Wearable fitness trackers are becoming increasingly popular as a means of tracking physical activity, heart rate and even sleep patterns. Studies show some surprising facts about the reliability of these devices. A review of 22 studies on fitness trackers found that step counting was most reliable, while using the devices to measure energy expenditure and sleep was less reliable. Another study on seven healthy subjects compared various brands of fitness trackers, with repeated experiments of walking, jogging on a treadmill and walking up/down stairs.
Average accuracy for steps counted in these activities was 97.2% (varying from 92.4 – 99.9%), average repeatability was less impressive at 0.79 (varying from 0.55 – 0.89). When comparing with energy expenditure measured by a portable metabolic response breathing device, there was on average 10-15% error in energy expenditure estimation. For sleep tracking, studies on adolescents shows good agreements with polysomnography for certain devices. Heart rate monitoring becomes less accurate during vigorous activity.
Thus it is difficult to say whether a certain device is reliable or not, since they all use varying hardware and software algorithms to make estimations. A trial investigating the effects of fitness trackers on dieting found that a group given fitness trackers lost LESS weight compared to a group who just tracked their results on a computer. Therefore you should be very skeptical to the numbers collected from your fitness tracker, but you can always use RELATIVE changes in the numbers to assess if you are more or less active than usual.
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