Some fitness trackers are inaccurately measuring running distance, according to research from the consumer watchdog Which?
It tested 118 trackers using a treadmill to complete the distance of a marathon - 26.2 miles (42km).
It found that the least reliable was the Garmin Vivosmart 4, which underestimated the distance by 10.8 miles – meaning the researcher actually ran 37 miles.
Garmin said it was because that particular tracker did not contain GPS.
It described the Vivosmart 4 as an “all-round smart fitness tracker” and suggested that marathon runners use its Forerunner range which is GPS-enabled.
Of the eight Apple models involved in the test, the Apple Watch series 1 was the most accurate, over-estimating the distance by 1%, while the series 3 overestimated by 13% - stating that the runner had completed the marathon distance after 22.8 miles.
“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,” said Natalie Hitchins, head of home product and services at Which?
Other results for the number of miles reached before the tracker recorded the official marathon distance included:
- Samsung Gear S2 – 36.2 miles
- Xiaomi Amazfit Bip – 34 miles
- Huawei Watch 2 Sport – 18.9 miles
A Huawei spokesman told the BBC “individual runner variances” could have affected the test results.
“With regards to running indoors, as this particular test was carried out on a treadmill," he said. "The algorithm of Huawei Watch 2 Sport calculates the user’s stride length from the acceleration sensor data while running at different speeds."
In January 2019 researchers at Aberystwyth University found that all the trackers they tested overestimated the number of calories burned during activity.
Gavin Taitt is a regular middle-distance runner from Earlston, in the Scottish Borders, who also coaches others. He said he and his group use a combination of Garmin Forerunner watches and the social fitness network Strava to measure and share results.
“The watches are quite expensive but have good feedback,” he said.
“Accuracy is very important.”
Another expert agreed that the calibrated treadmill test was not the best method because all the devices would have had to rely on step-counting algorithms rather than GPS (for those which had it) to calculate distance.
"This is a real shame as a real world (on-road) test would have been more useful for consumers," said Dr Dale Esliger, senior lecturer in physical activity and health at Loughborough University.
He added that when investing in a tracker, people should think about which metric is going to be most useful to them in terms of measuring their progress.
"Step-counting has become a key metric for many; however, devices are now coming with heart-rate monitoring capability which relates to activity intensity and provides insight into cardiovascular health," he explained.
"In our research, this [heart rate] is the metric that seems to be the potent driver for behaviour change."