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Hi all,

I am fairly new to OpenSim and modeling in general and was wondering if anyone could tell me the difference between a WeldConstraint, a WeldJoint and Locking a joint. In particular I would like to know how it would impact Inverse Dynamics results.

To give some context, I am using a model (Musculoskeletal Model of the Lumbar Spine) that has separated the trunk into the lumbar portion and the thoracic portion, which have associated masses and moment of inertias. The problem is I want to add head and neck masses and moment of inertias to the thoracic portion of the trunk but I am not sure whether to create new rigid bodies containing the head and neck and use either:

1. Weldjoints,
2. CustomJoints that have a WeldConstraints or,
3. CustomJoints that are locked.

Ideally I would just have a lumped mass and moment of inertia for a single combined thoracic, neck and head rigid body but I am unable to find such anthropometric values in literature.
The model also has a sacrum with no mass or moment of inertia properties “WeldJointed” to a pelvis with non-zero mass and moment of inertia properties. The author of this model has stated the properties of the pelvis represent the total of the pelvis and sacrum. If I were to run inverse dynamics, would I still need to assign a very small mass (e.g 0.001 kg) to the sacrum for it to give accurate results?

Thanks!

Laurence

Hi Laurence,

This's a modeling decision you have to make for example:
- If you plan at some point in the future to have an actual joint, separate body for the head then it makes sense to introduce a joint (if you have kinematics in mind you can create the joint and lock it for now or if you don't just create a weld joint). In this case OpenSim will take care of combining the mass properties/inertias of the two attached bodies. One of these could be massless and wouldn't affect Inverse Dynamics.
- WeldConstraint is a different creature that you'd use primarily to close loops since it relates two bodies that are already in the model, so it can't be used to add bodies.

Hope this explains and please let us know if you have any questions.

Good luck,
-Ayman

Thanks Ayman for the clarification!

I also noticed a similar post where the author claimed that moments calculated from ID and SO were different between using a weldjoint and locking a custom joint.
https://simtk.org/forum/message.php?msg_id=5082

However there has been no reply to this message so I am not sure whether this is a problem. Could you please clarify this?

Thanks,
Laurence

Hi Laurence,

Every joint relates two frames (one on each body). This relation is fixed for Weld but a custom joint has coordinates, that you lock to a specific value (WeldJoint has no coordinates). The answers for ID, SO should be identical if the relative position/orientation of the two body frames after locking the CustomJoint is the same as the WeldJoint and would be different otherwise.

Hope this helps,
-Ayman