How accurate are phones and trackers on a treadmill?
Case and colleagues (JAMA, 2015) had 14 adults walk 500 and 1,500 steps on a treadmill while a researcher counted. Smartphone apps in a trouser pocket read between 6.7% under and 6.2% over the true count. Wearable devices ranged from 22.7% under to 1.5% under.
Hochsmann and colleagues (2018) tested 20 adults aged 18 to 25 and 45 to 70 at 1.6, 3.2, 4.8 and 6.0 km/h, using video as the reference. An iPhone and a Garmin wrist tracker both had small errors at 3.2 km/h and faster. A Samsung phone and a hip-worn research monitor were only accurate at 4.8 km/h and faster. A wrist-worn research monitor had large errors at every speed.
Phone in your pocket, bag or hand
In the 2018 study, it did not matter whether the iPhone was in a trouser pocket, a shoulder bag or a backpack: all three gave accurate counts at normal speeds. Your hips and body move the same way on a belt as on the ground, so a phone carried on the body picks up each step.
Holding the phone still in your hand, or resting it on the treadmill console, was not tested there and is a poor idea. A phone that does not move with your body sees much less of each step, and motor vibration can confuse motion-based counting.
Our browser step counter counts only while its page is open and the screen is on. On a treadmill, keep the phone in a pocket or armband with the page open, not on the console.
Holding the handrails
Wrist trackers estimate steps partly from the swing of your arm. Sears and colleagues (2017) found every wrist-worn device they tested undercounted treadmill walking, from about 3% to 10% under, while a hip-worn pedometer was within 0.4%.
Fitbit's own help pages say a wrist device still counts steps when your hands are on a pushchair or trolley, but the total may be slightly lower than usual. Gripping treadmill handrails keeps your wrists still in the same way, so expect a lower count from a wrist tracker. Let your arms swing if you can do so safely, or carry your phone as a second counter.
Why the treadmill and your counter disagree on distance
The treadmill works out distance from belt speed and time. A step counter works out distance from steps multiplied by a step length: ours uses 0.415 x your height, or 0.75 m if you have not entered a height. If your real step on the belt is shorter or longer than that, the two distances drift apart even when the step count is right.
Treadmill walking on a flat belt costs about the same energy as walking on flat ground: the Compendium of Physical Activities gives 4.8 METs for both at 3.5 to 3.9 mph (5.6 to 6.3 km/h). Adding an incline raises the cost.
Check your own count in five minutes
- Set a steady, comfortable walking speed.
- Count 100 steps in your head, or count for one minute.
- Compare with the phone or watch reading for the same stretch.
- Repeat once with your arms swinging and once holding the rails.
- If one device is consistently off by a similar amount, trust the one that matches your count.