## Use Fiber Force as input in Forward Dynamics tool

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Thomas Uchida
Posts: 1183
Joined: Wed May 16, 2012 11:40 am

### Re: Use Fiber Force as input in Forward Dynamics tool

Our main goal is to represent the real behaviour of our subjects, in order to classify the severity of the disorder of each one
If you don't know the severity (for example, the amount each muscle contracts when it's stretched), then it seems like you could end up with a problem that cannot be solved. An analogy would be knowing the displacement of a rope in a tug-of-war but not knowing how many people are pulling each end. How are you going to ensure the problem has a unique solution?
if we simply run CMC with the subjects movement data (joint coordinates and external forces) it will compute the real muscle excitation, aren't we?
CMC will compute muscle excitations that produce an observed motion, resolving redundancy using a least-squares solution. This solution may not be appropriate for your study, but it depends on how the problem is posed, how you have modified the muscle model, etc.
I don't understand how OpenSim will distinguish between a healthy subject performing an slow movement and a subject with spasticity.
You will need to build either a model or a solver that takes this into account.

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Eduardo Sáenz Aldea
Posts: 18
Joined: Thu Oct 25, 2018 2:47 am

### Re: Use Fiber Force as input in Forward Dynamics tool

If you don't know the severity (for example, the amount each muscle contracts when it's stretched), then it seems like you could end up with a problem that cannot be solved. An analogy would be knowing the displacement of a rope in a tug-of-war but not knowing how many people are pulling each end. How are you going to ensure the problem has a unique solution?
We were thinking about making an iteration between the real subject movement and forces(we are using a robot to get this data) and the model in OpenSim, so we can try different spasticity gains in the software in order to get the one that better fits the real subject values. It's similar to the Ashworth scale technique that physiotherapists use in their diagnosis:
https://scireproject.com/outcome-measur ... fb7a3-f550
This solution may not be appropriate for your study, but it depends on how the problem is posed, how you have modified the muscle model, etc.
You will need to build either a model or a solver that takes this into account.
That's exactly why I'm building a new model class based on Thelen2003Muscle, so we'll be able to modify the excitations the spastic muscle is generating. Furthermore, the project is meant to work only with patients with spasticity, so we won't need to differentiate between healthy subjects and those with spasticity. And this is why I'm looking for a method in Thelen2003Muscle or relate classes that will be called during a CMC and can modify these excitations for each state. By the way, as I've seen in previous researches, this excitation is proportional to muscle fiber velocities.

I'm really sorry if I didn't explain it clear in the first time.

Thank you!

Thomas Uchida
Posts: 1183
Joined: Wed May 16, 2012 11:40 am

### Re: Use Fiber Force as input in Forward Dynamics tool

I think you could write a new MuscleFirstOrderActivationDynamicModel class (https://simtk.org/api_docs/opensim/api_ ... Model.html) that takes fiber velocity into account and replace the default activation model in Thelen2003Muscle with yours. The new activation model will need to know fiber velocity so it might take some work to figure out the details. Essentially, you would be replacing the "Activation Dynamics" block in Figure 2 of Millard et al. (https://nmbl.stanford.edu/publications/ ... rd2013.pdf), but now one of the inputs to that block will be v^M.