Ever wondered about the heart-rate accuracy of the cheapest Apple Watch compared to a medical-grade ECG chest strap? I did — and to find out, I wrote a Python script that compares heart-rate data from an Apple Watch SE (logged via WorkOutdoors.app on WatchOS) with data from a Polar H10 chest strap (recorded via Strava.app on iOS).
Results? Surprisingly close. The Apple Watch gives a very accurate overall picture. However, it’s not as detailed, lags quite noticeably (slow to respond to sudden starts and stops), and struggles with low heart rate (data canyons) more than with high heart rate (data peaks).
The Polar H10 samples your heart internally at 130 Hz (that’s 130 samples per second or 1 sample every 8 milliseconds), saving it at 1 Hz (1 sample per second). By contrast, the Apple Watch, in Workout mode, samples at a rate between 0,167 and 0,25 Hz — so, roughly 1 sample every 3-6 seconds.
There’s nothing like seeing it for yourself. If you’re comfortable exporting your own .fit files and running a Python script, head over to my GitHub and take a closer look into the accuracy of your running, cycling, or swimming gear metrics for some peace of mind.
One of the myths I’d put to rest: supposedly, the Apple Watch struggles with high heart rates — the opposite happens, probably because Apple’s algorithm expects slower heart reaction times. Caveat: I didn’t push into my own zone 5, trying to replicate a more “normal” heart load of my age group “run-walking to health”.
Yes, Apple lags behind if you want real-time readout, but otherwise — it’s a surprisingly accurate, comfortable and affordable tool to train your heart. Half the people working the checkout counters of my local supermarket wear one. No big bucks required to care for your main (and only) blood pump. 😉
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