Definition of shoulder joint reaction forces

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1

Hello,

I currently try to get the shoulder reaction forces split up in compressive and shear forces.
In the SelectedOutput of the Bodymodel are the predefined floats

  • GlenoHumeral_DistractionForce,
  • GlenoHumeral_InferoSuperiorForce and
  • GlenoHumeral_AnteroSuperiorForce.

These are defined as the x, y and z-components of the FTotalLocal-vector from GHReactions.
FTotalLocal is defined in the system Scapula.gh. I did not find a rotation of Scapula.gh so its axes remain those of the scapular segment.

My question is, why this reference frame is used.

In GHReactions the glenoid cavity is defined using a circular two dimensional simplification and the x-axis of Scapula.gh is not perpendicular to this glenoid-plane. In my understanding, this way the x-component (used as compressive force) has a shear component.
Would not a representation in a alternative system make much more sense? For instance the RefNode GHReactionCenterNodeRotated in GHReactions, which is used for the definition of the glenoid cavity.
Am I missing the intention behind this definition?

For better understanding:
left: the camera view orientated towards the z-axis of GHReactionCenterNodeRotated (yellow).
right: the camera view orientated towards the x-axis of Scapula.gh (green).

Glenoid view
Glenoid view1920×723 74.7 KB

I am very grateful for AnyAnswer.

Best regards
Leonhard

AnyBody version: 7.3.4.8518
AMMR version: 2.2.3

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Hi Leonhard

I think you are absolutely right. It seems that the three output variables are given with respect to the orientation of the scapula coordinate system. I wonder if once the the Scapula.gh node once had a different orientation.

Would not a representation in a alternative system make much more sense?

Yes. Think it will. I will suggest that we make an update to the AMMR so it get fixed in AMMR 2.5.

I your case I would recommend that you create your own corrected output variables. You could also change the AMMR directly, but unless you use a version control system, it will make it harder to update your models to the newest AMMR later.

1 Like
3

Hello Mr Lund,

Thank you for your quick reply despite the summer holidays.
You helped me a lot.

Best regards
Leonhard

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