Abstract
BackgroundRecent 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.MethodsA 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.ResultsFitbit 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.ConclusionIn 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.
Highlights
Recent advances in sensor technologies have promoted the use of consumer-based accelerometers such as Fitbit Flex in epidemiological and clinical research; 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+
Since no significant differences in moderate-tovigorous PA (MVPA) and SED were observed between gender and Body mass index (BMI) groups, and these comparisons were not integral to the intended analysis, we combined data from the entire sample (n = 65) for the remainder of the analysis
This study examined the accuracy of the Fitbit Flex PA monitor against a previously validated accelerometer, the ActiGraph GT3X+, for classifying SED and MVPA in free-living settings
Summary
Recent advances in sensor technologies have promoted the use of consumer-based accelerometers such as Fitbit Flex in epidemiological and clinical research; 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+. Using regression equations with accelerometer counts (i.e. counts/60 s) as a predictor, several activity count cut-points have been developed to estimate the amount of time spent in sedentary behavior (SED) as well as different intensities of PA, namely moderate-to-vigorous intensity PA (MVPA). Two sets of cut-points developed by Freedson et al, one using single axis data (Freedson) and the other vector magnitude data (VM3), and one set developed by Troiano et al are widely utilized cut-points for estimating time spent in SED and varying intensities of PA including, light intensity PA and MVPA [13, 15, 16]. The cut-points defined SED and MVPA as follows: 1) < 100 counts and ≥ 1952 counts/60-s, 2) < 100 counts and ≥ 2020 counts/60-s, and 3) ≥2691 counts/60-s (for MVPA only) for the Freedson, Troiano, and VM3 cut-points respectively
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