Angle normalization?

[Mohammadreza Rezaie]

Dear Experts

I have some queries (problems) about angles normalization and static position and hope you would help me.
1) As I know and read, the angles (IK) are relative to static positions and the static posture is the reference. So it seems that the static pose must be loaded before running IK or be added as an input to IK setup file which isn't. But I saw that the results of IK are the same with and without static pose (zero degree coordinates) which might not be reasonable. One possibility might be the adjusted markers (after scaling).

2) As the static posture is not the same between individuals, Isn't it necessary to normalize the angles relative to static posture (before reporting the results)? I saw the normalization with the lower limb length, but what about the static pose? Some researchers agree to subtract the static pose from angles but some others strongly disagree with this type of normalization.

I wonder if anyone would clarify these issues for me.

Kindly Regards

[Michael Asmussen]

Hi Mohammadreza,

For Question 2, maybe reply in a separate post because your question seems to be more about common/best practices in biomechanics.

With respect to your first question, are you referring to common practices for determining joint angles from a static posture or determining joint angles from an opensim model after performing the model constrained inverse kinematics routine? Also, what IK tasks are you using for the inverse kinematics? These inputs will also affect your joint angle outputs

But I saw that the results of IK are the same with and without static pose (zero degree coordinates) which might not be reasonable.

Could you clarify this point? Is this all completed within opensim? Maybe check out the documentation on IK and that might help answer your question. https://simtk-confluence.stanford.edu:8 ... tics+Works

I hope this is helpful!

Mike

[Mohammadreza Rezaie]

Hi Michael,

Thanks a lot,

I post my second issue on a new post as you asked for.

My first query was about using the static_output.MOT file (which is generating after Scale tool) for IK. Why the result of IK with and without this file is the same? I mean if this file is loaded to model or not (all coordinates set to zero), the results of IK with the same IKTaskSet are exactly the same.
My assumption is that this file must be loaded before running IK so that the zero values in IK become the static coordinates. Maybe OpenSim considers the static position by new markers placement after Scaling. I don't know.

Any help for clarifying this issue is much appreciated.

Regards

[Michael Asmussen]

Hi Mohammadreza,

Are the outputs the exact same? Do you mind uploading the files?

Just to clarify, you are running the IK with an unscaled model and a scaled model and getting the same results? Can you maybe share your set-up file for IK?

All the best,

Mike

[Mohammadreza Rezaie]

Hi Michael,

Sorry for the confusion,

Not unscaled model. After scaling, there is a static position output (mot file) which is one line for all coordinates. If I load these static positions before running IK or not (all coordinates are zero), The IK results are the same. There is no specific file I can share. Here I used default gait_2392 model files. Have you ever tried that?

These images are one scaled model with and without static coordinates.

2.png (29.02 KiB) Viewed 546 times

1.png (30.55 KiB) Viewed 546 times

But how it is possible to have a same IK result?

3.png (75.02 KiB) Viewed 546 times

Best,
Mohammadreza

[Michael Asmussen]

Hi Mohammadreza,

The IK needs a model to run procedure. My thoughts are that you scaled you model and the .mot file is the posture (coordinates) of your scaled model. When you run IK, you need a model, which if I understand correctly, is your scaled model. If you run IK with or without the .mot file, it would make sense why you have the same results because the procedure is based off that scaled model and your IK tasks (weights).

I hope that answers you question.

All the best,

Mike