Many people have questioned the accuracy of activity monitors and fitness trackers after wearing one. Dr. Nate Meckes began to study the devices after wearing one from a shipment that arrived at Arizona State University. Dr. Meckes said that after wearing the device throughout the day, including to a meeting where he stood up and paced, he was surprised to find that “It had recorded that I was not moving at all.”
The most common types of these devices are accelerometers, designed to measure a person’s movement and energy expenditure. The devices use electronics to determine bodily movement and intensity and feed data into the device’s electronic brain, where equations determine the amount of energy someone is expending or how many calories have been burned. Home users can choose from a variety of devices, including those worn on the hip and others that are worn on the arm or wrist. Users have taken the resulting measurements on faith as there have been no unbiased, comparative studies of the devices available.
Dr. Meckes set up an experiment examining how reliable these fitness trackers actually were. Dr. Meckes gathered 16 adult volunteers for his experiment and had each of them wear three different monitors; two on the hip and one around the arm. The volunteers were also asked to wear portable masks that measure oxygen consumption, long considered the gold standard for measuring energy output.
The volunteers then participated in a variety of activities in the university’s physiology lab. These activities included typing at a computer, cleaning a simulated kitchen, standing up, walking on a treadmill and playing a board game. Matching the results to those of the oxygen-consumption monitor, Dr. Meckes and his colleagues found all three of the devices accurately measured energy expenditure when the volunteers walked briskly, but the devices were far less reliable in tracking the energy costs of light-intensity activities, like standing or cleaning.
Dr. Meckes’s results join those of a number of other new studies in raising concerns about the accuracy of activity monitors. Several other new studies also found that these devices are subject to miscalculations. One involved 74 adults of various ages that wore an armband accelerometer and a portable oxygen-consumption gauge while walking, jogging, riding a stationary bicycle, and completing daily living activities. The accelerometer measured the more strenuous bodily movements fairly accurately, but it significantly underestimated subtler activities. The lesson at the moment for anyone who owns a fitness tracker is that the device’s measurements are likely to be imperfect.