definition of knee joint coordinate system

[judith piet]

Dear Dr. Johnson and users of the rat hindlimb model,
I want to estimate knee contact forces in the rat knee (after IK, ID and SO), and I am confused about the location the knee joint coordinate system.
The child frame for the knee joint is located at the distal end of the tibia (near the ankle), and when calculating the center of the knee joint based on the simm spline function, it is also located around the ankle.
However, the muscle moment arms seem to indicate that the knee joint center is located around the anatomical knee joint (towards the proximal end of the tibia). For instance for knee flexion, the muscles RF and BFp have moment arms of opposite signs, which corresponds to their opposition action.
Could you please clarify how the knee joint center is defined?
Thank you.
Kind regards,
Judith Piet