1. Department of Physical Therapy, Faculty of Medicine, University of Iceland,
Reykjavík, Iceland
2. Directorate of Health,
Reykjavík, Iceland
3. Department of Social Sciences, Faculty of Social and Human Sciences, University of Iceland,
Reykjavík, Iceland
4. Centre of Public Health Sciences, Faculty of Medicine, University of Iceland,
Reykjavík, Iceland
5. Department of Epidemiology, Harvard School of Public Health,
Boston, MA, USA