Validity of the Fitbit wearable activity monitor to estimate step counts in free-living conditions in ambulatory children and youth living with disability.

To assess the validity of the Fitbit ChargeHR versus a research-grade accelerometer (ActiGraph GT3X) for estimating daily step counts in free-living conditions in ambulatory children and youth living with physical and non-physical disabilities. Children and youth living with disability (n = 29; median age 10 years (IQR: 8-13), 55% boys; n = 2 with mobility aid) wore the GT3X ActiGraph accelerometer (hip) and the Fitbit ChargeHR (wrist) for seven days. Inter-device agreement in steps/day was assessed by intraclass correlation coefficients (ICCs) and Bland-Altman plots. A receiver operating curve (ROC) was used to determine a Fitbit step-count cut-point that corresponds to meeting physical activity guidelines (defined as ≥60 min of moderate-to-vigorous physical activity per day). Overall, we found an ICC = 0.861 (p < 0.001) between daily step counts measured by the two devices. Bland-Altman analyses revealed a mean difference ("bias") between the devices with the Fitbit recording, on average, 1,388 more steps/day than the accelerometer (Limits of Agreement (LoA) 1,741 to -4,518 steps per day). The ROC revealed a Fitbit cut-point of 12,272 steps/day corresponding to meeting guidelines. Fitbit ChargeHR devices tend to overestimate daily step counts, but may still provide useful estimates of step counts and patterns in children and youth living with disability.