Reflections on four decades of physical activity epidemiology

• Broad topics in physical activity (PA) epidemiology have remained the same over 40 years; specific issues and questions have changed and challenges remain; • Accelerometry has made many advances in PA measurement but created new methodological challenges; • The benefit of regular PA for a wide array of health outcomes has been well established, but specificity is still needed about dose response relations; • Interventions in the population have led to increased prevalence of recommended levels of PA, but prevalence is still less than optimal and there are racial and ethnic disparities.