Inverse optimal control based identification of optimality criteria in whole-body human walking on level ground

Understanding the underlying concepts of human locomotion is important for many fields of research. Based on the assumption that human motions are optimal, we propose an inverse optimal control (IOC) based approach to identify the optimality criteria in human walking. To this end, human walking is modeled as a non-linear optimal control problem with a linear combination of elementary optimality functions as objective and a hybrid dynamics multi-body system as constraints. The developed IOC-framework is set up in a modular way and exploits the natural bi-level structure of the problem. It allows for a great flexibility in the choice of outer optimization techniques and inner dynamic models. In the present work, we use the developed IOC approach to identify weights of seven elementary criteria for seven walking motions captured from six different subjects. The considered optimality criteria address the minimization of joint torques for four sets of joints, head stabilization, the step length, and the step frequency. For all trials the algorithm performs successfully. Even though the identified weights differ observably between subjects, which explains the different walking styles, the correlation matrix gives rise to the hypothesis that there exists a significant correlation of optimality across subjects. The identification of optimality criteria in human walking is a very important issue for all disciplines, where a prediction of human behavior is needed. For example in medical applications to improve therapies or to develop new mobility devices, in sport science to improve training plans or in humanoid robotics to develop new walking strategies.