Fitbit’s heart rate tracking is wildly inaccurate, study finds

May 1, 2016


Fitbit’s heart rate monitoring wristbands are wildly inaccurate, according to researchers.

A study has found that Fitbit devices’ heart rate readings could be as much as 23 beats per minute (bpm) off compared to the number recorded by an electrocardiogram (ECG).

“The PurePulse technology embedded in the Fitbit optical sensors does not accurately record heart rate, and is particularly unreliable during moderate to high intensity exercise,” said the study.

The researchers from California State Polytechnic University studied the company’s heart rate monitors after a lawsuit alleged the devices “do not and cannot consistently and accurately record wearers’ heart rates during the intense physical activity for which Fitbit expressly markets them”. The study was commissioned by the plaintiffs’ lawyers.

To assess the claims, the researchers tested the Fitbit Surge and Charge HR, two products designed to monitor users’ heart rates, steps walked, calories burned and sleep quality, on 43 healthy patients.

They tracked the adults across 65 minutes as they conducted different activities, including jogging and running. The results showed the greater the level of activity, the more inaccurate the devices’ readings were.

Not only did the Charge HR and Surge show different heart rate readings from the ECG, they were also different to one another on the same patient.

The PurePulse technology on the back of the Fitbit Surge CREDIT: FITBIT

Unlike regular heart rate monitors, the Fitbit devices have a PurePulse sensor that uses LEDs reflected onto the skin to detect changes in blood volume. Software then calculates the difference to gauge the wearer’s heart rate.

After the lawsuit, filed in January, claimed that Fitbits consistently undershoot actual heart rates, the study found the devices read on average 9 bpm less than wearers’ actual heart rate.


During exercise this average jumped to almost 17 bpm inaccuracy, and in one instance the Surge recorded a heart rate that was an average of 23 bpm lower than that recorded by the ECG.

A spokesman for Fitbit described the study as “biased, baseless, and nothing more than an attempt to extract a payout from Fitbit”.

“It lacks scientific rigour and is the product of flawed methodology,” said the spokesman. “It was paid for by plaintiffs’ lawyers who are suing Fitbit, and was conducted with a consumer-grade electrocardiogram – not a true clinical device, as implied by the plaintiffs’ lawyers.”

Fitbit spent three years developing the PurePulse technology and conducts “extensive internal studies” that test the accuracy of its products, the spokesman said.

While Fitbit insists the devices are not meant to be scientific or medical-grade, the lawsuit said: “The defect in the PurePulse Trackers presents a safety hazard because Class members’ could jeopardize their health by relying on the inaccurate heart rate readings and potentially achieving dangerous heart rates.”


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