1. Program for Human Movement Science, Faculty of Social Sciences and Technology Management, Norwegian University of Science and Technology (NTNU), Trondheim, Norway, , Tambartun National Resource Center for the Visually Impaired, Melhus, Norway
2. Department of Epidemiology, The Rollins School of Public Health, Emory University, Atlanta, GA, flanders@ sph.emory.edu, Department of Community Medicine, Section of Epidemiology, Faculty of Medicine, Norwegian University of Science and Technology (NTNU), Trondheim, Norway