1. M-LB Lenssinck, PT, BC, is Physiotherapist and Research Assistant, Department of General Practice, Erasmus Medical Centre Rotterdam, Rotterdam, the Netherlands
2. AC Frijlink, PT, BC, is Physiotherapist, Department of Physiotherapy, Hogeschool Rotterdam, Rotterdam, the Netherlands
3. MY Berger, PhD, MD, is Epidemiologist and Senior Researcher, Department of General Practice, Erasmus Medical Centre Rotterdam
4. SMA Bierma-Zeinstra, PT, PhD, is Physiotherapist and Associate Professor, Department of General Practice, Erasmus Medical Centre Rotterdam
5. K Verkerk, PT, MSc, is Physiotherapist and Assistant Professor, Department of Physiotherapy, Hogeschool Rotterdam
6. AP Verhagen, PT, PhD, is Physiotherapist, Epidemiologist, and Senior Researcher, Department of General Practice, Erasmus Medical Centre Rotterdam, PO Box 1738, 3000 DR Rotterdam, the Netherlands .