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The tutorial below is designed for use with OpenSim version 4.0 and later. A version of the tutorial compatible with OpenSim version 3.3 is available here. |
I. Objectives
Purpose
The purpose of this tutorial is to demonstrate how OpenSim solves an inverse kinematics and dynamics problem using experimental data. To diagnose movement disorders and study human movement, biomechanists frequently ask human subjects to perform movements in a motion capture laboratory and use computational tools to analyze these movements. A common step in analyzing a movement is to compute the joint angles and joint moments of the subject during movement. OpenSim has tools for computing these quantities:
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To set up an inverse kinematics analysis: - Open the Inverse Kinematics Tool dialog window by clicking the Tools menu and selecting Inverse Kinematics.
- Load an inverse kinematics tool setup file by clicking Load, selecting the file subject01_Setup_IK.xml, and clicking Open.
Note: In the file browser, ensure that you are in the Gait2354_Simbody folder,
subject01_Setup_IK.xml contains pre-configured settings for the inverse kinematics tool. Notice the text boxes in the dialog window are now are filled with values. A detailed explanation of the Inverse Kinematics Tool can be found on the Inverse Kinematics page of the documentation. |  |
Navigate to the Weights tab. - View which markers are selected for use in the inverse kinematics analysis, and their corresponding weights.
- Enable the tracking for the marker R.Knee.Lat. Notice the row turns red and the Run button is now greyed out. You will be unable to run the inverse kinematics tool because there is no experimental data found for the R.Knee.Lat marker in the subject_walk1.trc file. Disable the R.Knee.Lat marker and notice the Run button is now clickable.
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To perform inverse kinematics: - Click Run. The model will begin to move as the inverse kinematics problem is being solved for each frame of the experimental data.
- Notice the progress bar in the lower right-hand corner of the program. Wait until the bar disappears before proceeding.
Note: Closing the inverse kinematics tool dialog during the analysis doesn't affect the Inverse Kinematics tool running.
To visualize the inverse kinematics solution, animate the model by using the motion slider and video controls. The model should walk through one full gait cycle.
Note: You can loop  and control the speed of the animation.
The inverse kinematics solution is saved to subject01_walk1_ik.mot, as specified in the setup file.
Note: Be sure to use the exact file name given, as this file is used later.
To compare experimental marker data with inverse kinematics results, in the Navigator panel, go to Motions and right-click on IKResults (which are what the Inverse Kinematics Tool just generated). Then choose Associate Motion Data... from the drop-down menu. Choose subject01_walk1.trc and click Open. Model markers are shown in pink and experimental markers are shown in blue. Hit play in the Motion Toolbar. The virtual markers should correspond closely to the experimental marker locations as the animation proceeds.
Note: If using a Virtual Machine on a Mac, Command + Ctrl + Left Click on each motion.
- Click the Window menu and select Messages. The Messages window records details of all steps you have performed. Take a minute to explore the Messages window. Then, scroll to the very bottom. The line above InverseKinematicsTool completed... provides the markers errors and model coordinate errors (e.g., joint angle errors) associated with the last frame of the motion.
Note: All marker errors have units in meters, and all coordinate errors have units in radians.
- Once you have answered Questions 4-6, below, close the Inverse Kinematics Tool Dialog by clicking Close.
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