Physical activity can be assessed by subjective methods such as questionnaires, surveys, interviews, etc., or by objective methods such as heart rate monitors, accelerometers, pedometers, etc. Subjective methods are much cheaper and more affordable than objective methods, but the validity of their output is questionable. Therefore, we decided to examine the differences between the subjective and objective methods of assessing physical activity, with particular focus on whether there are such differences between children in terms of the body fat mass.
The study used data from 100 children aged 11 who participated in the ARTOS study. As subjective method of physical activity assessment, we used the proxy-report CLASS questionnaire that was completed by parents. As objective method we used the BodyMedia SenseWear Armband PRO 3 device, which contains a two-axis accelerometer and several other sensors.
The analysis showed that parents greatly overestimated the time their children spend in moderate to vigorous physical activity - setting the limit fort this activity at 4 MET. On the average, parents overestimated this time by 42% in boys and 40% in girls.
To determine the differences between the subjective and objective methods in regard of body fat mass, the sample was divided into quartiles. The first quartile included the children with the lowest body fat mass and the fourth quartile the children with the highest body fat mass. With each quartile parents increasingly overestimated the time their children spend in moderate to vigorous physical activity. In the first quartile the parents overestimated that time by 27%, in the second quartile by 34%, in the third by 41% and in the fourth by 69%.
Our findings are important for critical reflection on the study and monitoring of children's physical activity with questionnaires based on parental reporting. What is more, the findings of such studies are used to set and evaluate policies in this area.
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