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Pretty much every fitness tracker and smartwatch has an optical heart rate sensor, but they're not always perfectly accurate. Not only can you get better accuracy by wearing a chest strap heart rate monitor, you can even use a chest strap without a fitness watch—just connect it to your phone. Chest straps don't break the bank, either; my favorite is a mere 25 bucks.
Chest strap monitors—the traditional kind, anyway—measure the electrical activity of your heart. They use electrodes that contact your skin, and sometimes you may need to wait until you've worked up a bit of a sweat before they get a good connection (although I haven't found that to be an issue in my workouts).
That makes them far more accurate than optical sensors, which are the type you'll see on the back of a smartwatch. Optical sensors use a (usually) green light, and they pick up your heart rate by detecting the light that bounces off your blood vessels as the blood pulses through. Relying on light means that they don't always work as well through tattoos and may struggle with accuracy on darker skin tones. They can also be confused by outside light, which is why they aren't as accurate if your watch is worn loosely or is too large. Runners sometimes experience "cadence lock," where the cadence of their footsteps (170 steps per minute, let's say) causes the watch to think your heart is beating at exactly that rhythm. If your app reports a heart rate that is the same as your cadence, there's a good chance it wasn't measuring your heart rate at all.
You can sidestep all of those issues by wearing a chest strap. As I've found in my device reviews, even the most accurate watches won't always record your heart rate perfectly 100% of the time. If you care a lot about your heart rate in training, just get the chest strap.
I've been wearing a cheap Coospo heart rate monitor for years. I didn't borrow it for a review, I just bought one because I wanted a cheap and easy way to track my heart rate without a watch. (I was doing a lot of kettlebell workouts, and the kettlebell rests gently on your wrist during overhead exercises, where a watch would be in the way.
I've kept using it since then. It's been happily paired with an iPhone, a Peloton, an Apple Watch, a series of Garmins, and probably a bunch of others I've forgotten about. When I write the "accuracy" section in my fitness watch comparisons and reviews, I use the Coospo (either paired to my phone or to a third watch) to get a gold-standard heart rate graph to see how the tested devices stack up.
The Coospo heart rate monitor I have is a slightly older version of the one linked above (since I've had it so long). It uses Bluetooth and also supports ANT+, and it gets its power from a coin cell battery that I have to change out roughly once a year. It doesn't have any on-device storage space, which I don't need, because it beams the data directly to the connected watch or phone. It's comfortable and easy to adjust. The device pops off so I can wash the strap. And even though there are more expensive straps out there (like the $100 Polar H10, which I've heard is great), I cannot think of a single chest strap feature I could possibly care about that I don't already have.
On my iPhone, I found the simplest way was to download the Polar Beat app (free on iOS and Android), which pairs with my strap over Bluetooth. If you’re going for a run or a bike ride while carrying your phone, it can also use your phone’s GPS to track your pace and record where you went on a map.
I tend to use mine for indoor cycling, though, and for kettlebell workouts (which I log as “other indoor”). Using a strap without a watch is especially handy with kettlebells, since the bell otherwise knocks into the watch in certain positions. But if you have a fitness watch and you like using it, most models will allow you to pair the watch with a chest strap directly. That way you can get the most accurate readings, and be able to check the numbers from your wrist.
