Potential misclassification in studies of physical activity

Regular physical activity is associated with a reduced risk of coronary heart disease, although debate still occurs over the level of intensity required for cardioprotection. The use in epidemiological studies of the arbitrarily defined categories of light, moderate, and hard, as proxy measures of relative intensity has administrative merit but risks potential misclassification, particularly in women and the elderly. This potential misclassification is the result of the inappropriate categorizing of activity based upon absolute intensity values regardless of age or gender. Coronary heart disease is more common in the elderly and recent activity more important than remote for cardiac benefit. It is thus essential to resolve the issue of the level(s) of exercise intensity needed to provide health benefits. Only when this information is available will it be possible to give rational and safe public health advice.