Your nervous system has a tough job. When your repetitive walking motion is disturbed by changes in terrain or contact with obstacles, your nervous system has to fire signals to muscles in just the right way to keep you from falling over. This isn't easy: your nervous system needs at least 50 milliseconds to adjust a muscle's force in response to a disturbance. Fortunately, your muscles are endowed with mechanical properties (force depending on length and velocity) that can make a muscle respond to a disturbance without delay. My colleagues and I showed that these mechanical properties of muscles help stabilize a walking human model when we push the model in various directions. This resolved a long-standing mystery in human movement science, showing how your body responds to disturbances despite delays in your nervous system's response mechanism.
For the new muscle model I created whose force-length-velocity properties have been removed, click here. For the rest of the software framework, see the tagged version 1.5.5 of the OpenSim source code. If you are not a member of the OpenSim project, please request access to the source code from one of the project leads listed on the OpenSim project page.