[1] MIT, Dept Mech Engn, Cambridge, MA 02139 USA
[2] MIT, Dept Elect Engn & Comp Sci, Cambridge, MA 02139 USA
[3] Harvard Univ, Sch Med, Brigham & Womens Hosp, Dept Orthopaed Surg, Boston, MA 02115 USA
[4] VA Boston Healthcare Syst, Rehabil Engn Res & Dev, W Roxbury, MA 02132 USA