1. Department of Biomedical Engineering, Northwestern University, Evanston, IL 60208, USA
2. Department of Physical Therapy and Human Movement Sciences, Northwestern University, Chicago, IL 60611, USA
3. Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104-6321, USA
4. Department of Neuroscience, University of Pennsylvania, Philadelphia, PA 19104, USA
5. Research Service, Edward Hines Jr VA Hospital, Hines, IL 60141, USA