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Dear OpenSim experts,

Through my work, I would like to use the Rajagopal's model on OpenSim to compute, for example, muscle-tendon units lengths during movements (gait or stretches). For that purpose, I will have to use joint angles during the movement.
We already have a MatLab code which can generate joint angles, based on markers placed on anatomical landmarks, and, in a first place, we do not want to use the OpenSim Inverse Kinematic pipeline.

Now, I would like to ensure that the kinematic model we use to compute joint angles correspond to the Rajagopal's model, so that the joint conventions agree.

Unfortunately, I do not find much information about the joint coordinate systems used to construct the model in the associated paper or in the Arnold's paper Mrs Rajagopal refer to.

Thus, would it be possible that anyone detail me how the different joint coordinate systems (which markers they are based on and how the axes are defined) of the Rajagopal's model are constructed, please?

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Otherwise, would it be possible that anyone explain me how I can modify the Rajagopal's model to match the Joint Coordinate System we are using.
For example, for the ankle joint, on the GUI, you can see that the ankle_r joint has one parent frame (tibia_r_offset) and one child frame (talus_r_offset) and these two seems to coincide. So which one am I supposed to modify in the .osim file, and which part of the .osim file does this correspond to?

Sorry for all these questions, any answer would be really appreciated.

Thanks in advance,

Regards

Hi Axel,

Each joint has a partent and child frame. If you want to match an OpenSim model with your clinical model you need to account for the offset (if there is one).
E.g. the pelvic frame in most clinical models is defined based on the ASIS and PSIS markers which creates an offset around 15 degrees to most OpenSim model.
If you open the OpenSim model in a text editor (notepad++), you can easily change the orientation of the parent and/or child frames
--> go to your joint, e.g. ground_pelvis, orientation and change the z value, the values are all in radiant

Cheers
Hans