1. University Children's Hospital and Medical Centre Utrecht, Child and Development and Exercise Centre, University Medical Centre Utrecht PO Box 85090, 3508 AB Utrecht (e-mail: h.hulzebos@umcutrecht.nl)
2. Department of Physiotherapy, Physical Functioning in Chronicity, School for Public Health and Primary Care (CAPHRI), Maastricht University, Maastricht, and Health∼Holland (Topsector Life Sciences and Health), The Hague, The Netherlands