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.

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.