Invited commentary: a challenge for physical activity epidemiology.

Numerous epidemiologic studies report an inverse association between increasing levels of regular physical activity and reductions in major morbidity and mortality. Clinical guidelines emphasize the importance of specific amounts of regular physical activity, yet the majority of US adults do not meet these recommended levels. There are comparatively less data on nonexercise, or lifestyle, physical activity that may occur throughout the day in an unstructured setting with variable duration and frequency. In this issue (Am J Epidemiol 2007;165:1343-1350), both regular exercise and other forms of nonexercise physical activity are examined in relation to total and cause-specific mortality in Chinese women. This work illustrates several important issues in physical activity epidemiology regarding the identification, measurement, and analysis of nonexercise components of physical activity. Unlike semiquantitative food frequency questionnaires for studies of individual foods and nutrients, physical activity epidemiology lacks a prevailing measurement tool and analytical technique by which to examine nonexercise physical activity. Our challenge is therefore to more accurately and consistently measure these more subtle components of nonexercise physical activity to better understand its potential clinical effects and role in disease prevention.