1. Precision and Microsystems Engineering, Department of Mechanical Engineering, Delft University of Technology, Delft 2628 CD, The Netherlands e-mail:
2. Department of Rehabilitation, Donders Center for Neuroscience, Radboud University Medical Center, Nijmegen 6500 HB, The Netherlands
3. Department of Physics and Medical Technology, VU Medical Center, Amsterdam 1081 BT, The Netherlands
4. Precision and Microsystems Engineering, Department of Mechanical Engineering, Delft University of Technology, Delft 2628 CD, The Netherlands