The assessment of physical activity in individuals and populations: why try to be more precise about how physical activity is assessed?

Simple epidemiological measures of physical activity have proved sufficient to demonstrate associations with many chronic disease outcomes, but they have infrequently separated physical activity into its different dimensions, nor have they allowed estimation of dose-response effects. Generating greater clarity about the nature of the exposure-disease relationship, is an important step in the development of an appropriate public health intervention. This clarity can only be achieved with reliable and valid measurement instruments, which objectively and quantitatively assess the dimension of physical activity that is of interest for a particular health outcome. Objective techniques, such as heart rate monitoring, which have been directly compared to gold standard assessment methods, may be of use in medium-sized epidemiological studies and as a validation tool for questionnaires to be used in larger studies. The combination of methods with uncorrelated error, would result in an improved estimation of the true exposure and is an important area for research. Improved assessment would be of use in aetiological studies, in tracking trends in physical activity within populations, making objective comparisons between populations and in monitoring the effect of interventions.