Objectives: The purpose of this study was to determine the number of monitoring days needed to reliably measure weekly activity behaviours with the ActivPAL (AP) monitor in adults. Design: Cross-sectional study.
Methods: Participants (90 adults (51.1% men); age = 39.1 ± 12.43 years) wore an AP for 7 consecutive days. Mean time spent sitting/lying, standing and stepping per day, and mean number of transitions from sitting to standing per day were calculated for each participant using 7 days of monitoring (reference). Estimates for these activities were also derived from a combination of randomly selected days (from 1 to 6 days), and randomly selected weekdays and weekend days, and compared with the reference using ANOVA, correlation coefficients and Bland-Altman methods Spearman–Brown Prophecy Formula, based on Intraclass correlation of 0.8, was used to predict the minimum number of days needed to represent activity behaviours as measured with the AP.
Results: At least five days of monitoring were necessary to achieve a reliability of 0.8 for all postures and transitions. Correlation coefficients between estimates derived from any combination of 5 days and the reference were high (rho = 0.96–0.98). When using a combination of weekdays and weekend days, mean biases were comparable with those shown by any combination of days, but 95% limits of agreement were narrower.
Conclusions: When using the AP for a week, data from a combination of any 5 days provided reliable estimates of all activities and transitions per day, but more precise estimates were achieved if at least 1 weekend day was included.