The long version of this is that I was testing a combined version of the latest MoBL-ARMs upper limb (https://simtk.org/projects/upexdyn) and hand/wrist model (https://simtk.org/projects/arms_hand_model) in 4.3. In testing I found that certain combinations of positive wrist deviation and negative wrist flexion angles caused the ECRL and/or ECRB muscle to do some crazy muscle wrapping. The extent of the crazy path varies with the joint angles.
- Wonky ECRL.png (176.24 KiB) Viewed 1322 times
- WonkyECRL2.png (95.93 KiB) Viewed 1322 times
To see if it was my own mess up, I found that this also happened with fresh downloads of the models (can recreate on windows by setting deviation to .435 and flexion to -1.22, may need to mess with the elbow flexion in the upper limb model, although the issue isn't constrained to just those values). It isn't just a visualizer error since the fiber-length and moment arms are wonky.
- UpperLimbValues.png (45.81 KiB) Viewed 1322 times
Since the MoBL-ARMS upper limb model is validated to 4.1 I decide to try all the models in 4.1 to see what happened. In 4.1 the geometry path doesn't result in any crazy paths and all the fiber length/moment arms curves are normal. So...figuring it was a version issue and I tried 4.2 and 4.4 as well, both result in crazy geometry paths that cause similar issues for fiber length/moment arms as in 4.3.
The fix that I eventually found was that the ECRL and ECRB wrapping surfaces at the wrist were causing the issue (ruled out the elbow wrapping surface since 1. toggling it off didn't fix the issue and 2. ECRB doesn't use that wrapping surface). The default settings had this as the wrapping path settings
- ECRLOGWrapFunction.png (40.51 KiB) Viewed 1322 times
- ECRLFixedWrapFunction.png (20.83 KiB) Viewed 1322 times
With the change I got identical force length/moment arm curves in 4.2/4.3/4.4 as the original models had in 4.1. I will note that the geometry path settings that work in 4.2/4.3/4.4 do cause some wrapping issues in 4.1, but it seems to just be a visualizer issue or small enough not to affect the fiber length/moment arm curves. Its also worth pointing out that the wrapping issues only occurred between points 1 and 2.
So ultimately I am curious if something changed in 4.2 that affects geometry path calculations that didn't make it into the release notes, since the original models have no issue in 4.1 but changing how the geometry path is calculated fixed the issue in 4.2 onwards. Or if this is some phantom wrapping issue. Also a heads up if someone else has this problem.
Cheers,
-Max