1. Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
2. Rehab Technologies Laboratory at Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
3. The Eric P. and Evelyn E. Newman Laboratory for Biomechanics and Human Rehabilitation, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA