MISSING A BEAT?

QHow accurate are optical heart rate monitors?

Steven Brown

ALet’s just say the acceptance of optical heart-rate sensors’ accuracy flaws is so accepted that when we recently interviewed Wahoo Fitness founder Chip Hawkins about their new multisport watch’s precision, he told us candidly, “It’s as good as anyone’s but optical has limitations that you can’t overcome. For instance, in really cold weather the capillaries in your skin contract and pull the blood in. That’s an accuracy challenge. And when you need it most in a sprint, you’re really swinging your arms, blood’s moving around and it can impair accuracy. I’d say it’s 95% perfect, 5% not.”

Whether you feel that 5% figure’s underplaying the situation somewhat remains to be seen, but Hawkins isn’t the only one who’s open with optical’s flaws. Garmin, for instance, has a dedicated activity-disclaimer page on their website (garmin.com) that states, “While our wrist HR monitor technology is state of the art, there are inherent limitations with the technology that may cause some of the heart rate readings to be inaccurate under certain circumstance. These circumstances include the user’s physical characteristics, the fit of the device, and the type and intensity of the activity.”

Why the inaccuracy? It’s down to heart-rate measurements from wearables deriving from photoplethysmography, an optical method for measuring changes in blood volume under the skin. That’s why potential areas stem from the watch being worn too loosely, rapid motion (as when sprinting) and skin tone – albeit while the theory went that darker skin generates less-accurate results, this wasn’t echoed by a 2020 study in Nature led by Brinnae Bent. Interestingly, that same study showed that devices that cost more and were released more recently enjoyed greater accuracy.

That’s a sentiment we’d echo on 220, though ignoring the slight impracticality, we’d recommend following the lead of two-time Olympian Molly Huddle who usually relies on a wrist monitor, but wears a chest strap for more intense efforts like hill reps and track intervals.

James Witts