GH rotation angles

I have a simple question.

There are 3 components in shoulder kinematic results, [0], [1], and [2].
What is the meaning and the direction of each GH rotation angle?

Plus, there are some other results in ‘InterfaceFolder’.
(ex: HumanModel.Left.ShoulderArm.InterfaceFolder.GlenohumeralFlexion.Pos)
What do they mean?


You have to refer to the corresponding kinematic measure on the script. Looking at HumanModel.Left.ShoulderArm.Jnt.GHRot you will see this:

AnyKinRotational GHRot = {
AnyRefNode &scapula_gh = …;
AnyRefNode &humerus_gh = …;
Type = RotAxesAngles;
Axis1 = y;
Axis2 = z;
Axis3 = y;

The sequence of rotation of this measure is Y,Z,Y. So the component [0] is a rotation about Yaxis, component [1] is about the Z axis and component [2] is agian a rotation about the Y axis. This is a specific way to describe the GH rotations.

If it is not the way you want you can use the more general one of the interface folder. Again have a look at the kinematic measure in the script. This rotation measure uses the three axis (X,Z,Y sequence) and the three components have been separated and named using descriptive anatomical directions.

So the two measures refer to the same rotational motion, but just describe it differently.

Best regards, Sylvain.

Thanks for your great answer, Sylvain.
I have some more questions.

  1. Which coordinate system is involved in these rotations(Y and Z), scapula_gh or humerus_gh?

  2. Can I regard these components [0], [1], and [2] as anatomical directions such as flexion, abduction, or external rotation?

  3. I plotted results from the interface folder, but their min/max values are different than expected.
    What is the neutral position for each rotation angle?


the reference coordinate system is the first mentioned in the measure, so in the case of GHRot it is scapula_gh.

You can consider the component as the anatomical directions you mentioned in the case of the interface folder measure. The GHRot (YZY) is a little different: fist component gives the plane of elevation, second is the elevation in the previously defined plane, and third is internal/external rotation.

You have to remenber that the sequence is important. In the interface folder the GH rotation is always described with first abduction (x) followed by flexion (z) followed by external rotation (y). If you have described the motion with a different sequence in your experiments for example, then it is normal that the angles differ. It has to be the same way of describing the motion if you want to compare it.
the neutral position places the arm along the body, you can just drivr it to zero in the FreePosture model to see it if you want. Another good thing is to display the two ref frames.

Best regards, Sylvain.