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 828 cpm). 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, childʹs body mass index (BMI) and accelerometer measures were not related to each other (Pearsonʹs 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 low precision among preschoolers under free-living conditions possibly due to slipping instruments. This might explain lacking findings of an association between PA and obesity in studies among preschoolers.