Validation of a consumer-grade activity monitor for continuous daily activity monitoring in individuals with multiple sclerosis

Mult Scler J Exp Transl Clin. 2019 Nov 21;5(4):2055217319888660. doi: 10.1177/2055217319888660. eCollection Oct-Dec 2019.


Background: Technological advancements of remote-monitoring used in clinical-care and research require validation of model updates.

Objectives: To compare the output of a newer consumer-grade accelerometer to a previous model in people with multiple sclerosis (MS) and to the ActiGraph, a waist-worn device widely used in MS research.

Methods: Thirty-one individuals with MS participated in a 7-day validation by the Fitbit Flex (Flex), Fitbit Flex-2 (Flex2) and ActiGraph GT3X. Primary outcome was step count. Valid epochs of 5-min block increments, where there was overlap of ≥1 step/min for both devices were compared and summed to give a daily total for analysis.

Results: Bland-Altman plots showed no systematic difference between the Flex and Flex2; mean step-count difference of 25 more steps-per-day more recorded by Flex2 (95% confidence intervals (CI) = 2, 48; p = 0.04),interclass correlation coefficient (ICC) = 1.00. Compared to the ActiGraph, Flex2 (and Flex) tended to record more steps (808 steps-per-day more than the ActiGraph (95% CI= -2380, 765; p < 0.01), although the ICC was high (0.98) indicating that the devices were likely measuring the same kind of activity.

Conclusions: Steps from Flex and Flex2 can be used interchangeably. Differences in total step count between ActiGraph and Flex devices can make cross-device comparisons of numerical step-counts challenging particularly for faster walkers.

Keywords: Physical activity; accelerometry; motor activity; multiple sclerosis; validation studies as topic (mesh heading).