JMIR Publications

ABSTRACT

Background:

Previous research has attempted to determine the minimum number of days of accelerometry required to reliably reflect an individual’s physical activity. However, human behaviors on a day-to-day basis can be highly variable. As a consequence, the number of days required to reliably predict habitual physical activity is dependent on the variability that exists within an individual. There is a concern that adopting generic recommendations from previous research could provide unreliable estimates.

Objective:

Thus, the main aim of this study was to determine if the number of days of accelerometry data required to reliably estimate short- (7 days) and medium-term (28 days) physical activity differed between clusters of individuals with distinct physical activity patterns.

Methods:

Accelerometry data was collected from 258 participants using the Withings Scanwatch. Agglomerative hierarchical clustering was used to identify clusters of individuals based on their physical activity. Intraclass correlation coefficients (ICCs) of step count were then calculated within each physical activity cluster. A series of ICCs were computed by separately comparing the average step count across the full periods (7 and 28, for the short- and medium-term analysis, respectively) to a series of averaged subsamples (ranging from 1–6 days and 1–27 days, for the short- and medium-term analysis, respectively). For each subsample, 500 random combinations were generated and compared, providing a distribution of ICCs for each subsample.

Results:

To achieve a mean ICC score greater than or equal to 0.80, using all randomized combinations, the number of days ranged from 2 to 6 days depending on the physical activity cluster, for the short-term analysis. For the medium-term analysis, the range was 6 to 11 days.

Conclusions:

Physical activity patterns influence the number of days required to estimate habitual physical activity. Thus, to avoid unreliable estimates of physical activity, researchers should be mindful of the physical activity patterns of their sample.