Comprehensive Testing Confirms Fitbit’s PurePulse™ Heart Rate Monitors ‘Highly Inaccurate’


San Francisco, CA – A comprehensive new study conducted by researchers at California State Polytechnic University, Pomona (“Cal Poly Pomona”) reveals that the PurePulse™ heart rate monitors in the Fitbit Surge™ and Charge HR™ bear an “extremely weak correlation” with actual users’ heart rates as measured by a true echocardiogram (ECG) and are “highly inaccurate during elevated physical activity.”

With this study, Dr. Brett Dolezal and Dr. Edward Jo set out to test the accuracy of the Fitbit heart rate monitoring devices and to determine once and for all whether they were statistically valid heart rate monitors. It is the most thorough examination of the Fitbit heart rate monitors performed to date, and includes results from 43 separate subjects tested for 65 minutes each during a variety of activities depicted by Fitbit when marketing the devices such as jogging, stair climbing, jumping rope, and plyometrics. While performing these activities, each subject wore a Fitbit heart rate device on different wrists, which were measured against a time-synchronized ECG, the gold standard of heart rate monitoring.

Jonathan Selbin of Lieff Cabraser stated: “We commissioned this comprehensive, peer-review quality study by academics with expertise in this area to further test our allegations, and it proved them to be true. These devices simply are not accurate at measuring heart rates during moderate or high intensity exercise.” Co-counsel, and former Assistant to the United States Solicitor General, Robert Klonoff added: “We are unaware of any comprehensive testing performed by Fitbit prior to releasing the Fitbit devices with the PurePulse™ technology or since.”

After carefully analyzing the more than 46 hours’ worth of comparative data— including hundreds of thousands of individual data points—that resulted from this testing, Dr. Jo and Dr. Dolezal concluded that the Fitbit devices simply do not accurately track users’ actual heart rates, particularly during exercise. At moderate to high exercise intensities, the average difference between the Fitbit devices and the ECG was approximately 20 beats per minute, well beyond any reasonable or expected margin of error. The authors also noted that their analysis “disregards the thousands of discrete data points in which the devices recorded literally no heart beat at all.” Incorporating those null data points increased the average discrepancy at moderate to high intensities to approximately 25 beats per minute, an even worse result. In addition, the study also reported a “startling inconsistency” between Fitbit devices simultaneously recording the same user’s heart rate on different wrists.

The report concludes: “Overall, the results of this investigation fail to support Fitbit® Surge™ and Charge HR™ wearable fitness trackers and the integrated PurePulse™ technology as valid methods for heart rate measurement.” In other words, “[t]he PurePulse Trackers do not accurately measure a user’s heart rate, particularly during moderate to high intensity exercise, and cannot be used to provide a meaningful estimate of a user’s heart rate.”

Although this is the by far the most comprehensive study of the devices’ accuracy, it is not the first. The data from the Cal Poly Pomona study corresponds closely with results obtained in February 2016 by researchers at Ball State University in Indiana, in an unrelated and independent study. They too found that the Fitbit heart rate devices were off by “20 or 30 beats per minute,” a margin of error they concluded was unacceptably high and “dangerous – especially for people at high risk of heart disease.” As that report put it, “The box for the Fitbit Charge HR says ‘every beat counts,’ but despite what the package says, the tracking device inside missed lots of them.”

The new study was included in an amended complaint filed in the consumer fraud class action case filed earlier this year by several purchasers represented by attorneys at Lieff Cabraser and co-counsel against Fitbit, Inc., over complaints that various Fitbit heart rate monitors—now including three Fitbit models equipped with the PurePulse™ technology, the Fitbit Blaze™, Fitbit Charge HR™ and the Fitbit Surge™— fail to accurately measure user heart rates during exercise, the precise use for which Fitbit markets them and for which it charges a premium price.