About coordinate system of 'joints3d'

[mmmmbaby]

Dear All,

I would like to apply 'joints3d' data (defined in def get_spin_joint_names() of kp_utils.py) to conduct motion analysis in OpenSim software. Yet,

1. it is not clear to me what coordinate system is 'joints3d' data defined in?
2. What transformation of the 'joints3d' data should I do to make them in a unified reference coordinate system (i.e. the ground)? Many thanks. Yuan