A class action lawsuit against Fitbit may have grown teeth following the release of a new study that claims the company's popular heart rate trackers are "highly inaccurate."
Researchers at the California State Polytechnic University, Pomona tested the heart rates of 43 healthy adults with Fitbit's PurePulse heart rate monitors, using the company's Surge watches and Charge HR bands on each wrist.
Subjects were then hooked up to a BioHarness device that produced an electrocardiogram (ECG), to record the heart's rhythm against the data being produced by Fitbit's devices.
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Comparative results from rest and exercise — including jump rope, treadmills, outdoor jogging and stair climbing — showed that the Fitbit devices miscalculated heart rates by up to 20 beats per minute on average during more intensive workouts.
"The 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," the study stated.
The study was commissioned by the Lieff Cabraser, the law firm behind the class action suit that is taking aim at three Fitbit models that use the PurePulse heart monitor, including the Fitbit Blaze, Fitbit Charge HR and Fitbit Surge.
Fitbit did not immediately respond to CNBC's request for comment, but a Fitbit statement posted by Gizmodo took aim at the study's results.
"What the plaintiffs' attorneys call a 'study' is biased, baseless, and nothing more than an attempt to extract a payout from Fitbit. It lacks scientific rigor and is the product of flawed methodology," the statement read.
"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."
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"Fitbit's research team rigorously researched and developed PurePulse technology for three years prior to introducing it to market and continues to conduct extensive internal studies to test the features of our products," the statement concluded.
However, a separate study by Ball State University in Indiana and journalists at NBC-affiliated TV station WTHR released in February also showed that the Fitbit Charge HR missed heartbeats, marking an average heart rate error of 14 percent.
"Calculating a heart rate that's off by 20 or 30 beats per minute can be dangerous — especially for people at high risk of heart disease," the report said.
Fitbit did send a written reply to WTHR, saying that its devices "are designed to provide meaningful data to our users to help them reach their health and fitness goals, and are not intended to be scientific or medical devices."