Reliability of physical activity measures from accelerometry among preschoolers in free-living conditions

Background & aims: Physical activity (PA) is a major determinant of obesity. Accelerometers have been reported to provide valid measures among adults. However, studies among preschoolers rarely report positive findings. To assess the day-to-day variability of accelerometers in preschoolers. Methods: Uni-axial accelerometer (Actigraph monitor AM 7164-2.2) counts including one weekend from the time of getting up in the morning until bedtime. Results: On average, boys showed 899 counts per minute (cpm) compared to 764 for girls (p<0.01; overall mean 828cpm). Intra-individual correlation for accelerometry data between single days of examination was low with Pearson correlation coefficients between r=0.31 and 0.51. Furthermore, child's body mass index (BMI) and accelerometer measures were not related to each other (Pearson's correlation coefficient r = -0.06). Subsequent analyses showed higher measures (+50%cpm; p<0.01) for instruments placed in front of the umbilicus compared to instruments placed at the right hip. Conclusions: Measurements of uni-axial accelerometers showed a low reliability among preschoolers. Uni-axial accelerometers placed on elastic belts might measure PA with tow precision among preschoolers under free-living conditions possibly due to slipping instruments. This might explain tacking findings of an association between PA and obesity in studies among preschoolers. (C) 2007 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.