1. Department of Physical Medicine and Rehabilitation, Harvard Medical School, 300 First Avenue, Charlestown, MA 02129, USA
2. School of Electrical and Electronic Engineering, University College Dublin, Belfield, Dublin 4, Ireland.
3. School of Engineering and Applied Science, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA.
4. Wyss Institute for Biologically Inspired Engineering, Harvard University, 3 Blackfan Circle, Boston, MA 02115, USA.