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Background Recent advances in sensor technologies have promoted the use of consumer-based accelerometers such as Fitbit Flex in epidemiological and clinical research; however, the validity of the Fitbit Flex in measuring sedentary behavior (SED) and physical activity (PA) has not been fully determined against previously validated research-grade accelerometers such as ActiGraph GT3X+. Therefore, the purpose of this study was to examine the concurrent validity of the Fitbit Flex against ActiGraph GT3X+ in a free-living condition. Methods A total of 65 participants (age: M = 42, SD = 14 years, female: 72%) each wore a Fitbit Flex and GT3X+ for seven consecutive days. After excluding sleep and non-wear time, time spent (min/day) in SED and moderate-to-vigorous PA (MVPA) were estimated using various cut-points for GT3X+ and brand-specific algorithms for Fitbit, respectively. Repeated measures one-way ANOVA and mean absolute percent errors (MAPE) served to examine differences and measurement errors in SED and MVPA estimates between Fitbit Flex and GT3X+, respectively. Pearson and Spearman correlations and Bland-Altman (BA) plots were used to evaluate the association and potential systematic bias between Fitbit Flex and GT3X+. PROC MIXED procedure in SAS was used to examine the equivalence (i.e., the 90% confidence interval with ±10% equivalence zone) between the devices. Results Fitbit Flex produced similar SED and low MAPE (mean difference [MD] = 37 min/day, P  = .21, MAPE = 6.8%), but significantly higher MVPA and relatively large MAPE (MD = 59–77 min/day, P  < .0001, MAPE = 56.6–74.3%) compared with the estimates from GT3X+ using three different cut-points. The correlations between Fitbit Flex and GT3X+ were consistently higher for SED (r = 0.90, ρ = 0.86, P  < .01), but weaker for MVPA (r = 0.65–0.76, ρ = 0.69–0.79, P  < .01). BA plots revealed that there is no apparent bias in estimating SED. Conclusion In comparison with the GT3X+ accelerometer, the Fitbit Flex provided comparatively accurate estimates of SED, but the Fitbit Flex overestimated MVPA under free-living conditions. Future investigations using the Fitbit Flex should be aware of present findings.

This study investigated the concurrent validity of two accelerometer-based physical activity (PA) monitors, the wrist-worn Fitbit Flex, and ActiGraph’s hip-worn GT3X+. Specifically, we examined the relationship, differences, and level of agreement between Fitbit and GT3X+ sedentary behavior (SED) and moderate-to-vigorous PA (MVPA) estimates. Sixty-seven adults (mean age: 47.1 ± 14.1, female: 73.1%) from North Dakota State University wore the Fitbit and GT3X+, and logged any sleep and non-wear time, for seven consecutive days in free-living conditions. GT3X+ estimates were calculated using ActiGraph Freedson, Troiano, and Freedson’s VM3 cut-points. Fitbit estimates were calculated via Fitabase. Only data during waking hours where both PA monitors were worn were analyzed. Fitbit and GTX+ estimates strongly correlated. Fitbit produced similar SED (mean difference = -35.83 minutes (min)/day), but significantly higher MVPA (mean differences = -59.7 – 77.41 min/day) to GT3X+. As the mean volume of MVPA increased, so did differences between Fitbit and GT3X+ estimates.