Inverse dynamics differences - OpenSim 4.2 vs OpenSim 4.1

[Dan Mulla]

Hi all,

I've noticed a big difference in inverse dynamics results between OpenSim 4.2 vs 4.0/4.1 using Ajay Seth et al.'s thoracoscapular model and was wondering why this may be happening. I know that the Scapulothoracic Joint is now built-in with OpenSim 4.2 as opposed to using a plug-in with previous versions (4.0/4.1), but was wondering if there was a change in dynamics with this joint that may have happened across versions. My understanding from going through the change and pull requests (2877 and 2932) is that the dynamics should not have changed (https://github.com/opensim-org/opensim- ... ANGELOG.md). I haven't noticed any big differences in IK or CMC results between the different OpenSim versions using this model (differences have been in the decimal places), so it's only the inverse dynamics results that seem to be affected.

I would attach the files here, but the attachments don't seem to go through due to a "board attachment quota", so I'll link the files I'm using here.

The model (ThoracoscapularShoulderModelSubject.osim) I'm using is the scaled model found in package here: https://simtk.org/frs/?group_id=1708. The motion file is abd01_IK.mot found under Results > IK found in the package downloaded from the above link. I filtered the coordinates at 3 Hz in the GUI and did not apply any external loads as it was a simple arm elevation motion.

To get the model to run in 4.1, I've been using the ScapulothoracicJointPlugin 4.1 found here: https://simtk.org/frs/?group_id=986

Any help or advice would be appreciated.

Thank you!

Dan

[Ayman Habib]

Hi Dan,

The model is still actively maintained/developed by Ajay Seth, who oversaw the integration into our code base and fixed a few issues along the way during integration into OpenSim 4.2 so I'd suggest you contact the model author(s) directly. As far as OpenSim is concerned, there were no material changes to the Inverse Dynamics tool from 4.1 to 4.2.

Hope this helps,
-Ayman

[Dan Mulla]

Hi Ayman,

Thanks for your quick response. I'll contact Ajay directly to figure out what may be happening.

Take care!

Dan