One plaintiff claims that her Fitbit Charge HR underestimated her heart rate by more than 70 beats per minute. She claims that she discovered the discrepancy after her personal trainer manually measured her heart rate. (Video via Fitbit)

She says she was reaching the maximum recommended heart rate for her age, so relying on the Fitbit's data could have put her in an unsafe position. (Video via Fitbit)

The lawsuit says Fitbit's PurePulse technology is to blame. The same technology is in the new Fitbit Blaze, which was released for presale on the same day the suit was filed.

But Fitbit told Ars Technica that it doesn't think the case has merit.

company spokesperson told the outlet: "It's ... 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."

This video includes images from Getty Images.