The purpose of Computed Muscle Control (CMC) is to compute a set of muscle excitations (or, more generally, actuator controls) that will drive a dynamic musculoskeletal model to track a set of desired kinematics in the presence of applied external forces (if applicable).

The Computed Muscle Control Tool is accessed by selecting Tools → Computed Muscle Control… from the OpenSim main menu bar. Like all tools, the operations performed by the Computed Muscle Control Tool apply to the current model.

Overview

The figure below shows the required inputs and outputs for the Computed Muscle Control Tool. Each is described in more detail in the following sections.


Each of the files in this figure can be categorized as either experimental data, an OpenSim file (.osim), a settings file, or a file generated by the workflow.

The file names are examples that can be found in the examples/Gait2354_Simbody directory installed with the OpenSim distribution.

Settings File

The subject01_Setup_CMC.xml file is a setup file for the CMC Tool, which specifies settings, inputs, and outputs that affect the behavior of the tracking controller to determine actuator (including muscles) controls. These can be defined using the GUI or by hand. Details of the settings are described in the section on the Graphical User Interface.

The setup file identifies the actuators (i.e., the residual actuators, as required for dynamic consistency) as well as the kinematic tracking tasks. Furthermore, control constraints on the actuators (to limit the maximum residual force) can be specified.

Inputs

Several data files are required as input by the Computed Muscle Control Tool:

subject01_walk1_RRA_Kinematics_q.sto: Contains the time histories of model kinematics, including the joint angles and pelvis translations from RRA.

gait2345_CMC_Tasks.xml: The tracking tasks file specifying which coordinates to track and the corresponding tracking weight (weights are relative and determine how "well" a joint angle will track the specified joint angle from RRA).

gait2345_CMC_ControlConstraints.xml: Contains limits on model actuators, which include muscles, reserve and residual actuators. The control constraints file specifies the maximum and minimum "excitation" (i.e., control signal) for each actuator. Control constraints can also be used to enforce when certain actuators are "on" or "off" and the range in which they can operate.

subject01_walk1_grf.xml: External load data (i.e., ground reaction forces, moments, and center of pressure location). See Inverse Dynamics for more details.

subject01_simbody_adjusted.osim: A subject-specific OpenSim model generated by scaling a generic model with the Scale Tool or by other means, along with an associated marker set containing adjusted virtual markers. The model must include inertial parameters. The model should have an adjusted torso center of mass to reduce residuals.

gait2345_CMC_Actuators.xml: Contains the residual and reserve actuators, as in RRA.

Outputs

The Computed Muscle Control Tool primarily reports the necessary controls:

subject01_walk1_controls.xml: Contains the excitations to individual muscles as well as controls for any residual and/or reserve actuators.

subject01_CMC_forces.sto (not shown): Muscle forces and reserve/residual forces and torques.

subject01_walk1_states.sto (not shown): Model states and muscle states of the simulated motion (i.e., joint angles AND velocities, muscle fiber lengths AND activations).

Best Practices and Troubleshooting Tips

 

 

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