Model Scaling, RSM and Maximum Marker Error

[Claudia Ghidini]

Hello everyone,

I am using opensim for gait analysis (kinematic) with lower-limb amputees but I am facing a problem when scaling my model.

I get high errors but then visually checking the experimental and model markers it looks like the error should be really low. Does anyone have any suggestions on what to change? I am only weighting markers on anatomical landmarks for scaling and I have slightly moved the markers on the general model to account for the prosthesis (e.g., femoral condyles are moved outer compared to able body people).

Also is there a way to export the markers error?

Thanks so much!

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Scaling Old prosthesis.PNG (220.69 KiB) Viewed 555 times

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Reported errors:
Frame at (t = 2.5): total squared error = 0.00967638, marker error: RMS = 0.0219959, max = 0.0488547 (L_MKNEE)

[Carmichael Ong]

There are some extra options with IK that you can use by editing an existing setup. You can create the setup file from the GUI by saving the settings, and then you can open up the setup file in a text editor (such as Notepad++).

One helpful way to see all the tags is the XML Browser (in the GUI, Help -> XML Browser) and then looking up the name that shows up in the setup file (in this case, InverseKinematicsTool). It will look something like this:

xmlbrowser-iktool.png (301.51 KiB) Viewed 465 times

Adjusting some of the flags, such as "report_errors" and "report_marker_locations" to true could be helpful for your use case here.

[Claudia Ghidini]

Thank you for your reply!

Anyways, I have found the problem in the scaling. I have modified the model but not exported the new markerset. So when I was scaling it was still using the unmodified markerset hence the error was still high even if the model looked visually correct.

For whoever needs to modify a model and use some manual marker measurement taken during data collection, remember to use the modified model + the new modified markerset when scaling.