Research Article: Validation of the Fitbit Charge 2 compared to the ActiGraph GT3X+ in older adults with knee osteoarthritis in free-living conditions

Date Published: January 30, 2019

Publisher: Public Library of Science

Author(s): Jamie E. Collins, Heidi Y. Yang, Taylor P. Trentadue, Yusi Gong, Elena Losina, Belinda Parmenter.


To evaluate physical activity (PA) and sedentary time in subjects with knee osteoarthritis (OA) measured by the Fitbit Charge 2 (Fitbit) and a wrist-worn ActiGraph GT3X+ (AGW) compared to the hip-worn ActiGraph (AGH).

We recruited a cohort of subjects with knee OA from rheumatology clinics. Subjects wore the AGH for four weeks, AGW for two weeks, and Fitbit for two weeks over a four-week study period. We collected accelerometer counts (ActiGraphs) and steps (ActiGraphs, Fitbit) and calculated time spent in sedentary, light, and moderate-to-vigorous activity. We used triaxial PA intensity count cut-points from the literature for ActiGraph and a stride length-based cadence algorithm to categorize Fitbit PA. We compared Fitbit wear times calculated from a step-based algorithm and a novel algorithm that incorporates steps and heart rate (HR).

We enrolled 15 subjects (67% female, mean age 68 years). Relative to AGH, Fitbit, on average, overestimated steps by 39% and sedentary time by 37% and underestimated MVPA by 5 minutes. Relative to AGH, AGW overestimated steps 116%, underestimated sedentary time by 66%, and captured 281 additional MVPA minutes. The step-based wear time Fitbit algorithm captured 14% less wear time than the HR-based algorithm.

Fitbit overestimates steps and underestimates MVPA in knee OA subjects. Cut-offs validated for AGW should be developed to support the use of AGW for PA assessment. The HR-based Fitbit algorithm captured more wear time than the step-based algorithm. These data provide critical insight for researchers planning to use commercially-available accelerometers in pragmatic studies.

Partial Text

Physical activity (PA) is associated with lower all-cause mortality and improved quality of life[1–4]. Guidelines outlined by the Centers for Disease Control and Prevention (CDC) recommend that older adults engage in ≥150 minutes of moderate-intensity PA, 75 minutes of vigorous-intensity PA, or a combination of the two weekly[5]. Knee osteoarthritis (OA), affecting over 14 million Americans,[6] is a leading cause of disability among older adults[7]. PA may help alleviate pain and improve function in knee OA[8]. However, <13% of men and <8% of women with symptomatic knee OA meet CDC PA guidelines[9]. We compared measures of PA obtained from the Fitbit Charge 2 and wrist-worn ActiGraph GT3X+ against the hip-worn ActiGraph in older adults with knee OA. Compared to AGH, we found that Fitbit overestimates steps by 39% and that AGW overestimates steps by 116%; Fitbit overestimates daily sedentary time by 37% while AGW underestimates sedentary time by 66%; and Fitbit underestimates daily MVPA by 50% while AGW reported considerably more MVPA. Our results confirm previous findings that AGW records significantly more steps than AGH[36] and that Fitbit sometimes overestimates steps in free-living settings[37, 38]. The overestimation of steps and MVPA suggests that in order to use AGW to measure PA, specific thresholds should be established and thresholds derived from hip-worn ActiGraphs should not be used to measure PA by wrist-worn ActiGraphs in older adults with OA. Our findings differ from literature reporting that Fitbit underestimates sedentary time[37]; this may be attributed, in part, to our specific sample of participants with musculoskeletal conditions that limit mobility and impact gait and, in part, to our consideration of HR, which allows us to clearly distinguish no wear from sedentary time.   Source:


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