1. S. Romkema, MSc, Department of Rehabilitation Medicine, University of Groningen, University Medical Center Groningen, CB40, Hanzeplein 1, 9700 RB, Groningen, the Netherlands.
2. R.M. Bongers, PhD, MSc, Center of Human Movement Sciences, University of Groningen, University Medical Center Groningen.
3. C.K. van der Sluis, MD, PhD, Department of Rehabilitation Medicine, University of Groningen, University Medical Center Groningen.