(CNN) Your favorite fitness tracker may not be totally accurate, according to a study used in an amended complaint filed Thursday against Fitbit. 
 
 The class-action lawsuit, filed earlier this year, argues that the PurePulse technology used in the Fitbit trackers that measure your heart rate doesn't do it as well as the company's marketing material promises, a claim Fitbit denies. The technology is used in the more expensive models of the device, the Surge, Blaze and Charge HR. 
 
 The lawsuit was filed on behalf of people who bought these Fitbits specially to help them track their heart rate, whether for health reasons or to make sure they are getting the most out of their workouts. 
 
 "We are not arguing that it is a medical device. I think that is irrelevant," said Jonathan Selbin , one of the attorneys who filed the lawsuit. "This is about the way they market it and that they charge a premium for the heart rate monitor, but it's not giving a meaningful measurement." 
 
 The suit alleges that a defect in the trackers is a safety hazard because people who rely on inaccurate heart rate readings for medical purposes could be at risk. The class-action members say there is no reasonable way for them to know that the devices can be inaccurate and that they might not have even bought them if they knew. 
 
 The amended complaint, filed Thursday, added a new study that looked at how the trackers worked for 43 healthy adults, compared with an ECG and against each other. The participants wore two Fitbits during the test. 
 
 The study -- which was funded by the legal team that brought the lawsuit -- found that the devices were off by, on average, 20 beats per minute during moderate- to high-intensity exercise. The lawsuit concludes that the "devices could not provide meaningful heart rate data" when someone is exercising. 
 
 "It works pretty well when you are resting," study co-author Edward Jo said. But Fitbit is "advertised to people who exercise," and ads suggest that the trackers are "counting every beat." 
 
 Researchers tested the device on 22 men and 21 women, who were observed in a single visit. They wore the devices, a Zephyr Technology BioHarness and a single-channel ECG sensor. Monitored for 65 minutes, the subjects were asked to perform physical activities that were similar to ones represented in the Fitbit ads, including a standing rest, a self-paced jog, jumping rope, climbing stairs, fast movement exercise, treadmill running and a seated rest. 
 
 The research was done by Jo and Brett Dolezal, scientists at California State Polytechnic University, Pomona. 
 
 Fitbit objects to the study, suggesting that it "lacks scientific rigor and is the product of flawed methodology." 
 
 According to a statement from a Fitbit representative, "It was paid for by plaintiffs' lawyers who are suing Fitbit, and was conducted with a consumer-grade electrocardiogram -- not a true clinical device, as implied by the plaintiffs' lawyers. Furthermore, there is no evidence the device used in the purported 'study' was tested for accuracy." 
 
 The statement goes on to say, "Fitbit's research team rigorously researched and developed PurePulse technology for three years prior to introducing it to market and continues to conduct extensive internal studies to test the features of our products. Fitbit Charge HR is the #1 selling fitness tracker on the market, and is embraced by millions of consumers around the globe." 
 
 Fitbit also cites an independent study by Consumer Reports in which it did its own tests and found the heart rate measurements to be accurate. That study, however, measured the heart rates of a very small sampling: only two subjects. 
 
 The Consumer Reports test also used a consumer-grade heart rate monitor. 
 
 The Fitbit representative added, "We stand behind our heart-rate monitoring technology and all our products, and continue to believe the plaintiffs' allegations do not have any merit." 
 
 Alex Montoye , an‎ assistant professor of clinical exercise physiology at Ball State University, has tested several wearables including Fitbit and says he is not surprised by the study results. He is not involved in the class-action suit. 
 
 "The (Fitbits) give a pretty decent ballpark estimate of activity levels, but these are difficult elements to measure perfectly," Montoye said. 
 
 To the company's credit, Montoye said, Fitbit technical specifications don't claim 100% accuracy. He noted that the company uses words like "track" rather than "measure" your steps. But the website does promise that you can "accurately track calorie burn all day," "check real-time heart rate" and "use simplified heart rate zones to tailor your workouts on the spot" or "see when your health is improving by analyzing your all-day and resting heart rate trends." 
 
