1. Department of Electrical and Computer Engineering, University of Washington, Seattle, WA 98195, USA.
2. Robotics Laboratory, SRI International, Menlo Park, CA 94025, USA.
3. Paul M. Rady Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO 80303, USA.
4. Schools of Physics and Biological Sciences, Georgia Institute of Technology, Atlanta, GA 30317, USA.
5. Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, BC V5A 1S6, Canada.