Some fitness trackers are so unreliable they can add up to 11 miles onto the distance of a marathon, a new study by Which? has found.
The consumer watchdog said fitness trackers, including Garmin, Fitbit, Apple and Huawei were unable to accurately track someone running a marathon distance.
The research has been published ahead of the London Marathon, which is due to take place on Sunday.
How much distance do the devices add to a marathon?
Garmin's Vivosmart 4, which costs around £119.99, was the least reliable, according to Which?.
They found the device clocked the marathon distance of 26.2 miles, when runners in fact ran 37 miles.
Samsung’s Gear S2 was also found to miscalculate the distance according to the research, saying the wearer had reached marathon distance only after they had run 36.2 miles.
The Misfit Ray, Xiaomi Amazfit Bip, Fitbit Zip and Polar A370 were all also named as devices which carried runners past the 30-mile mark in testing.
Huawei’s Watch 2 Sport left runners short of a full marathon, the research claimed, telling the wearer they had reached the milestone after only 18.9 miles.
The Apple Watch Series 3 said the target had been hit at 22.8 miles.
How did Which? carry out their research and what do they say?
Which? said it carried out its analysis using a calibrated treadmill to compare the ability of different trackers to log steps taken and distance travelled.
It urged people to research any device before purchasing and warned that buying a well-known brand did not guarantee accuracy.
Natalie Hitchins, head of home products and services at the consumer watchdog, said: “Running a marathon is no mean feat, so runners who are putting in the months of training beforehand will want to know their fitness tracker is trustworthy, and not jeopardising their finish times.
“Our tests have found a number of models from big-name brands that can’t be trusted when it comes to measuring distance, so before you buy, make sure you do your research to find a model that you can rely on.”
How have the brands responded?
In response to the research, Garmin highlighted that the Vivosmart 4 does not use GPS and was therefore not their recommended device for marathon runners.
“The Vivosmart 4 is an all-round smart fitness activity tracker used to monitor wellness, health and fitness for a range of activities,” a company spokesman said.
“As the Vivosmart 4 does not incorporate GPS, our recommendation for someone who is running long distances such as a marathon, would be to choose a tracking device such as our Forerunner range which is dedicated to running and incorporates GPS.”
In a statement, a Huawei spokeswoman said: “The test results may vary depending on testing conditions such as indoor and outdoor environments and individual runner variances.
“With regards to running indoors, as this particular test was carried out on a treadmill, the algorithm of Huawei Watch 2 Sport calculates the user’s stride length from the acceleration sensor data while running at different speeds. Watch 2 Sport tracks the distance by evaluating the data of user’s stride length and steps.
“The results may deviate owing to individual runner variances. Huawei is fully committed to providing accurate and better running experience for users, and we will continuously work to optimise our existing and new generation products.”