1. Intelligent Control Systems Group, Max Planck Institute for Intelligent Systems, 70569 Stuttgart, Germany
2. Optimization, Robotics and Biomechanics, Institute of Computer Engineering, University of Heidelberg, 69120 Heidelberg, Germany
3. Department of Movement Science, Institute of Sport and Exercise Sciences, University of Münster, 48149 Münster, Germany
4. Dynamic Locomotion Group, Max Planck Institute for Intelligent Systems, Stuttgart, Germany