FitBit is being sued under the accusation of false advertising, after their monitoring devices have proved to be inaccurate.

The complaint filed in San Francisco’s federal court on Tuesday claims that the fitness trackers are significantly misreading heart rates.

A Colorado woman said that her device reported her heart rates at a speed up to 50 percent lower than it actually was after a period of intense physical exercise. Her actual heart rate has been manually measured by her personal trainer.

The law suit has been brought as a class action after more customers complained that their devices are not measuring accurate. All three plaintiffs claim that the wrist-based heart monitors developed by Fitbit do not work accurately as they are being advertised. The three customers are referring to “Surge” and “Charge HR” two devices that cost between $150 and $250.

FitBit’s ads claim that their heart measuring monitors can accurately measure heart rate even at elevated levels such as during exercise. The ad says that “every breath counts”.

The three customers say that they have been deceived by the ad’s claims as their heart monitors are incapable to measure elevated heart rates, underestimating their levels. Underestimated heart rates during exercise can be very harmful and even deadly.

Teresa Black, one of the plaintiffs, claims that her Charge HR underestimated 78 beats per minute one time when she was exercising. The Fitbit device was reading 82 bpm while her personal trainer recorded 160 bpm. If the trainer’s measurement was accurate Black was reaching the maximum heart rate recommended for her age which could have posed a serious danger on her safety and health if she continued exercising, relying on the inaccurate measurement of Fitbit’s Charge HR.

A similar problem has been reported with the Surge device by plaintiff David Urban from Wisconsin. The man says that while exercising his Surge device has never shown more than 125bpm, constantly underestimating his heart rate by 15-25 bpm. Urban compared Surge’s readings with those of a chest strapped triathlon monitor. The man says he bought the device to make sure that he’s heart won’t exceed 160 bpm giving his family history of heart conditions.

According to the lawsuit a cardiologist compared the heart rate measurements of Fitbit’s devices with those from an electrocardiogram to find that at elevated heart beats the devices were off by 25 bpm on average while the maximum was a misreading of 75 bpm.

On the other side, a spokeswomen of the company said that FitBit intends to vigorously defend their products as internal studies have shown that their technology performs at its best.

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