Number of days required to estimate objectively measured physical activity constructs in different age groups

Purpose: The present study aims to estimate the minimum number of accelerometer measurement days needed to estimate habitual physical activity (PA) among 6, 18 and 30 years old participants, belonging to three population-based Brazilian birth cohorts. Method: PA was assessed by triaxial wrist worn GENEActiv accelerometers for 4-7 consecutive days, including at least one weekend day. Accelerometer raw data were analyzed with R-package GGIR. Description of PA measures (overall PA, MVPA, LPA) between weekdays and weekend days were conducted, and statistical differences were tested with chi-squared and Kruskal-Wallis tests. Intraclass Reliability Coefficient (IRC) was applied through the Spearman Brown Formulae to test reliability of different number of days of accelerometer use. Results: Differences between week and weekend days regarding LPA, MVPA and Overall PA, were only observed among 30-year-olds. More MVPA (p=0.006) and Overall PA (p