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. 2024 Jun 17:10:20552076241262710.
doi: 10.1177/20552076241262710. eCollection 2024 Jan-Dec.

Fitbit's accuracy to measure short bouts of stepping and sedentary behaviour: validation, sensitivity and specificity study

Julie Delobelle  1   2 , Elien Lebuf  1   2 , Delfien Van Dyck  1 , Sofie Compernolle  1   2 , Michael Janek  3 , Femke De Backere  4 , Tomas Vetrovsky  3
Affiliations

Fitbit's accuracy to measure short bouts of stepping and sedentary behaviour: validation, sensitivity and specificity study

Julie Delobelle et al. Digit Health. .

Abstract

Objective: This study aims to assess the suitability of Fitbit devices for real-time physical activity (PA) and sedentary behaviour (SB) monitoring in the context of just-in-time adaptive interventions (JITAIs) and event-based ecological momentary assessment (EMA) studies.

Methods: Thirty-seven adults (18-65 years) and 32 older adults (65+) from Belgium and the Czech Republic wore four devices simultaneously for 3 days: two Fitbit models on the wrist, an ActiGraph GT3X+ at the hip and an ActivPAL at the thigh. Accuracy measures included mean (absolute) error and mean (absolute) percentage error. Concurrent validity was assessed using Lin's concordance correlation coefficient and Bland-Altman analyses. Fitbit's sensitivity and specificity for detecting stepping events across different thresholds and durations were calculated compared to ActiGraph, while ROC curve analyses identified optimal Fitbit thresholds for detecting sedentary events according to ActivPAL.

Results: Fitbits demonstrated validity in measuring steps on a short time scale compared to ActiGraph. Except for stepping above 120 steps/min in older adults, both Fitbit models detected stepping bouts in adults and older adults with sensitivities and specificities exceeding 87% and 97%, respectively. Optimal cut-off values for identifying prolonged sitting bouts achieved sensitivities and specificities greater than 93% and 89%, respectively.

Conclusions: This study provides practical insights into using Fitbit devices in JITAIs and event-based EMA studies among adults and older adults. Fitbits' reasonable accuracy in detecting short bouts of stepping and SB makes them suitable for triggering JITAI prompts or EMA questionnaires following a PA or SB event of interest.

Keywords: Fitbit; JITAI; accelerometry; ecological momentary assessment; event-based ecological momentary assessment (EMA); just-in-time adaptive intervention; physical activity; sedentary behaviour; step; wearable.

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Conflict of interest statement

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figures

Figure 1.
Figure 1.
Bland–Altman plots of Fitbit Ionic (on the left) and Inspire 2 (on the right) compared to ActiGraph for adults. The mean difference is shown by the middle line. Positive values indicate an overestimation by Fitbit. The dotted lines represent the limits of agreement.
Figure 2.
Figure 2.
Bland–Altman plots of Fitbit Ionic (on the left) and Inspire 2 (on the right) compared to ActiGraph for older adults. The mean difference is shown by the middle line. Positive values indicate an overestimation by Fitbit. The dotted lines represent the limits of agreement.
Figure 3.
Figure 3.
ROC curve for detecting 30 min SB in older adults with Fitbit Inspire 2.
See this image and copyright information in PMC

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