INTRA-INDIVIDUAL VARIATION IN ESTIMATES OF USUAL PHYSICAL ACTIVITY

Abstract

24 Background: Intra-individual variation in estimated physical activity (PA) may occur because of actual variability in activity or because it is difficult to represent usual PA accurately, making estimation of usual PA imprecise. Methods: The magnitude of intra-individual variation, as determined by 3 methods of PA assessment, was estimated to determine the optimal number of repeated measures needed to reliably measure usual activity. Data were obtained from 77 healthy adults enrolled in the Study of Physical Activity, Fitness, and Exercise (SAFE). PA was measured by Caltrac accelerometers worn 48-hours every 26 days for one year, concurrent 48-hour PA records (PAR), a corresponding series of four-week PA histories (FWH). Repeated measures regression were used to analyze the data. Results: PA tended to be slightly higher in the summer months as compared with the winter months, but this seasonality explained little of the intra-individual variation. The intra-individual standard deviation (SD) estimated for the FWH (181 MET-min/day) was 39% of the mean annual MET-min/day as compared with the Caltrac (186 MET-min/day or 10%) and PAR (259 MET-min/day or 11%). Given the intra-individual variation, 3 repeated observations of the FWH, or 7 to 8 measurements of the Caltrac or PAR over the course of the year were needed to achieve 80% reliability in estimation of mean annual PA. Conclusion: Intra-individual variation in PA is substantial and must be accounted for in estimates of usual PA. The number of repeated observations needed to reliably estimate usual PA depends on the method used.