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Posted: **Thu Jan 21, 2021 4:43 pm**

Hi,
I am working with a neck model, and I want to add an external load as a point force. We have an experiment where our test subject is pushing their head back against a resistance, and I want to simulate that to see what happens in my model, although I have no data for the force yet. Preferably via Matlab, but in the GUI is good too.
I looked in the tutorials, and I found the files specifying the ground forces and how I can change the force over time, but I can not figure out how to add a new force to the model. How do I create the file from scratch, so that it applies to my model? How do I specify where the force is applied, and at what angle?
If it is shown in the tutorials, I don't understand it and if it is documented somewhere else, I have not found it. Can someone please help explain or point me towards the appropriate documentation?
Thank you in advance,
Karin

Posted: **Fri Jan 22, 2021 1:37 am**

Hi

Maybe this code help! You can check the XML related components and the documentation related to PrescribedForce.

Code: Select all

         //------------------------------------------------------------------
         // Exteranal Force
         //------------------------------------------------------------------
         float extForceValY = 50 ; // 50 N external force
         PrescribedForce *myExtForce = new PrescribedForce("myExtForce", *myBdy);
         myExtForce->setName("myExtForce");
         double extForceTime[2] = {0, finalTime};                    // time nodes for linear function

         double fXofT[2]   = {extForce,  extForce};  // force values at t1 and t2
         // Create linear function for the force components
         PiecewiseLinearFunction *extForceValY = new PiecewiseLinearFunction(2, extForceTime, fXofT);
         // Set the force and point functions for the new prescribed force
         myExtForce->setForceFunctions(new Constant(0.0),extForceValY, new Constant(0.0));

         double fXofTPt[2] = {extForceMrkPtY, extForceMrkPtY};  // force values at marker point extForceMrkPtY
         PiecewiseLinearFunction *extForcePtY = new PiecewiseLinearFunction(2, extForceTime, fXofTPt);
         myExtForce->setPointFunctions(new Constant(0.0), extForcePtY, new Constant(0.0));

         myExtForce->setTorqueFunctions(new Constant(0), new Constant(0), new Constant(0));
         osimModel.addForce(myExtForce);

Posted: **Thu Jan 28, 2021 8:31 am**

Hi,
Thank you for your very fast answer! Would I use this script in MatLab then?
How do I find the coordinates in the frame that I want to use the force? I know where it is visually from a photo of our test subjects, but how do I know which coordinates it is and that I am in the correct frame?

Posted: **Sun Jan 31, 2021 3:48 am**

Would I use this script in MatLab then?

I don't use matlab but I think there should be equivalent way to do this.

How do I find the coordinates in the frame that I want to use the force?

There other software where you cam load a mesh e.g. stl file and fins location of different points, personally I use 3D slicer but I think you can use blender or something similar. Another way is to create a marker in opensim then keep changing its location until it is in the correct location.

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How do I make an external load point force on the neck?

How do I make an external load point force on the neck?

Re: How do I make an external load point force on the neck?

Re: How do I make an external load point force on the neck?

Re: How do I make an external load point force on the neck?