Fitbit’s accuracy to measure short bouts of physical activity and sedentary behavior: validation, sensitivity and specificity study (Preprint)
Authors: Delobelle Julie , Lebuf Elien , Van Dyck Delfien , Compernolle Sofie , Janek Michael , De Backere Femke , Vetrovsky Tomas 
Published: December 2022

BACKGROUND

Adopting a physically active lifestyle results in substantial health benefits. However, many adults and older adults are insufficiently physically active and engage in high levels of (SB). Just-in-time adaptive interventions (JITAIs) are promising in targeting these health behaviors by providing real-time supportive prompts during opportunistic moments. Developing effective JITAIs requires better insight into the dynamic determinants of PA and SB, which can be obtained through event-based ecological momentary assessment (EMA). In both JITAIs and event-based EMA research, real-time monitoring of participants’ behavior is needed to enable the detection of the events of interest (e.g., 5 minutes of sustained walking or 30 minutes of prolonged sitting).

OBJECTIVE

To ensure Fitbits are suitable devices for use in event-based EMA studies and JITAIs, we explored the concurrent validity, accuracy and inter-device agreement of two Fitbit models (Inspire 2 and Ionic) in measuring short bouts of stepping and SB among free-living adults and older adults.

METHODS

Thirty-seven adults (18-65y) and 32 older adults (65+) from two countries (Belgium and the Czech Republic) wore four devices simultaneously for three consecutive days. In addition to two Fitbit models at the non-dominant wrist, they wore an ActiGraph GT3X+ at the hip and an ActivPAL at the thigh as reference devices for step count and SB, respectively. To investigate the concurrent validity for measuring step count during sustained walking (≥ 60 steps/min), the mean error and mean absolute error were calculated for 1-, 5-, 10-, and 20-minute epochs. In addition, concordance correlation coefficients were calculated and Bland-Altman plots were constructed. To explore Fitbit’s accuracy in detecting short bouts of stepping, the sensitivities and specificities were calculated for various thresholds (60 to 120 steps/minute) and bout durations (1 to 20 min). Finally, to define Fitbit’s optimal thresholds to capture prolonged bouts of sitting (1 to 60 min), receiver operating characteristics curves were constructed.

RESULTS

The Fitbit appeared to be valid for measuring steps on a short time scale (i.e., 1, 20, 30 and 60 minutes) compared to the ActiGraph. Except for stepping above 120 steps/min in older adults, both Fitbits detected stepping bouts of various durations in both adults and older adults with sensitivities and specificities greater than 87% and 97%, respectively. In addition, optimal cut-off values to detect prolonged bouts of sitting were identified, obtaining sensitivities and specificities greater than 93% and 89%, respectively.

CONCLUSIONS

This paper provides practical considerations for using Fitbit devices in JITAIs and event-based EMA studies in adults and older adults. Considering Fitbits' reasonable accuracy in detecting short bouts of stepping and SB, they can be used to trigger JITAI prompts or EMA questionnaires following an event of interest (e.g., 5 minutes of sustained walking).