Simulation with external force and SO

[Maurice Lang]

Hi everyone,
I'd like to perform a simulation with arm26.osim model, in particular I'd like to see how adding a force to the model the joint reactions and the muscle activations change, but I have some doubts about the process that I have to follow.

I've create a motion file and with it I've run Inverse Dynamic tool adding the force in the panel "External Loads" and selecting the body where the force acting. Is it correct?

After that I've run Static Optimization tool and in this way,if I understand correctly,I find the individual muscle forces at each instant in time and their activation.

To see Joint reaction I've run the Analysis tool -->joint reaction editing with the muscleforce.sto obtained with the SO.

I don't understand in the analysis tool the voice about "MuscleAnalysis" because I obtain a lot of files but to see how muscle activations change with or without a force I don't know which of this files is necessary, or maybe if is the file obtained as a results with the static Optimization thaat I've to see

Thank you

Maurice

[Dimitar Stanev]

Hi Maurice,

Please consider spending some time into understanding how OpenSim works, it will help you solve some of your problems:

https://simtk-confluence.stanford.edu:8 ... %27s+Guide

The documentation is very good and contains the underlying theory, examples and tutorials.

I've create a motion file and with it I've run Inverse Dynamic tool adding the force in the panel "External Loads" and selecting the body where the force acting. Is it correct?

If you are interested to see how a force affects the motion, muscle and reaction forces then you do not want to perform ID. Think of this in the simplest form of Newton's equation m*a = f. If you specify f you can solve for a, this is called forward dynamics. If you specify a, then you solve for f, this is called inverse dynamics. You provided a motion and an external force, thus the joint forces and muscle activations will be higher (depends on the force). Is this what you are interested for or would you like to observe the resultant motion by applying a force? If this is the case then you need to implement a prescribed force and perform forward dynamics.

https://simtk.org/api_docs/opensim/api_ ... Force.html

This can be done in xml too.

After that I've run Static Optimization tool and in this way,if I understand correctly,I find the individual muscle forces at each instant in time and their activation.

Yes.

To see Joint reaction I've run the Analysis tool -->joint reaction editing with the muscleforce.sto obtained with the SO.

Moreover, you need to be careful with the rest of the parameters (e.g. step_interval). They must be the same both for StaticOptimization and AnalysisTool.

https://simtk-confluence.stanford.edu:8 ... tion+Loads

I don't understand in the analysis tool the voice about "MuscleAnalysis" because I obtain a lot of files but to see how muscle activations change with or without a force I don't know which of this files is necessary, or maybe if is the file obtained as a results with the static Optimization thaat I've to see

MuscleAnalysis records additional properties of the muscle that may be of interest.

https://simtk.org/api_docs/opensim/api_ ... ml#details

Best

[Maurice Lang]

Thank you Dimitar for your reply.

I read again the guide to better understand how OpenSim works and the link that you wrote to me.

So,if I want to observe what motion my force will produce I have to performe Forward Dynamic adding a prescribed force,
but, as in my case, if I'm interested to see joint forces and muscle activations given a motion and an external force I have only to perform SO and Analysis tool.
and in this case I have to add the force in the "external load" box selecting the body and so on. Correct?

ExternalLoadBox.JPG (27.7 KiB) Viewed 361 times

Thanks
Maurice

[Dimitar Stanev]

Yes.