Inverse dynamics - MTP, shoulder

[Jiyoun Kim]

Hi.
I have two questions about inverse dynamics.
First, I want to make sure if the moments are calculated sequentially from the MTP joint when GRFs are applied to the toes during inverse dynamics. When I compared the ankle moments with and without MTP joint locked, those were same. (I divided the GRFs and applied the force to the toes when simulating without MTP joint locked.)
Second, how can the inverse dynamics of a shoulder without GRF data be done?

I think those questions are basic, but I couldn't find the right answers. (I've searched it in the OpenSim Documentation but there's only a simple equation.)
It would be very appreciated and helped a lot if someone answers me.

Thanks.

[Dimitar Stanev]

You have to check if the MTP joint angles are prescribed when it is not locked. This you can check during solving inverse kinematics (second tab).

For the shoulder model this might not be a problem, if you create a floating base between the thorax and ground. This is because the ground reaction forces do not pass through the arm (kinematic chain) as is the case with the lower limb model. The residual forces applied on the base are in essence the reaction forces that must be applied if you had a measurement.

[Ton van den Bogert]

For the ankle joint, it does not matter whether you apply the GRF to the foot segment or the toe segment. Also the toe segment mass is very small (possibly even zero) so locking of the toe segment will not affect the inertial terms in the inverse dynamic analysis.

The MTP joint moment, however, can't be trusted, because you don't know how much of the GRF is applied to the toes. The force platform measures the combined force applied to the toes and foot.

For the shoulder, GRF measurements are only needed if an external force is applied to the arm or hand. As Dimitar said, you can apply a floating base (6 DOF) to the thorax and measure or prescribe those motions. Another common method is to weld the thorax to ground so it does not move, if that corresponds to how you do your experiments.

[Quinten Humphrey]

"For the shoulder, GRF measurements are only needed if an external force is applied to the arm or hand. As Dimitar said, you can apply a floating base (6 DOF) to the thorax and measure or prescribe those motions. Another common method is to weld the thorax to ground so it does not move, if that corresponds to how you do your experiments."

Could you explain this in laymen terms, please?