Fitbit may have been one of the most popular gifts this Christmas, but not everyone is happy with the fitness monitor.

A national class action lawsuit filed this week claims the Fitbit monitors "Charge HR" and "Surge" do not and cannot accurately measure heart rate. Both of the bands are equipped with what Fitbit called "PurePulse" LED-based technology that's supposed to track heart rate as people exercise.

"Fitbit's representation is repeated and echoed throughout its advertising of the PurePulse Trackers, which employs such deceptive slogans as "Every Beat Counts" and "Know Your Heart." But the representation is false," the lawsuit claims.

Plaintiffs said the bands mis-record heart rates by a "very significant" margin, something they claim was backed up by testing. The lawsuit cites examples by different people whose Fitbit bands underestimated heart rates, in some cases causing users to over exert themselves and push themselves towards dangerous rates.

"Unfortunately, the PurePulse Trackers do not work, and their heart rate readings are wildly inaccurate," the lawsuit claims.

The bands are sold nationwide for around $150 - $250.

In a statement provided to Ars, Fitbit denied the claims:

"We do not believe this case has merit. Fitbit stands behind our heart rate technology and strongly disagrees with the statements made in the complaint and plans to vigorously defend the lawsuit. Fitbit is committed to making the best clip and wrist-based activity trackers on the market. Our team has performed and continues to perform internal studies to validate our products' performance... But it's also important to note that Fitbit trackers 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."