Static optimization is an extension to inverse dynamics that further resolves the net joint moments into individual muscle forces at each instant in time. The muscle forces are resolved by minimizing the sum of squared (or other power) muscle activations.
To launch the Static Optimization Tool, select Static Optimization… from the Tools menu. The Static Optimization Tool dialog window, like all other OpenSim tools, operates on the current model open and selected in OpenSim.
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Overview
The figure below shows the required inputs and outputs for the Static Optimization Tool. Each is described in more detail in the following sections.
Inputs and Outputs of the Static Optimization Tool. Experimental data are shown in green; OpenSim files (.osim) are shown in red; settings files are shown in blue; files generated by the workflow are shown in purple. To run static optimization, you use the analyze command.
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The file names are examples that can be found in the examples/Gait2354_Simbody directory installed with the OpenSim distribution. |
Inputs
Three files are required as input by the Static Optimization Tool:
subject01_walk1_ik.mot: Motion file containing the time histories of generalized coordinates that describe the movement of the model. This can be kinematic data (i.e., joint angles) from IK or states (i.e., joint angles AND velocities) from RRA for the time range of interest.
subject01_walk1_grf.xml: External load data (i.e., ground reaction forces, moments, and center of pressure location). Note that you must measure or model all external forces acting on a subject during the motion to calculate accurate muscle forces. The .xml file describes how to apply the measured ground reaction forces to the model during the analysis.
subject01_simbody.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 (segment masses, etc.).
x: The exponent for the activation-based cost function to be minimized (i.e., the criterion used to solve the muscle force distribution problem).
Outputs
The Static Optimization Tool generates three files in a specified folder:
subject01_walk1_StaticOptimization_controls.xml: Contains the time histories of muscle activations. These controls were minimized by the Static Optimization Tool.
subject01_walk1_StaticOptimization_activation.sto: Storage file containing the time histories of muscle activations.
subject01_walk1_StaticOptimization_force.sto: Storage file containing the time histories of muscle forces.
Best Practices and Troubleshooting
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