It was very useful for me to how I can find joint position. However, I would like to know the axes of joint coordinate (for flexion/extension, and supination/pronation).

On the other hand, using the following code, I could figure out the segment frames of ulna and radialis;

By the way, forearm segments’ coordinates are, of cause, different to that of the joint. These drawn frames were not helpful for me find out the joint coordinate.

You can access the orientation matrix of a node in global coordinate system with the Axes member. So for example for the humerus node of the elbow joint it will be Main.Model.HumanModel.Right.ShoulderArm.Seg.Humerus.fe.RotNode.Axes

And if you want to visualize the axes in the model view use the following code:

However, I want to know is that the coordinate system of the “whole elbow joint”, not the individual segment’s coordinate such as humerus, ulna, or radialis.
Does not Anybody model have the coordinate system of elbow joint as a one complex joint? not as a combination of several joints.

The reason why I am trying to find out the coordinate is to know the rotation direction of pronation/supination at elbow joint, however it’s hard to figure out with only humerus fe point of the human model.

The elbow joint in the repository is in fact constituated of two separate joints. One is the flexion/extension joint between humerus and ulna, the second is the pronation/supination joint between ulna and radius. So they are not located exactly at the same place and have slightly different orientations.

I gave you the code for the Humerus.fe node as example, but you can use the same approach for all the other joints. If you are interested in the pronation joint then have a look at the Ulna.ps2.RotNode.

Your answer is very helpful.
Then I may regard that the rotation axis of supination/pronation at elbow is the x-axis of Ulna.ps2.RotNode.
Thank you very much.