Fitness technology company Fitbit has been accused of putting customers in danger with "wildly inaccurate" readings from their heart rate trackers.
Fitbit sells products which claim to monitor users' heart rates, the number of calories they burn, steps they take and level of sleep quality.
But some users in the US are trying to sue the firm, saying the devices do not "do not and cannot consistently and accurately record wearers’ heart rates during the intense physical activity for which Fitbit expressly markets them".
They are now trying to sue the firm, which has strongly denied the allegations.
Here's everything we know.
- What are the allegations?
The claimants say the devices pulse trackers "do not work, and their heart rate readings are wildly inaccurate".
The "significant" margins of error make the trackers "effectively worthless" and could lead to "dangerous heart rates", the lawsuit claims.
Fitbit - which has promoted the products using slogans such as "Every Beat Counts" - is also accused of "defrauding the public and cheated its customers".
One claimant alleges that her personal trainer recorded her heart rate at 160 beats per minute but her Fitbit product only gave a reading of 82 bpm.
Another claimant - whose doctor had advised him not to exceed 160 bpm because of health problems - claims his device underestimated his heartrate by as much as 25 bpm.
One Twitter user, Nick Bailey, took to social media to question the readings from the Fitbit he purchased for his wife.
- How are the trackers meant to work?
Fitbit products are said to use LEDs that reflect on to the skin to monitor changes in blood flow through a user's wrist.
Algorithms then turn this data into a heart rate measurement.
- How have Fitbit responded?
The firm has vehemently denied the claims.
In a statement, it said: