A group of U.S. customers has launched a class action suit against Fitbit, alleging that its wrist-based Charge HR and Surge heart monitors do not and cannot measure heart rates accurately, especially during intense physical activity, contrary to claims made by the company in national advertisements. One of the lawyers in the case says that Fitbit has tried to protect itself from being held accountable through a hidden arbitration clause and a class action ban on its website. The lawsuit requests injunctive relief. Fitbit points out that it never claimed that Surge would be a medical monitoring device, but its stock has taken a hit. It has declined by 37 percent since the start of the year to less than $19, below the $20 price of its IPO.