Fitbit's Heart Heart Rate Monitors Are Off By Up To 20 BPM: Lawsuit

Your Fitbit may not be as accurate as you think.

The heart rate monitor found in Fitbit’s Surge and Charge HR models may be off by as much as 20 beats per minute, according to a new study commissioned by a group suing the maker of the most popular health-trackers on the market.

It’s the latest study to call into question the reliability of Fitbit's and other consumer-grade fitness-trackers' data.

The study was conducted by two researchers at Cal State Ponoma. They took 43 healthy adults and put them through exercises used in Fitbit advertisements such as jogging, running and stair-climbing. The subjects were each outfitted with a Fitbit Surge on one wrist and a Charge HR on the other. Those results were compared against data from an independent device hooked up to hard-wired electrical signals.

The study was paid for by a law firm that is part of a class-action lawsuit against Fitbit, filed in January, that says the devices don’t provide accurate heart rate data.

Here’s what the researchers found:

• Overall, the Charge HR’s measurements differed from the electrical ones by about 6.1 beats per minute. During the periods of intense exercise that number was off by an average of 12.2.
• The Surge was even more inaccurate. It was off by 11.6 beats per minute overall and 20.8 during the exercise.

In a statement to Patch, the company questioned the study’s reliability and the accuracy of Zephyr Technology’s BioHarness, the monitor that was being compared to Fitbit.

BioHarness was used in 2011 to track the health of 33 Chilean miners while they were trapped underground for 69 days.

“What the plaintiffs’ attorneys call a ‘study’ is biased, baseless, and nothing more than an attempt to extract a payout from Fitbit,” the statement said. “It lacks scientific rigor and is the product of flawed methodology. 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. Furthermore, there is no evidence the device used in the purported ‘study’ was tested for accuracy.”

After the lawsuit was filed, Consumer Reports did an independent test of its own using Polar H7, which the consumer magazine characterized as "a chest-strap monitor with proven accuracy."

"The new testing confirmed our earlier results: Both the Charge HR and Surge were very accurate when compared to the reference Polar H7 ECG monitor," Consumer Reports concluded. "During nearly every trial, the variance between the chest strap and the Fitbit devices amounted to no more than three heartbeats per minute."

The discrepancy did increase slightly as the exercise picked up its pace but only by as much as six beats per minute, not exactly the 10 or 20 recorded in the recent study.

The American Heart Association says that the resting heart rate for the average adult should be anywhere between 60 and 100 beats per minute. For a 30-year-old, the target heart rate during exercise should be between 95 and 162, depending on how vigorous the exercise is. For a 40-year-old, it’s between 90 and 153.

So, a monitor being off by 10 or 20 beats per minute is not insignificant.

But Fitbit shouldn't feel alone in any potential inaccuracies its devices may have. An American Council on Exercise study surveying five popular fitness-trackers, including Fitbit, showed that step-counters are mostly accurate at slow speeds and short distances but are unreliable during faster, agility exercises.

As Caitlin Stackpool, who led the study, said on ACE's website, “These activity trackers work best for lower-intensity activities such as walking. It gives them a way to assess where they are, set goals and see improvements.”

This isn’t the first time Fitbit’s accuracy has been called into question either.

An article in Berkley Science Review published in 2014 detailed various studies that showed how Fitbit’s step-counting accuracy gets worse as speeds of exercise increase, sometimes by more than 20 percent. In February, the TV station WTHR in Indiana partnered with Ball State University to do Fitbit research and found similar results.

In response to the WTHR story, Fitbit said that its products shouldn’t be used as a “scientific or medical device.”

“Overall, the success of Fitbit products comes from empowering people to see their overall health and fitness trends over time — it's these trends that matter most in achieving their goals,” the company said at the time.