simtk.org • Anyway to determine STA metrics in OpenSim - Page 2

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Re: Anyway to determine STA metrics in OpenSim

Posted: Sun Apr 23, 2023 2:31 am

by kernalnet

Yes, it changes relative to global reference frame (ground) during motion at each time frame. For example, you can get the 3*3 rotation matrix of pelvis in global reference frame:

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model.getBodySet().get('pelvis').getTransformInGround(state).R().asMat33()

or as angle:

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model.getBodySet().get('pelvis').getRotationInGround(state).convertRotationToBodyFixedXYZ()

and you will see that it changes over IK time.

Similarly, the position of markers changes in global reference frame:

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marker.getLocationInGround(state)

But it is steady in local reference frame.

Is that what you are asking?

Re: Anyway to determine STA metrics in OpenSim

Posted: Sun Apr 23, 2023 3:14 am

by vigneshrk

Hi,

Yes thank you. I wanted to determine how much each marker moves in their respective body reference frame (definition of STA) but in OpenSim it remains the same ( I assume due to rigid body assumption).

Is there a way to check which transformation matrix is bad? For example, residual error is higher for one marker compared to another. Is there a way to relate it to how much the skin moves or how much the segment moves ?