Muscle Force from Static Optimization vs. Force Reporter

[Alex MacIntosh]

Hello,

I am hoping for some clarification on the difference between the muscle forces from Static Optimization and from the Force Reporter in the Analysis tool. I Believe I understand the method for Static Optimization, I am unsure however about the Force Reporter.

In the ForceReporter.cpp source code there is a comment about being all forces exerted on a model during a simulation. Unfortunately however, I do not yet understand the language well enough to follow what specifically is happening.

If anyone would help me to understand why a muscle would have a different force value from the Force Reporter compared to the Static Optimization result it would be greatly appreciated.

Thanks,

Alex

[jimmy d]

Hi Alex,

The forcerReporter asks the component (in this case, a muscle) what are the forces acting upon it. A muscle component will output its force as a tension across the muscle-tendon unit.

The differences between static opt and force reporter would most likely be accounted for by the inputs into the analysis. For example, if you run the force reporter analysis on just the kinematics, you will get back the passive forces from the muscle component.

Could you clarify when/where you are using the analysis?

cheers,
-james

[Luca Modenese]

Hi James,

I am running some simulations with a model including constraints and using static optimization and I have noticed an issue similar to what Alex reported. In my case the muscle forces from ForceReporter are way larger than in the SO simulations.

I thought a workaround could have been assigning the states resulting from static optimization as input through a storage file in the Analysis panel, but I have noticed that the muscle states from SO are all reported as a default value (muscle length 0.1, activation 0.05, I am using Thelen2003 muscles). In your opinion is there a way of setting ForceReporter so that it will use forces occurring in SO simulations?

Thank you in advance for your help,

Luca

[jimmy d]

Hi Luca,

The problem most likely revolves around SO not generating a true state. Its a state-like object that some people requested, but it has no information on fiber length among other things.

So whatever you put into the Analysis tool from SO is not going to give you a good answer. Really, the Analysis tool shouldn't even take incomplete states. Its a design flaw on our part and we are working to improve it in the next release.

Hope that helps,
- James

[Luca Modenese]

Thank you James!