Fitbit has been hit with another consumer class action asserting false advertising claims, this time alleging that the fitness tracker fails to accurately monitor the users' heart rates.

"The heart rate monitoring function of the PurePulse Trackers is a material—indeed, in some cases, vital—feature of the product," the plaintiffs told the court. "Not only are accurate heart readings important for all of those engaging in fitness, they are critical to the health and well-being of those Class members whose medical conditions require them to maintain (or not to exceed) a certain heart rate."

According to the federal court action filed by three Fitbit users, the "PurePulse" technology in Fitbit's Charge HR and Surge models misreports the number of heartbeats, particularly during exercise. In a test performed by a cardiologist comparing the Fitbit technology to an electrocardiogram, the Fitbit was off by an average of about 25 beats per minute, the complaint alleged.

One plaintiff claimed the error was potentially dangerous. Colorado resident Teresa Black's personal trainer manually recorded her heart rate during a training session at 160 beats per minute. At the same time, her Charge HR indicated her heart rate was only 82 beats per minute, she claimed. "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 suit alleged.

Despite these inaccuracies, Fitbit engaged in a widespread national advertising campaign touting its activity trackers with the heart rate monitoring feature with slogans such as "Every Beat Counts" and "Know Your Heart," the plaintiffs said. To reinforce the claims, commercials depicted the Fitbits being used for activity and fitness, including high-intensity workouts.

In additional counts, the plaintiffs challenged Fitbit's use of an arbitration agreement featuring a class action ban as a separate and actionable unfair and deceptive trade practice. Consumers are not informed of the agreement prior to purchase, the suit said, and are instead forced to agree to the Terms of Service pursuant to an online registration required for the PurePulse products to work.

"Fitbit's attempt to bind customers who bought PurePulse Trackers through third party online and brick and mortar stores to an arbitration clause and class action ban post-purchase when they register the product—which is required to make the product function as intended—is unconscionable, invalid, and unenforceable," the plaintiffs wrote.

Seeking to certify a nationwide class of purchasers, as well as subclasses in California, Colorado, and Wisconsin, the complaint requested injunctive relief and restitution, as well as compensatory, exemplary, punitive, and statutory penalties and damages.

To read the complaint in McClellan v. Fitbit, Inc., click here.

Why it matters: The company pledged to fight the case, noting in a statement 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." Last summer, Fitbit faced similar false advertising allegations about sleep measurement claims for its products.