 "You shouldn't think you are buying a medical-grade device, and the company never says that," Montoye said. 
 
 People who buy them, though, may have higher expectations. 
 
 Montoye says some people in his campus exercise program have complained that they went to the grocery store but didn't get credit for their steps because they were pushing a cart. The device relies in part on a change in motion, and for the motion pattern to recognize a step, it must be large enoug h. Pushing a cart or even walking on a soft surface like a plush carpet may undercount your movement. It can also overcount your steps if you are riding on a bumpy road, according to Fitbit's website. 
 
 Montoye said for participants who need to track their heart rate he advises using a different device that is more specific. 
 
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 "If they had just been honest," Selbin said, "and said it can give you a ballpark figure most of the time, or if the marketing emphasized that you can use these when you are aspiring to be healthier, that would have been OK, but that's not how they market it, and they charge a premium for it." ||||| SAN FRANCISCO--(BUSINESS WIRE)--The following statement is being issued by Lieff Cabraser Heimann & Bernstein, LLP: 
 
 A comprehensive new study conducted by researchers at California State Polytechnic University, Pomona (“Cal Poly Pomona”) reveals that the PurePulse™ heart rate monitors in the Fitbit Surge™ and Charge HR™ bear an “extremely weak correlation” with actual users’ heart rates as measured by a true echocardiogram (ECG) and are “highly inaccurate during elevated physical activity.” 
 
 With this study, Dr. Brett Dolezal and Dr. Edward Jo set out to test the accuracy of the Fitbit heart rate monitoring devices and to determine once and for all whether they were statistically valid heart rate monitors. It is the most thorough examination of the Fitbit heart rate monitors performed to date, and includes results from 43 separate subjects tested for 65 minutes each during a variety of activities depicted by Fitbit when marketing the devices such as jogging, stair climbing, jumping rope, and plyometrics. While performing these activities, each subject wore a Fitbit heart rate device on different wrists, which were measured against a time-synchronized ECG, the gold standard of heart rate monitoring. 
 
 Jonathan Selbin of Lieff Cabraser stated: “We commissioned this comprehensive, peer-review quality study by academics with expertise in this area to further test our allegations, and it proved them to be true. These devices simply are not accurate at measuring heart rates during moderate or high intensity exercise.” Co-counsel, and former Assistant to the United States Solicitor General, Robert Klonoff added: “We are unaware of any comprehensive testing performed by Fitbit prior to releasing the Fitbit devices with the PurePulse™ technology or since.” 
 
 After carefully analyzing the more than 46 hours’ worth of comparative data—including hundreds of thousands of individual data points—that resulted from this testing, Dr. Jo and Dr. Dolezal concluded that the Fitbit devices simply do not accurately track users’ actual heart rates, particularly during exercise. At moderate to high exercise intensities, the average difference between the Fitbit devices and the ECG was approximately 20 beats per minute, well beyond any reasonable or expected margin of error. Moreover, this analysis disregards the thousands of discrete data points in which the Fitbit devices recorded literally no heart beat at all. As the authors note, incorporating those null data points increased the average discrepancy at moderate to high intensities to approximately 25 beats per minute, an even worse result. In addition, the study also reported a "startling inconsistency" between Fitbit devices simultaneously recording the same user’s heart rate on different wrists. 
 
 The report concludes: “Overall, the results of this investigation fail to support Fitbit® Surge™ and Charge HR™ wearable fitness trackers and the integrated PurePulse™ technology as valid methods for heart rate measurement.” In other words, “[t]he PurePulse Trackers do not accurately measure a user’s heart rate, particularly during moderate to high intensity exercise, and cannot be used to provide a meaningful estimate of a user’s heart rate.” 
 
