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Outdoors, smartwatches do a great job of telling you how far you ran or walked. Indoors, they often disagree with the readout on the gym’s treadmill. Which leaves a lot of us wondering—should we trust the treadmill, or the watch?
I won’t make you wait: The answer to this question is the treadmill. While a watch guesses at your speed based on wrist movements, the treadmill will always have a better idea than the watch of what your feet are doing. But I don’t blame you for doubting everything you see on a gym machine’s readout. After all, their calorie counts are notoriously inaccurate.
How treadmills determine distance
Whenever we’re evaluating accuracy, it’s important to know whether a device is measuring something directly, or interpreting data to make a guess. In this case, the treadmill is measuring distance, while the watch is guessing at it. (Calories are always interpreted, which is why they are never truly reliable.)
Distance is pretty simple: It’s the number of miles (or meters) of treadmill belt that passes under your feet while you run. The treadmill knows how long its belt is, and how fast the motor is running, so it’s pretty directly measuring how much belt it’s sending under your feet. (Sticklers will note that technically there is still some interpretation going on here, since the relationship between the motor’s performance and how much belt it moves can change over time, which is why gyms should recalibrate their treadmills from time to time. But the treadmill’s number is likely to be close to accurate even if it’s not perfect, and is still your best bet for the purposes we discuss here.)
How watches determine distance
Your watch, by contrast, is on your wrist. There’s no way it can know what your feet are doing. It can make some guesses based on how much your wrist is swinging back and forth, and thus pick up a cadence (each arm swing corresponds to one footstep). You’ll swing your arms a bit harder and higher when you’re running faster, so it can use the oomph in your arm swing to make a guess at the speed you’re running. From speed and time, it calculates distance.
Some devices use outdoor, GPS-tracked activities to estimate the length of your average stride. That number then goes into the distance calculation, which helps to improve accuracy. But your stride length isn’t the same for every speed you might run or walk, or for every type of terrain. The calculation is pretty limited in its usefulness.
How to get the most accurate numbers on a treadmill
Neither is guaranteed to be perfect, but the treadmill is a lot more likely to get the right idea. A treadmill can be slightly miscalibrated, and give you a distance that’s a smidge off from what you actually ran. But a watch is just riding on your wrist, guessing. I’ve noticed that my arms don’t swing all that differently when I’m running at a fast versus a slow pace.
In fact, while following an interval workout on the treadmill on a Garmin, I’ve found that the watch thinks I’m going slower than I am during the fast parts. If I’m following a Garmin workout that asks me to run specific paces, it often buzzes to chide me (“Pace low”) even if I’m going at the right speed. I can often convince it I’m in range by pumping my arms a little harder. This isn’t really a solution, though.
Instead of trying to get more accurate numbers from my watch during the run, I approach my treadmill workouts with the assumptions that the following things are accurate:
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The treadmill speed during the workout
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The treadmill distance at the end of the workout
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The heart rate from my watch (especially if I’m using a chest strap)
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The time from either or both devices (which should be the same if I started them at the same time)
I then follow my workouts appropriately. If it’s a heart-rate-based workout, I let my watch handle everything. (Garmin might tell me to do 32 minutes at 139 to 167 beats per minute. Perfect. All you, Garmin.)
But if I want to do a speed- or distance-based workout, I manage that manually, with my human brain. A quarter mile at 7.5 miles per hour, repeated eight times? I boop the buttons on the treadmill accordingly. At the end of the workout, I’ll either “calibrate” (from the watch) or edit the distance on my app to make sure it recorded the correct total distance.
