The model and optimization framework for simulating locomotion.

<p>The OpenSim musculoskeletal model has 9 degrees of freedom and 8 musculotendon actuators per leg. The dynamics of each musculotendon actuator are governed by force-length-velocity properties and first order excitation-activation dynamics. Ground contact is represented by two compliant spheres on each foot with Coulomb friction. The model’s controllers include stretch and force feedback, as well as proportional-derivative control. The parameters of these controllers and 18 initial position and velocities of the model constitute design variables that we optimized to generate simulations of locomotion.