Fitbit, the world’s biggest seller of wearable technology, is being sued in the US over claims that its heart rate monitors are inaccurate and place exercisers at risk.

The proposed nationwide class action lawsuit was filed this week by three plaintiffs who say their Fitbit wrist-worn devices – the ‘Surge’ and ‘Charge HR’ – do not accurately measure heart rate as advertised. The lawsuit claims the devices – which use Fitbit’s ‘PurePulse Tracker’ technology whereby LED lights reflect on the skin to detect changes in capillary blood volume – dangerously underestimate heart rates during workouts and place users at risk of overexerting themselves.

One of the plaintiffs in the lawsuit, Teresa Black, claims that in one instance, her personal trainer recorded her heart rate at 160 bpm, while her Fitbit read 82 bpm.

The lawsuit states: “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.”

Fitbit has indicated that it will “vigorously defend the lawsuit” and said that it’s devices are intended to provide data to help users reach their fitness goals, rather than serve as scientific or medical devices.

Accuracy of fitness trackers is a contentious issue and there have been numerous pieces of research into trackers since the technology first came to prominence. One recent study by researchers at the University of Iowa looked at devices from eight leading fitness tracker brands and found Fitbit to be among the most accurate, second only to the BodyMedia FIT device.

To read an interview with Fitbit’s Gareth Jones about motivating people through challenges and rewards – from the June 2015 edition of Health Club Management – click here.