1. Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK
2. Division of Biomechanics and Engineering Design, K.U. Leuven, B-3001 Leuven, Belgium
3. Institut für Mathematik, Martin-Luther-Universität Halle-Wittenberg, 06099 Halle (Saale), Germany
4. Magdalen College, University of Oxford, Oxford OX1 4AU, UK