Heart Rate Monitors Don't Lie (But Your Wrist Might)

The moment my Garmin’s wrist HR (red) realised it forgot to train for this sprint.

I bought the Polar H10 because a study said it was 99% accurate. That, and I could already tell it was picking up data faster than my wrist - I just wanted the numbers to confirm it. So after six months of wearing a chest strap like some kind of cyborg, here’s what I actually found.

For steady stuff like Zone 2 rides, resting HR, sleep tracking - the wrist is fine. The Garmin keeps up fine. But throw an interval at it and the whole thing falls apart. The graph says it better than I can: wrist HR lags, sometimes never fully catches up, and by the time it does you’re already on the way back down. Not ideal when you want to see if your HR is lower or higher than the last time you did that sprint.

The good news is you don’t need to spend £70 to fix this. I’ve been running a £20 Coospo H9Z from AliExpress alongside the Polar and genuinely didn’t expect this - it’s been spot on. Full comparison coming soon. I’m also running the HW9 Armband for Muay Thai and yeah, same story there. Spot on.

One more thing you might notice on the graph: there are actually four lines. I was also curious whether my Wahoo Kickr V5 and my 4iiii Precision 3 crank powermeter would agree on power output. They do. That particular anxiety is resolved.

Bottom line: chest strap if you care about data for hard efforts, wrist if you don’t.