1. Robotics and Rehabilitation Lab, Department of Mechanical Engineering, Columbia University, New York, NY 10027 e-mail:
2. Robotics and Rehabiliation Lab, Department of Mechanical Engineering, Columbia University, New York, NY 10027 e-mail:
3. Fellow ASME Professor Robotics and Rehabiliation Lab, Department of Mechanical Engineering, Columbia University, New York, NY 10027 e-mail: