UPDATE Scientists have tested a pair of wearable fitness gadgets from Fitbit and found they get heart rates wrong by as much as 25 beats per minute.

The study (PDF) was commissioned by law firm Lieff Cabraser, which is running a class action against Fitbit over inaccurate heart rate readings.

The study didn't use a colossal sample – just 43 people were tested – and only tested subjects once.

In those tests the subjects were hooked up to an electrocardiograph known to produce valid readings, then provided with a Fitbit Charge HR on one wrist and the Fitbit Surge on the other. Authors Edward Jo, PhD and Brett A. Dolezal, PhD did not place the same device on the dominant wrist, to avoid bias.

Participants were then asked to undertake 65 minutes of exercise and once the numbers had been crunched the authors concluded that “the PurePulse™ technology embedded in the Fitbit optical sensors does not accurately record heart rate, and is particularly unreliable during moderate to high intensity exercise.” The pair therefore conclude that FitBit kit has “significant limitations … for biometric monitoring during exercise … cannot be used to provide a meaningful estimate of a user’s heart rate.”

The authors don't say why, only offering speculation that there's a problem with FitBit's algorithms rather than the photoplethysmography technology used to do the measurements.

This study will scare the many athletes - serious competitors and weekend warriors alike – who aim to train at certain heart rates. If the devices over-report heart rates, users will have trained at lower heart rates than they wanted to achieve. If the devices under-report, users may be straining to reach heart rates beyond their optimal peak levels. Which can end badly.

Either way, it's not hard to see why there's a class action in the works. ®

UPDATE: Fitbit's been in touch, through its PT firm, with a statement that says, in part, that the study "lacks scientific rigor and is the product of flawed methodology ... and was conducted with a consumer-grade electrocardiogram – not a true clinical device, as implied by the plaintiffs’ lawyers." The statemetn also contend "there is no evidence the device used in the purported 'study' was tested for accuracy."

Fitbit "stand[s] behind our heart-rate monitoring technology and all our products, and continue to believe the plaintiffs’ allegations do not have any merit. We are vigorously defending against these claims, and will resist any attempts by the plaintiffs’ lawyers to leverage a settlement with misleading tactics and false claims of scientific evidence.”

Bootnote: No, we are not starting a photoplethysmography desk. Stories on the topic are like buses: none for ages then two at once.