Inter-Device Agreement between Fitbit Flex 1 and 2 for Assessing Sedentary Behavior and Physical Activity.

Area of Science:

• Wearable technology
• Human movement analysis
• Biomedical engineering

Background:

• Accurate measurement of sedentary behavior (SED) and physical activity (PA) is crucial for public health research.
• Consumer-grade wearable activity trackers are increasingly used, necessitating validation studies.
• Understanding the agreement between different models of the same device brand is important for data consistency.

Purpose of the Study:

• To assess the inter-model agreement between the Fitbit Flex (FF) and Fitbit Flex 2 (FF2) for estimating SED and PA.
• To determine if the FF2 can be used as a substitute for the FF in free-living conditions.
• To evaluate measurement differences, agreement, and systematic bias between the FF and FF2.

Main Methods:

• 33 healthy adults participated in a 14-day free-living study.
• Participants wore both FF and FF2 devices on their non-dominant wrist.
• Data analysis included Pearson's correlation, Mean Absolute Percent Error (MAPE), Bland-Altman plots, and equivalence testing.

Main Results:

• The FF2 demonstrated strong correlations with the FF in estimating time spent in SED and PA.
• FF2 provided similar estimates for SED and PA compared to the FF.
• Low measurement discordance and no significant systematic bias were observed for SED and moderate-to-vigorous PA estimates.

Conclusions:

• The Fitbit Flex 2 exhibits good inter-model agreement with the Fitbit Flex for measuring sedentary behavior and physical activity.
• The FF2 is a viable alternative for measuring SED and PA in longitudinal research, especially when the FF is not commercially available.
• These findings support the use of FF2 in research settings requiring consistent PA and SED monitoring.