Agreement between Fitbit and ActiGraph Estimates of Physical Activity in Young Children

Physical activity (PA) estimates from the Fitbit Flex 2 were compared to those from the ActiGraph GT9X Link in 123 elementary school children. Steps and intensity-specific estimates of PA and 3-month PA change were calculated using two different ActiGraph cut-points (Evenson and Roma nzini). Fitbit estimates were 35% higher for steps compared to the ActiGraph. Fitbit and ActiGraph intensity-specific estimates were closest for sedentary and light PA, while estimates of moderate and vigorous PA varied substantially depending upon the ActiGraph cut-points used. Spearman correlations between device estimates were higher for steps (r(s) = .70) than for moderate (r(s) =.54 to .55) or vigorous (r(s) =.29 to .48) PA. There was low concordance between devices in assessing PA changes over time. Agreement between Fitbit Flex 2 and ActiGraph estimates may depend upon the cut-points used to classify PA intensity. However, there is fair to good agreement between devices in ranking children's steps and MVPA.

Automated summary

This study compared PA estimates from Fitbit Flex 2 to those from ActiGraph GT9X Link in elementary school children. Fitbit estimates were 35% higher for steps compared to ActiGraph. Fitbit and ActiGraph estimates for sedentary and light PA were closest, while estimates for moderate and vigorous PA varied depending on the ActiGraph cut-points used. There was higher correlation between devices for steps than for moderate or vigorous PA, and low concordance in assessing PA changes over time.