Validity of the Fitbit Ace and Moki Devices for Assessing Steps During Different Walking Conditions in Young Adolescents

Purpose: Using wearable monitoring devices is increasingly ubiquitous, including among young people. However, there is limited evidence of the validity of devices which are aimed at children and adolescents. The purpose of this study was to evaluate the validity of Fitbit Ace and Moki monitors in healthy young adolescents. Methods: This cross-sectional study included 17 young adolescents (ages 11-13 y) ambulating between 3 different walking conditions (incidental [similar to 6 min], controlled, and treadmill [each 3 min], while wearing wrist-worn devices [Fitbit Ace, Moki] on each wrist [left and right, respectively]). Data from the devices were compared with observer counts (criterion). Bland-Altman plots and mean absolute percentage errors were computed. Results: Analyses identified that the Fitbit Ace showed higher levels of bias across conditions compared with the Moki device: (mean difference [SD]; Fitbit Ace: 30.0 [38.0], 3.0 [13.0], and 13.0 [23.0] steps and Moki: 1.0 [19.0], 4.0 [16.0], and 6.0 [14.0] steps, incidental, controlled, and treadmill, respectively). Mean absolute percentage errors ranged from 3.1% to 9.5% for the Fitbit Ace and 3.0% to 4.0% for the Moki device. Conclusion: The Fitbit Ace and Moki devices might not provide acceptable validity under all walking conditions, but the Moki provides more accurate estimates of incidental walking and might therefore be a good choice for free-living research or school-based interventions.