Validity and wear compliance of wrist-wearable activity trackers in free-living schoolchildren: validation study

IntroductionWearable activity trackers are widely available for promoting physical activity (PA) and reducing sedentary behavior (SB). However, studies assessing their validity for PA and SB measurement in free-living conditions in school-age children remain limited. ObjectiveThis study aimed to evaluate the criterion validity and wear compliance of the Fitbit Ace, a wrist-worn wearable activity tracker designed for children, in measuring PA and SB under free-living conditions. MethodsA total of 102 children (mean age: 10.2 years; 44.1% girls) simultaneously wore a waist-worn accelerometer (Omron Active Style Pro HJA-750c; ASP HJA-750c) and a Fitbit Ace for 11 consecutive days. Step count and time spent in SB, light PA (LPA), moderate PA (MPA), and vigorous PA (VPA) were collected from both devices. Pearsons and Spearmans correlations assessed criterion validity, while Bland-Altman plots, mean absolute percentage error (MAPE), and intraclass correlation coefficients were used to evaluate agreement. Wear compliance was compared between the two devices. ResultsThe Fitbit Ace showed a very strong correlation with step count and strong correlations with SB, LPA, and moderate-to-vigorous PA (MVPA). However, significant differences were observed for all PA variables. The Fitbit Ace overestimated step count, SB, VPA, and MVPA, whereas it underestimated LPA and MPA. Agreement between devices was low, with proportional errors in step count, LPA, VPA, and MVPA. MAPE exceeded 20% for all variables, including step count (21.7%), SB (31.2%), LPA (30.3%), and MVPA (120.7%). The proportion of children meeting the criterion of [≥]10 h/day wear criterion was higher for the Fitbit Ace (83.2%) than for the ASP HJA-750c (57.8%), as was adherence to [≥]4 days (97.0% vs. 62.2%). ConclusionWhile caution is needed when using the Fitbit Ace for precise measurement of individual PA, it is a useful tool for assessing overall patterns of step count, SB, and LPA in school-aged children.