Phantom bodies in upper extremity model

[Woojae Kim]

Hello,

I was wondering if there is any post that explains what the "phantom bodies" are. They are bodies without center of mass and inertia. I use MoBL-ARMS dynamic upper limb model - OpenSim 3.2+. In the model, they are called scapphand, humphant, humphant1, and clavphant. What are their function? Do they affect inverse kinematics and dynamics results?

Thank you for your attention.

Jason(Woojae) Kim

[jimmy d]

In 3.3, joints can only be added with a body. If you look in the model file, you will see that each joint is nested inside a corresponding body. However, for complicated motions, you may want a complicated motion that involves multiple joints without having to add extra bodies (mass and inertia). In this case, you would create a 'phantom' body, one without mass or inertia, so that you can include the additional joint. This is not a great strategy for numerous reasons, but it is the choice of the person building the model.