Fitbit hit back at a US study which said that its pulse tracking technology “cannot be used to provide a meaningful estimate of a user’s heart rate”.

While the test was hardy exhaustive – it “includes results from 43 separate subjects tested for 65 minutes each” – it was found that at moderate to high exercise intensities, the average discrepancy between Fitbit device and an electrocardiogram was “approximately 20 beats per minute, well beyond any reasonable or expected margin of error”.

Also noted was a “startling inconsistency” between Fitbit devices simultaneously recording the same user’s heart rate on different wrists.

“Overall, the results of this investigation demonstrate that the PurePulse technology integrated in Fitbit’s heart rate monitoring devices is not a valid method for heart rate measurement, and cannot be used to provide a meaningful estimate of a user’s heart rate,” the report said.

But Fitbit hit back, stating that the study was “biased, baseless, and nothing more than an attempt to extract a payout from Fitbit” – it was commissioned by a law firm involved in a class action suit against the wearables company centering on the accuracy of its trackers.

Other objections were that a “consumer-grade electrocardiogram” had been used rather than a clinical device, and that there was no evidence that it had been tested for accuracy.

Fitbit says devices such as Charge HR offer “continuous, strap-free heart rate monitoring”, including the ability to “check real-time heart rate” to monitor workout progress.

The class action suit was filed early this year.