^a Department of Mechanical Engineering, The University of Texas, Austin, TX, USA

^b Rehabilitation R&D Center, VA Palo Alto HCS, Palo Alto, CA, USA and Department of Functional Restoration, Stanford University, Stanford, CA, USA

^c Rehabilitation R&D Center, VA Palo Alto HCS, Palo Alto, CA, USA and Departments of Functional Restoration and Mechanical Engineering, Stanford University, Stanford, CA, USA