Why your fitness tracker may not be as accurate as you think

Researchers say manufacturers’ claims should be taken with ‘healthy dose of scepticism’

Wearable fitness trackers undercount how many steps a person takes each day, a major analysis of the technology’s accuracy has found.
Researchers from University College Dublin said that the devices generally underestimated step counts by about 9 per cent.
It suggests that if an individual walked or ran 6,000 steps in a day, their smartwatch or band would register 5,500 steps.
The findings also suggest that the four million Britons who use the devices can now add 182,500 steps to their annual average count – equivalent to walking or running an extra 130 km.
The research – based on an “umbrella” analysis of data from 249 studies covering 430,500 people – also assessed how well the devices measured heart rates, the number of calories burned and sleep quality.
The academics said that the accuracy could be affected by multiple factors such as the algorithms used in the devices, where they were worn, the temperature and humidity of the surroundings, and the body age, shape and skin tone of the user.
Researchers found wearable devices measured heart rate with an error rate of plus or minus 3 per cent. They also accurately recorded heart rate variability and could sensitively detect arrhythmia, a problem with the rate of a person’s heartbeat.
However, the technology was found to be less effective at recording a person’s energy expenditure. The devices tested underestimated the number of calories burned by as much as 21.27 per cent and overestimated by as much as 14.76 per cent, amounting to a 30 per cent error range.
The wearables which were examined tended to overestimate total sleep time and sleep efficiency, typically by more than 10 per cent.
They also tended to underestimate sleep onset latency – the time it takes to fall asleep – and wakefulness after sleep onset.
Errors ranged from 12 per cent to 180 per cent, compared with the gold standard measurements used in the scientific method of sleep study known as polysomnography.
The researchers noted that fewer than one in 20 devices had been validated for the range of physiological factors they claimed to measure. They suggested there should be standardised tests to give consumers a better idea of the accuracy of their devices.
“As wearable technologies continue to permeate various facets of health and lifestyle, it is important to approach manufacturers’ claims with a healthy dose of scepticism,” wrote Cailbhe Doherty, an assistant professor and lead researcher, in an article for The Conversation.
He added: “Gaps in research, inconsistent methodologies and the rapid pace of new device releases underscore the need for a more formalised and standardised approach to [the] validation of devices.”
The American College of Sports Medicine previously identified wearable technologies as the “number one fitness trend” for 2024, based on a survey of 4,500 health and fitness professionals.
The global market for wearable health technology is expected to reach £140 billion by 2030, expanding at an annual rate of 14.6 per cent.
The International Federation of Sports Medicine has called for a global standard for sport and fitness wearables amid rising concerns over quality assurance for the products.