A class action lawsuit against Fitbit may have grown teeth following the release of a new study which 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. 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.

Lisa Werner | Getty Images

"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 document stated.

The study was commissioned by 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. The suit claims the devices do not accurately measure heart rate, and the study was included as part of an amended complaint filed last week. In a statement, Fitbit called the study "biased" and "baseless," saying it lacked "scientific rigor." "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."