Fitbit hides inaccurate heart rate tracking in plain sight

No, that's not your heart skipping a beat—it's your fitness tracker.

Credit: Fitbit

Fitbit just can't seem to catch a break. Its Blaze fitness smartwatch was panned by investors due to Apple Watch comparisons, there's a class-action lawsuit against the company, and now a new study claims that its heart rate trackers can be wildly inaccurate.

Reports of inaccurate fitness trackers are nothing new, but this new study from California State Polytechnic University, Pomona, provides ammunition for the class action lawsuit that started back in January. And that’s exactly what it was designed to do. According to CNBC, the study was commissioned by the law firm behind the suit.

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On the surface, the researchers’ methodology seems sound. The study compared generated electrocardiogram readings against readings from the PurePulse heart rate sensors in Fitbit’s Charge HR and Surge trackers, and found that they can be off by up to 20 beats per minute during rigorous exercise.

Despite these findings, the suit could still face an uphill battle since Fitbit's product manuals for the Charge HR, Surge, and Blaze explicitly warn of these inaccuracies:

With high-intensity interval training or other activities where your wrist is moving vigorously and non-rhythmically, the movement may prevent the sensor from finding an accurate heart rate. Similarly, with exercises such as weight lifting or rowing, your wrist muscles may flex in such a way that the band tightens and loosens during exercise. Try relaxing your wrist and staying still briefly (about 10 seconds), after which you should see an improved heart rate reading.

That’s from the Fitbit Charge HR manual, and the Surge and Blaze manuals include nearly verbatim versions of the same statement.

Here at, we happen to have the Fitbit Blaze on-hand, and we recently conducted our own test of its heart rate sensor using a surgical-grade pulse oximeter. Our data showed that the Blaze was off by just one to two beats per minute for both resting and active heart rate.

It’s important to note that our active heart rate readings were conducted while our subject was sitting still, a few seconds after completing rigorous exercise. That’s exactly what Fitbit recommends in its manual. The new study, in contrast, tested heart rate accuracy while its subjects were actively exercising, which runs counter to Fitbit’s advice.

Fitbit deserves the heat it’s getting, if only for burying such a huge disclaimer deep in its manuals. Tweet It

This might not be the fairest comparison: We used a different kind of heart-rate monitoring device and a different Fitbit tracker. Still, it suggests that further independent tests are needed to settle the accuracy debate.

In any case, Fitbit deserves the heat it’s getting, if only for burying such a huge disclaimer deep in its manuals. If customers are purchasing these trackers believing they’ll be accurate during exercise, the company is doing them a huge disservice.

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Fitbit Blaze Smart Fitness Watch

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