Peter Federolf1, Katharine Boyer2, Thomas Andriacchi3. 1. Department of Neuroscience, Faculty of Medicine, Norwegian University of Science and Technology NTNU, 7489 Trondheim, Norway; Department of Physical Performance, Norwegian School of Sport Sciences, Oslo, Norway. Electronic address: peter.federolf@ntnu.no. 2. Department of Kinesiology, University of Massachusetts-Amherst, Amherst, MA, USA. 3. Department of Mechanical Engineering, Stanford University, USA; Department of Orthopedic Surgery, Stanford University, USA; Veterans Administration, Palo Alto, CA, USA.