1. Swiss Paraplegic Research, 6207 Nottwil, Switzerland
2. Institute of Sports Medicine, Swiss Paraplegic Centre Nottwil, 6207 Nottwil, Switzerland
3. Department of Health Sciences, University of Lucerne, 6002 Lucerne, Switzerland
4. Institute of Social and Preventive Medicine, University of Bern, 3012 Bern, Switzerland