Validation of the Fitbit Zip® as a Measure of Pre-school Children’s Step Count: A Cross-Sectional Study.

Objectives: Validation of physical activity measurement tools is essential to determine the relationship between physical activity and health in pre-school children, but research to date has not focussed on this priority. The aims of this study were to ascertain inter-rater reliability of observer step count, and inter-device reliability and validity of Fitbit Zip accelerometer step counts in pre-school children.
Methods: Fifty-six 3-4-year-old children (29 girls) recruited from 10 nurseries in North Wales, UK, wore two Fitbit Zip accelerometers whilst performing a timed walking task in their childcare settings. Accelerometers were worn in secure pockets inside a custom-made tabard. Video-recordings enabled two observers to independently code the number of steps performed in 3 minutes by each child during the walking task. Intra-class correlations, concordance correlation coefficients, Bland-Altman plots and absolute percent error were calculated to assess the reliability and validity of the consumer-grade device.
Results: An excellent intra-class correlation was found between the two observer codings (ICC = 1.00) and the two Fitbit Zips (ICC = 0.91). Concordance between the Fitbit Zips and observer counts was also high (r = 0.77), with an acceptable absolute percent error (6–7%). Bland-Altman analyses identified a bias for Fitbit 1 of 22.8 ± 19.1 steps with limits of agreement between -14.7 and 60.2 steps, and a bias for Fitbit 2 of 25.2 ± 23.2 steps with limits of agreement between -20.2 and 70.5 steps.
Conclusions: Fitbit Zip accelerometers are a reliable and valid method of recording pre-school children’s step count in a childcare setting.