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Hello,

i am a starter in OpenSim. i am working right now to modify the elbow part of the Arm26 model. My prof asked to change the elbow flexion axis from a fixed one to an unfixed one. Which means, when the arm slowly flexion and extension, the forearm rotates around an axis at distal humerus and this axis keeps changing both in position and orientation. How can i program this in the osim file?

THX
Jiang

Hi,

You can define this through the SpatialTransform property in the Joint of the child body. About flexion/extension it seems easy, but you need to be careful with the axis of rotation for the ulna and radius. The following paper explains how you can define complex movements for your joints, and this is the current implementation in OpenSim. You can also check the knee joint of the gait2354 model to see how they defined the translation of the tibia as a function of the knee angle.

https://nmbl.stanford.edu/publications/pdf/Seth2010.pdf

Thx very much!