 Although this is the by far the most comprehensive study of the devices’ accuracy, it is not the first. The data from the Cal Poly Pomona study corresponds closely with results obtained in February 2016 by researchers at Ball State University in Indiana (available at http://www.wthr.com/story/31285468/sometimes-your-fitness-tracker-lies-a-lot-fitbit-jawbone-garmin-ifit-misfit-accuracy), in an unrelated and independent study. They too found that the Fitbit heart rate devices were off by “20 or 30 beats per minute,” a margin of error they concluded was unacceptably high and “dangerous – especially for people at high risk of heart disease.” As that report put it, “The box for the Fitbit Charge HR says ‘every beat counts,’ but despite what the package says, the tracking device inside missed lots of them.” 
 
 The new study was included in an amended complaint filed in the consumer fraud class action case filed earlier this year by several purchasers represented by attorneys at Lieff Cabraser and co-counsel against Fitbit, Inc., over complaints that various Fitbit heart rate monitors—now including three Fitbit models equipped with the PurePulse™ technology, the Fitbit Blaze™, Fitbit Charge HR™ and the Fitbit Surge™—fail to accurately measure user heart rates during exercise, the precise use for which Fitbit markets them and for which it charges a premium price. 
 
 Consumer Protection Attorneys at Lieff Cabraser 
 
 If you purchased a Fitbit Heart Rate Monitor (Fitbit Charge HR™, Blaze™, or Surge™), we invite you to visit our Fitbit Heart Rate Monitor lawsuit page to contact a consumer attorney at Lieff Cabraser. We welcome the opportunity to learn of your experiences with your Fitbit Heart Rate Monitor and to answer any questions you may have about your legal rights. 
 
 About Lieff Cabraser and Counsel 
 
 Recognized as “one of the nation’s premier plaintiffs’ firms” by The American Lawyer, Lieff Cabraser Heimann & Bernstein, LLP, has successfully litigated and settled hundreds of class action lawsuits in federal and state courts, including dozens of cases requiring manufacturers to remedy a defect, extend warranties, and refund to purchasers the cost of repairing the defective product. It has recovered billions of dollars for consumers in such cases. With sixty-plus attorneys in offices in San Francisco, New York, Nashville, and Seattle, we are among the largest law firms in the United States that represent only plaintiffs. 
 
 Robert Klonoff is the Jordan D. Schnitzer Professor of Law at Lewis & Clark Law School and served as Dean of the Law School from 2007-2014. He has vast experience as a class action attorney and expert, and has written leading textbooks in the field. He is a former Assistant to the Solicitor General and has argued numerous cases in the U.S. Supreme Court. 
 
 The consumer plaintiffs are also represented by Lori Feldman of Levi & Korsinsky LLP.
– The heart rate tracker technology in fancier models of Fitbit starts to become inaccurate when people start exercising, according to a study included in an amended class-action lawsuit against the company. In a press release, plaintiffs' lawyers say California State Polytechnic University put both Fitbits and ECG sensors on 43 separate subjects tested for 65 minutes of moderate to high-intensity exercise, and found that the PurePulse technology in the Fitbits had an "extremely weak correlation" with the heart rate recorded by the ECGs. The researchers say the Fitbit measurement was up to 20 beats per minute off during exercise. In some cases, Fitbits on both wrists recorded different rates—and heart rates of zero were recorded on subjects who were definitely not zombies. Fitbit argues that the study, which was funded by the lawyers, "lacks scientific rigor and is the product of flawed methodology." The company notes that the ECGs were consumer-grade ones, not clinical devices. Alex Montoye, an assistant professor of clinical exercise physiology at Ball State University tells CNN that his own research has found that Fitbits are pretty good at giving a "ballpark estimate of activity levels, but these are difficult elements to measure perfectly." He says the Fitbit is not a medical device and doesn't claim to be one, which is why the company tends to use words like "track" instead of "measure" in marketing materials and never promises 100% accuracy. (Doctors say heart rate data from this man's Fitbit helped save his life.)