Comparison of Physical Activity Measures Derived From the Fitbit Flex and the ActiGraph GT3X+ in an Employee Population With Chronic Knee Symptoms

OBJECTIVE: We examined the accuracy of data from an affordable personal monitor (Fitbit Flex) compared with that of data from a research‐grade accelerometer worn simultaneously for 7 days; high accuracy would support substitution with this less‐expensive personal activity monitor in future community‐based arthritis research. METHODS: Subjects (N = 35) with chronic knee symptoms were recruited for a pilot intervention study using Fitbits to increase physical activity in employees with chronic knee symptoms at an urban corporation. Subjects simultaneously wore for 7 days a Fitbit Flex (wrist‐worn) and ActiGraph GT3X+ (waist‐worn). Fitbit Flex data were regularly stored on a research storage service (Fitabase) by participants. Bland–Altman plots were constructed to examine the agreement between the mean daily times spent in light activity and in bouted moderate‐to‐vigorous physical activity (MVPA). Comparisons were calculated by matching Fitabase data from calendar days the Fitbit was worn with data from valid monitoring days (greater than or equal to 10 hours wear time) of the ActiGraph. RESULTS: Participants at baseline were mostly female (69%) and white (57%) and had a mean age of 52 years and body mass index of 32 kg/m(2). Bland–Altman analyses indicated systematic bias overall (the Fitbit overestimated both light‐intensity activity and MVPA compared with the ActiGraph). The average error varied in magnitude and direction with changing activity amounts. CONCLUSION: The Fitbit Flex does not appear to be an adequate substitute for research‐grade accelerometry (which represents the gold standard for objective research monitoring of all physical activity intensity levels) in this population of persons with chronic knee symptoms.