A group of three Fitbit users have filed a class-action lawsuit against the fitness tracker giant on the grounds that the heart rate monitors in three of its models (the Surge Charge HR , and Blaze ) are inaccurate. They state that the monitors' readings are off by a "significant margin" and that they don't "count every beat." These issues are more than just an annoying feature of Fitbit's PurePulse technology — the inaccuracy could ultimately pose major risks to certain users.The users leading the lawsuit claim that the monitors are particularly inaccurate during periods of intense exercise, which, of course, is generally when one would demand the most accuracy from a heart monitor. While doctors don't deem it necessary for all people to watch their heart rate while exercising, they certainly recommend that specific groups do. For example, people who have or are prone to heart disease should pay closer attention to their heart rates when they're active. It's this group that would be put at risk with a faulty monitor.Meanwhile, Fitbit announced that it "strongly disagrees with the statements made in the complaint and plans to vigorously defend the lawsuit," adding that PurePulse is more accurate than gym equipment that measures heart rates. Fitbit's statement ends with a vital addendum: Its trackers are intended for fitness and shouldn't be expected to the take the place of a medical device.Beyond using any kind of tracker to gauge the intensity of your workout, be sure to know your personal limits (and anything a doctor has told you to watch out for) before you begin — if you find that you're completely out of breath only a few minutes in or you need to stop exercising well before you planned to stop, rethink your regimen and consider how to adjust it for your abilities. And know that you don't have to push yourself past comfortable limits to get a great workout.