Concurrent Validity Of Wearable Activity Monitors Designed For Children In Estimating Moderate-to-vigorous Physical Activity

Moderate-to-vigorous physical activity (MVPA) benefits both physical and mental health in children. Wearable activity monitors (WAMs) for children can help children self-monitor their daily MVPA time and increase their activity levels through fun and motivating graphics. However, it is still unclear if WAMs for children accurately estimate children's MVPA time in free-living conditions. PURPOSE: To assess the validity of the Garmin Vivofit Jr. 3 (VFJ 3) and Fitbit Ace 3 (Ace 3) in estimating children’s MVPA time under a free-living condition. METHODS: 25 children (Girls: 56%, Age: 10.1 ± 2.5 years, BMI: 17.1 ± 2.4 kg/m2) wore the VFJ 3 and Ace 3 on wrists and an Actigraph accelerometer (wGT3X-BT) on the waist simultaneously while performing daily activities for seven consecutive days. Data from wGT3X-BT was used as a criterion measure. Pearson correlations, mean absolute percent errors (MAPE), Bland-Altman (BA) plots, and equivalence tests were conducted to systematically evaluate the validity of the two WAMs against criterion measures. RESULTS: VFJ 3 and Ace 3 had strong positive correlations with wGT3X-BT in estimating MVPA time (range: r = 0.72 to 0.79) and overestimated children’s daily MVPA time against the criterion measure (VFJ 3: +38.6 and Ace 3: +2.9 min/day). MAPEs of VFJ 3 and Ace 3 were ≥ 30%. BA plots showed that Ace 3 had no apparent systematic bias for agreement, but VFJ 3 constantly overestimated MVPA time in children. Equivalence tests showed the 90% Confidence Interval of the estimates from VFJ 3 and Ace 3 fell within ±93.1% and ± 35.3% of the criterion measure, respectively. CONCLUSIONS: Our findings demonstrated that the Ace 3 may outperform the VFJ 3 in estimating daily MVPA time. Due to the Ace 3's large measurement error at the individual level, it is not yet possible to confirm that it is a valid WAM for monitoring children's MVPA time in free-living conditions.