Inverse Dynamics - Torque and Forces

[Jazmin Cruz]

I have performed inverse dynamics before using the tutorial provided on the OpenSim Documenation.

However, I noticed that not all joints result in both moment and force values. As in, not all joints report both values in the .sto file.

My question is, what is the underlying reason why a torque or force is reported for a specific coordinate? Is there a sort of "toggle" or "switch" in the code that says whether to report the torque or force at a joint?

I assume that if the joint is rotational then moment would be reported. Likewise, if the joint is translational then a force is reported. If the joint has both rotation and translation, then both would be reported.

If this is an obvious question, please direct me to the documentation I need to read. I really appreciate the help.

[Carmichael Ong]

No worries at all, I don't know where this would be in the documentation. Overall, you're correct. Depending on the type of each individual joint in the model, inverse dynamics will return a moment or force, or both for each joint. And as you noted, if the coordinate related to a joint is rotational (like all the coordinates for the ball joint at the hip), then moments are reported. Likewise, the same is true for coordinates that are translational (like some of the coordinates for the pelvis) which return a force.

[Jazmin Cruz]

Thanks for the reply! That really helps.

Okay, great. Here is my scenario: Using the lumbar spine model, each of the vertebrae is defined as a 3 DOF rotational joint. However, a constraint is used to control the overall flexion/extension of the spine. When ID is run, it reports a force for the constraint name "flexion/extension", but reports joint moments for each individual joint that is associated with the constraint. Are the values calculated IK and ID values for constraint coordinates supposed to be neglected?