Fitbit’s health trackers are among the most popular wearable gadgets, with everyone from President Obama to celebrities like Ryan Reynolds donning them. The devices measure wearers’ activity metrics like heart rate, steps taken and more.

But some customers are alleging the devices aren’t as accurate as Fitbit claims. A class-action lawsuit to that effect was filed earlier this year. The parties bringing that lawsuit recently pointed to the results of a study that calls the Fitbit Surge and Charge HR’s heart rate monitors “highly inaccurate” during elevated physical activity.

Here are four things Fitbit owners should know about the lawsuit:

1. The complaints in the lawsuit apply specifically to Fitbit’s constant heart rate monitoring technology, which the company calls “PurePulse.” It does not make any claims regarding how Fitbit’s products perform in other areas, such as step counting, distance tracking, and sleep monitoring. The Surge, Charge HR, and Blaze are the only Fitbit devices that include the technology in question.
2. Fitbit is being accused of misrepresenting how accurately its devices monitor users’ heart rate during physical activity. The newly released study, funded by the plaintiffs’ legal team, was conducted by researchers at the California State Polytechnic University, Pamona. The researchers had 43 test subjects wear a Fitbit device on either wrist while exercising for 65 minutes. The results from the Fitbit gadgets were then tested against those from an electrocardiogram (ECG). The average difference between the Fitbit numbers and the ECG results was approximately 20 beats per minute. In some instances, Fitbit devices recorded no heartbeat at all. “Overall, the results of this investigation demonstrate that the PurePulse technology integrated in Fitbit’s heart rate monitoring devices is not a valid method for heart rate measurement, and cannot be used to provide a meaningful estimate of a user’s heart rate,” the study reads.
3. Fitbit rejects the study’s results. In a statement given to TIME, the company calls the study “biased” and “baseless,” adding that it “lacks scientific rigor and is the product of flawed methodology.” The company says it researched and developed PurePulse for three years before introducing it in its products, and that it “continues to conduct extensive internal studies.” A January test from Consumer Reports concluded that the Charge HR and Surge were “very accurate” when compared to a chest strap heart rate monitor, but that study included a small sample size of just two participants.
4. Other health wearable firms have faced similar accusations of inaccuracy. While the class action lawsuit against Fitbit may be attracting attention, experts have been skeptical about the accuracy of wrist-worn devices for years. In 2014, CNET tested five fitness trackers made by Samsung, Garmin, Basis, and Withings, finding that they were sometimes inaccurate when measuring heart activity. This is largely because it’s difficult to capture precise heart rate metrics from the wrist. Medical-grade devices, typically worn around the chest, measure electrical impulses that travel through the heart, while gadgets like the Fitbit and Apple Watch shine a light through the wearer’s skin to detect blood activity in the wrist to measure one’s heart rate.