# Inverse dynamics of a model without joints

Hi,
I was wondering if the following approach is possible. In the end, I would like to model one chewing cycle of the mandible. Problem is that I have no idea how I should define the joint. But I have the whole movement of the mandible recorded, so I have the kinematics. Question is if, when I attach the muscles, I can analyse the response of the muscles on the movement regardless on the joint?

I started with an easier example (see the attachment) – a bar which moves in the vertical direction, therefore no joints needed (prescribed translation, fixed rotation). The kinematics works fine, but if I add one muscle to the top node and one to the bottom node and I want to see , how the muscles would perform such movements, it does not work (kinematics works, but for inverse dynamics I get: “No solution found : There are fewer unknown forces (muscles and reactions) than dynamic equations.“). I have no idea, which reaction I should switch on/off, which kinematic measure I should include, etc.

Or can the problem be, that the muscles are aligned with the bar, therefore the muscle´s arms are zero? So if I had two instead on one muscle in each location, it could work?

Could anybody have a look at it?

Thanks
Michala

I have now 4 muscles below and 3 muscles above the bar (so that all DOF are coverd; now the musclse are not alligned with the bar), but I am still getting the same error message for the inverse dynammics (kinematics is still running ok).

What I have also found concerning this error is that “could be missing muscles for a certain motion, missing reactions in a joint or to ground”. I think I covered all the DOF now, but since I don´t have the joint, the reaction force could be a problem. How to introduce it?

Michala

Hi,
I may have found the solution. Now it is the marker which is governed, not the point on the moving segment. The kinematics and the inverse dynamics work.

So do I understand it right, that the muscle activities/forces I am getting are needed to perform that movement? If yes, than I found the way how to do what I need for my mandible example.

Thanks
Michala

Hi Michala,

yes you are right, the muscle are activated to move the pendulum. The muscle force is needed to perform this motion.