How reliable is heart rate monitoring on smartwatches?

08 Tem 17:29

Just a couple of days before my marathon, I casually put on my brand-new Garmin Venu 2S, and it ended up making me laugh-cry. I had my pre-race heart rate set around 170, but when I checked it before starting, the watch showed 158 bpm—even though I’d already been warming up in the locker room for about ten minutes. Then, after stepping outside for a quick walk and testing it on the treadmill, Garmin’s own reading shot up to 175, while the wrist monitor stubbornly stayed at 160. Halfway through the run, exhausted, it suddenly spiked to 192 bpm! The actual heart rate monitor beside me was only showing 173. Come on, really? I was about to puke—why was it still "accurately" counting? Later, I opened it up and realized the optical heart rate sensor had too little space between the chip and the strap. The constant jostling during running made the readings completely off. Garmin’s PPG algorithm is decent, but without proper contact, accuracy drops by half. Ironically, if I pressed my fingertip on the green sensor for a few seconds mid-run, the numbers finally matched my real pulse. Now I get it—smartwatch PPG sensors can scrape by when you're sitting still, but once you're moving—especially with big motions, sweat, and strap jiggle—errors can jump by 15–25 bpm. I’ve even seen someone on an Apple Watch in the gym drop to 40 bpm mid-run and claim they were “about to pass out”—total nonsense. So if you're serious about training with a watch, at least pair it with a proper heart rate strap. Don’t rely solely on wrist readings.