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I have a query regarding the way to obtain the pose matrix of the rigid bodies.
When defining the CustomJoint transforms, the transform of the child in the parent has to be defined as a function of the generalized coordinates. In the OSim documents it is specified how the joint frames are defined (i.e. assigning the 6 location and orientation parameters) but it is not clear what kind of angles are used for the orientations (Euler angles? If so which sequence?), to define the pose matrix of the child in the parent body.
Without this information it is not possible to correctly reproduce a motion in another simulation environment, since I have not found any workaround for exporting the trajectories of at least 3 points fixed to each body.
Thank you.

have you solved your problem? I have the same problem now.

Dear Rui,

I had a similar problem some years ago:

viewtopicPhpbb.php?f=91&t=6619&p=17221&start=0&view=

I think that this is correct: "I would say that for rotation2 we have a 'floating' axis in the sense of Grood & Suntay (1983)".
The translations are along the fixed JCRS (joint coordinate reference system) rigidly attached to the parent body and the Euler rotations are about two body fixed axes and one floating axis. The floating axis is perpendicular to both body fixed axes.

I also concluded that the order of the spatial transform for the custom joint is translations first followed by rotations (this is in contrast to what OpenSim user guide says).

Please correct me if I'm wrong!