With some Fitbit devices, every beat may not get counted, according to claims in a proposed nationwide class action lawsuit filed Tuesday.

Three plaintiffs claim that their Fitbit wrist-based heart monitors, “Charge HR” and “Surge,” do not and cannot accurately measure heart rate as advertised. Those sales pitches claim that both products, which are sold for around $150 and $250, respectively, can continuously and accurately monitor heart rate, even during exercise—under tag lines such as “every beat counts.” But the lawsuit claims that the heart rate monitors, which tout “PurePulse Tracker” technology, seem particularly incapable of accurately measuring elevated heart rates, often reading dangerously underestimated rates during workouts.

In the lawsuit, plaintiff Teresa Black, of Colorado, claimed that her Charge HR device was off by 78 beats per minute (bpm) during one workout. Her personal trainer recorded her heart rate at 160 bpm, while her Fitbit read 82 bpm. “Plaintiff Black was approaching the maximum recommended heart rate for her age, and if she had continued to rely on her inaccurate PurePulse Tracker, she may well have exceeded it, thereby jeopardizing her health and safety,” the lawsuit stated.

Another plaintiff, David Urban, of Wisconsin, reported similar problems with his Surge device. Compared to readings from a chest strap-based triathlon monitor, Urban claimed that the Surge consistently under-reported his heart rate by 15-25 bpm as he was exercising and never displayed a reading above 125 bpm. Due to a family history of heart disease, Urban reported buying the device to ensure his heart rate didn’t exceed 160 bpm, as recommended by his doctor.

The third plaintiff, Kate McLellan, a PhD rehabilitation scientist from California, made similar claims, comparing her heart rate readings from a Charge HR with the measurements from a stationary cardiovascular exercise machine.

The lawsuit also states that a board-certified cardiologist compared the Fitbits’ heart rate measurements with those from an electrocardiogram (ECG). The doctor reportedly found that for heart rates above 110 bpm, the Fitbits were off by an average of 25 bpms, with some readings wrong by as much as 75 bpm.

The lawsuit concludes that the devices, which are advertised to use LED lights to detect changes in capillary blood volume, are “effectively worthless as heart rate monitoring devices.”

In a statement to Ars, a Fitbit spokesperson wrote, "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."

In another lawsuit, still pending, customers claimed that their Fitbit devices underestimated sleep.