Knee joint axis dependant on C3D markers

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1

Hello,

I am trying to make the knee joint axis of rotation dependant of C3D markers.
I have a problem with the RotMat Function, in the definition of ARel of the joint.

As told in the Reference manual, I must set 3 points or reference:

the origin: I took the average between the coordinates of a knee external and a knee internal markers (to have the center of the knee, dependant of my 2 markers).

the x axis: I took R/LKNE marker (but I think this marker is for z axis)

the y axis: I took the HipJointRight/Left TD

The results is not satisfying so I gess the error comes from the x axis is not correcty defined?
Sould I have an additional maker in the front of the knee for example to set correctly the x axis?

Is it possible to change the coordinates system of the joint, to make it rotate about the x axis (and keep the same movement)?

For now, I set the ARel propertie of the knee joint in the HumanModel folder, in order to separate Right en Left.

I use AMMRV1.4.
I joint the model I’m working with.

Thank you for helping,

Best regards,

Claude Pothrat

2

Hi Claude,

When using the RotMat 3 points function the X axis is always the principal (the one realy defined by the nodes). If you want it to be another axis you can simply add a serie of 90 deg rotations to switch around the axis. For example:

ARel = RotMat(node1, node2, node3) * RotMat(pi/2, y);

That way Z axis is taking the place of X axis and goes through the KNEE marker.

Best regards, Sylvain.