Validity and Wear Compliance of Wrist-Worn Consumer Activity Trackers Among Japanese School-Aged Children Under Free-Living Conditions

<b>Background</b>: Wrist-worn consumer activity trackers are widely used to promote physical activity (PA) and reduce sedentary behavior (SB). However, evidence regarding their validity for measuring PA and SB in free-living school-aged children remains limited. This study evaluated the concurrent validity and wear compliance of a wrist-worn consumer activity tracker in school-aged children under free-living conditions with protocol-defined wear requirements. <b>Methods</b>: A total of 102 children (mean age: 10.2 years; 44.1% girls) wore a wrist-worn device (Fitbit Ace) and a waist-worn accelerometer (Omron Active Style Pro HJA-750c, ASP-750c). Of the 1122 person-days collected over 11 days, 135 person-days meeting inclusion criteria for both devices were included (≥10 h/day wear time and an inter-device wear time difference of ≤60 min). Step count and time in SB, light (LPA), moderate (MPA), vigorous (VPA), and moderate-to-vigorous PA (MVPA) were assessed. Correlations, mean absolute percentage error (MAPE), agreement, and wear compliance between the two devices were examined. <b>Results</b>: Correlations were strong for step count (r = 0.86), SB (r = 0.72), and LPA (r = 0.71); however, agreement was poor, with systematic overestimation of step count, SB, VPA, and MVPA and underestimation of LPA and MPA by the Fitbit Ace, and MAPE exceeding 20% for all PA variables. Wear compliance (≥10 h/day on ≥4 days) was higher for the Fitbit Ace (97.0%) than for the ASP-750c (62.2%). <b>Conclusions</b>: Although the Fitbit Ace may be useful for characterizing general patterns of LPA and SB in school-aged children, caution is warranted for accurate individual-level PA assessment.