U.S. (KXAN) — The fitness tracking device Fitbit has been hit with a lawsuit over two of its wristbands.

Several customers claim the heart-rate monitor on the Fitbit “Charge HR” and “surge” do not and can not accurately measure heart-rates as advertised. The group notes that the monitor doesn’t work, especially during times of intense exercise.

One of the plaintiffs told Mashable, her health was threatened by following the Fitbit results, which was way off from her personal trainer’s heart rate reading. According to the woman the Fitbit measured her heart rate at 82 beats per minutes (bpm), while her trainer recorded it at 160 bpm.

Fitbit says they stand by their technology. However, they do warn that the wristbands are not intended to be medical devices.

Fitbit released a statement following the lawsuit, “PurePulse provides better overall heart rate tracking than cardio machines at the gym, as it tracks your heart rate continuously — even while you’re not at the gym or working out.”