1. Department of Rehabilitation Medicine, University of Groningen, University Medical Center Groningen, 9713 GZ Groningen, The Netherlands
2. Center for Bionics and Pain Research, 431 30 Mölndal, Sweden
3. Department of Orthopaedics, Institute of Clinical Sciences, Sahlgrenska Academy, University of Gothenburg, 405 30 Gothenburg, Sweden
4. Department of Bioengineering, Faculty of Engineering, Imperial College London, London SW7 2AZ, UK
5. Department of Human Movement Sciences, University of Groningen, University Medical Center Groningen, 9713 GZ Groningen, The Netherlands