.
Run For Time Instead of Distance
If even a minor distance discrepancy is going to mess with your mind, change your metric and run by time instead of distance. After all, the duration of the run is ultimately what your body responds to. For example, if your normal easy run pace is approximately a 9-minute mile and you want to run 5 miles, run for 45 minutes on the treadmill. If your watch reads short or long, it really doesn’t matter – you will have achieved your prescribed workout.
For speedwork, use time-based interval workouts such as fartleks rather than distance-based intervals. Faster paces may exaggerate cadence and form differences. In this case, your watch will likely be more inaccurate during interval runs. If you are using the interval function on your watch, time-based intervals will align with the time readouts on the treadmill. Try a fartlek workout or one of these treadmill workouts for runners.
Remember the Purpose of Your Workout
In the age of GPS watches and social training logs, we can become slightly obsessive over the preciseness of data and numbers. Ultimately, what matters is the workout itself: how much time you spend running at a particular intensity. Distance and pace are one method of measuring volume and intensity, but so are duration and perceived effort.
For example, 5 mile run and 5.3 mile run at an easy effort render the same workout. Choose which way you want to measure that run on the treadmill and then disregard any minor discrepancy. Don’t obsess too much if your watch and treadmill are slightly different on distance, especially if it’s an easy run. The discrepancy does not negate the purpose of your workout.
Treadmill Inclines Will Affect Accuracy
When you run uphill or downhill, your gait naturally changes. However, your watch cannot differentiate flat, uphill, or downhill gradient on a treadmill. Rather, it simply notices the changes in your stride. If you change the incline often on the treadmill, this can affect the Garmin indoor mode accuracy. However, this is no reason to avoid inclines on the treadmill! Simply use auto lap (see below) and calibrate your watch after the run. (For more on the benefits of treadmill incline workouts, read here!)
Utilize the AutoLap
Want accurate splits for your Strava or Final Surge data? Utilize the auto lap function on your watch. You can hit auto lap each mile based on the treadmill reading. You will still want to calibrate the distance at the end, but then at least the splits are accurate.
A Final Note on Indoor Mode
In addition to the treadmill, Run Indoor mode functions well for the other type of indoor running, the indoor track. Run Indoor mode will be more accurate on the indoor track than the treadmill. This is because indoor track running is overground running, just like running outside. You can also use the precisely measured distance of an indoor track (a majority are 200ms per loop) to confirm the distance and pace of your run. (If you are training on the indoor track, try one of these indoor track workouts!)