SAN FRANCISCO (CBS) — The makers of Fitbit are being sued by people who allege that the popular fitness device isn’t accurately keeping track of their heart rate while exercising.

The lawsuit filed in federal court this week claims that the wrist-based activity tracker is consistently misrecording users’ heart rates by a “very significant margin.” It also takes aim at Fitbit commercials with slogans like “Every Beat Counts” and “Know Your Heart.”

Read The Complaint (PDF)

“Far from ‘counting every beat,’ the PurePulse Trackers do not and cannot consistently and accurately record wearers’ heart rates during the intense physical activity for which Fitbit expressly markets them,” the lawsuit states.

The plaintiffs say accurate heart readings are critical during exercise, and are seeking punitive damages.

In a statement shared with CBS News, Fitbit says the case does not have 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,” the company said.

Fitbit products sell for about $150 to $200.

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