Objective methods can improve accuracy of physical activity measurement in field studies but uncertainties remain about their use.
Children age 11 years from the Avon Longitudinal Study of Parents and Children (ALSPAC), were asked to wear a uni-axial accelerometer (MTI Actigraph) for 7 days.
Of 7159 children who attended for assessment, 5595 (78%) provided valid measures. The reliability coefficient for 3 days of recording was .7 and the power to detect a difference of 0.07 SDs (P ≤ .05) was > 90%. Measures tended to be higher on the first day of recording (17 counts/min; 95% CI, 10–24) and if children wore the monitor for fewer days, but these differences were small. The children who provided valid measures of activity were different from those who did not, but the differences were modest.
Objective measures of physical activity can be incorporated into large longitudinal studies of children.
Mattocks, Tilling, Deere, Saunders, Kirkby, and Smith are with the Dept of Social Medicine, University of Bristol, UK. Ness and Leary are with the Dept of Oral and Dental Science, Bristol Dental School, BS1 2LY, UK. Blair is with the Dept of Exercise Science, University of South Carolina, Columbia, SC 29208. Shield is with the Institute of Child Life and Health, United Bristol Healthcare Trust, Bristol, BS2 8AE, UK. Wells is with the Childhood Nutrition Centre, Institute of Child Health, London, WC1N 1EH, UK. Wareham is with the Medical Research Council Epidemiology Unit, Cambridge, CB2 0QQ, UK. Reilly is with the Division of Developmental Medicine, Yorkhill Hospitals, University of Glasgow, G3 8SJ, UK. Riddoch is with Sport and Exercise Sciences, School for Health, University of Bath, BA2 7AY, UK.