Intra-individual variation and estimates of usual physical activity

PURPOSE: The magnitude of intra-individual variation, as determined by thre e methods of physical activity (PA) assessment, was estimated to determine the optimal number of repeated measures needed to reliably measure an indiv idual's usual PA habits. METHODS: Data were obtained from 77 healthy adults enrolled in the Survey o f Activity, Fitness, and Exercise (SAFE). PA was measured by: 1) Caltrac ac celerometers worn 48-hours every 26 days for one year; 2) concurrent 48-hou r PA records (PARs); and 3) a corresponding series of four-week PA historie s (FWH). Repeated measures regression analyses were used to analyze the dat a. RESULTS: PA tended to be slightly higher in the summer months as compared w ith the winter months, but this seasonality explained little of the intra-i ndividual variation. The intra-individual standard deviation (SD) estimated for the FWH (180 MET-min(d-1)) was 39% of the mean annual MET-min(d-1), as compared with the Caltrac (163 MET-min(d-1) or 9%) and PAR (259 MET-min(d- 1) or 11%). Given the intra-individual variation, three repeated observatio ns of the FWH, six Caltrac, or nine PAR over the course of the year were ne eded to achieve 80% reliability in estimation of mean annual PA. CONCLUSIONS: Intra-individual variation in PA is substantial and must be ac counted for in estimates of usual PA. Ann Epidemiol 1999;9:481-488. (C) 199 9 Elsevier Science Inc. All rights reserved.