Tutorial 3 - Scaling, Inverse Kinematics, and Inverse Dynamics

In this tutorial, you will be using a generic musculoskeletal model with 23 degrees of freedom and actuated by 54 muscles entitled 3DGaitModel2354.
Note: Detailed information about the model can found on the Gait2392 and 2354 Models page

To load the generic musculoskeletal model into OpenSim:

• Click the File menu and select Open Model.

• Find the Gait2354_Simbody folder in your default OpenSim resources directory— \Documents\OpenSim\Models for PC and Mac.  
  Note: When you first launch OpenSim, you are prompted to provide a path to install the resources folder, the default is in your systems Documents folders.

• Open the Gait2354_Simbody folder, select the file gait2354_simbody.osim, and click Open.

Subject-specific modeling involves (i) scaling a generic musculoskeletal model to modify the anthropometry, or physical dimensions, of the generic model so that it matches the anthropometry of a particular subject and (ii) registering the markers placed on the model to match the locations on the subject. Scaling and Registration are the most important steps in solving inverse kinematics and inverse dynamics problems because IK and ID solutions are sensitive to the accuracy of the scaling and registration.

To scale the generic model and register the markers: 

• Click the Tools menu and select Scale Model.
  
  
  

• At the bottom of the Scale Tool dialog, click Load to input a settings file.
  
  
  

• In the file browser, ensure that you are in the Gait2354 folder, select the file subject01_Setup_Scale.xml and click Open.
  
  
  

This Scale Setup file is an xml file that contains pre-configured settings to scale and register the generic gait2354 musculoskeletal model to the dimensions of a subject that we have experimental data for. A detailed explanation of the Scale Tool can be found on the Scaling page of the documentation.

In OpenSim, the scaling step adjusts both the dimensions of the body segments, as well as the mass properties (mass and inertia tensor). Scaling can be performed using a combination of two methods:

(1) Manual Scaling: Scaling that allows the user to scale a segment based on some predetermined scale factor. Manual scaling is sometimes necessary when suitable data are not available, or if the scale factors were determined using an alternative algorithm.

(2) Measurement-based Scaling: Scaling that determines scale factors for a body segment by comparing distance measurements between specified landmarks on the model, known as model markers, and the corresponding experimental marker positions.

In OpenSim, the registration step adjusts the location of model markers to match the location of markers on the subject. To do this, you must first estimate a pose for the model that closely resembles the pose of the subject during the experimental static trial.

To complete the scale step:

• In the Scale Tool dialog, click Run.
  
  
  

• When complete, a new, scaled and registered model entitled subject01 will appear in Visualizer window. Notice the pink model markers around the new model. 
  
  
  

• To save the scaled model, either click File and select Save Model, or right-click on the model name, subject01, in the Navigator window, and select Save As.

• Save the scaled model as gait2354_scaled.osim, and click Save.
  Note: ensure that you are in the Gait2354 folder.
  
  
  

• Once you have answered Questions 1-5, below, close the Scale Tool Dialog by clicking Close. At this point, you may close the generic model (right-click the model name in the Navigator window, and select Close) or hide the model (right-click the model name, and select Display -> Hide).