Adding New Muscles to Existing Models

[JPi2323]

Hi there, I am extremely new to OSC but very excited to learn and use the platform. I am interested in adding new muscles to the existing FullBodyModel_Hamner2010 (with simple arms model). Currently working on adding the bicep and triceps muscles using a combination of PathPoints, Conditional PathPoints and Moving Pathpoints. I am also using the Arm26 model as a guide, and noticed that there are Wraps that are not shown on the FullBody model around the knee or any other joints. The issue I am running into is that the muscle doesn't seem to shorten/lengthen as I flex/extend the elbow, but instead the Moving Pathpoint at the end bends and deforms into/through the bone. I have been able to position it so that it is embedded into the bone and it looks "natural" but wondering if this is the best way to do this. I would like to learn in order to create a full skeletomuscular render of the body, but would like to make sure I am going about it the right way. Any advice on best practice would be greatly appreciated. Thank you kindly. :)

[JPi2323]

Additionally, wondering how to add in WrapOjects (Ellipsoids and Cylinders) and wrap geometry to have muscles bend and wrap around joints, rather than going through bone. Thanks in advance for all help on this matter.

[adamkewley]

From what I understand of OpenSim, it's immaterial whether a path point is "moving" or not when it comes to wrapping around bones/geometry, so use what you like. Contextually, MovingPathPoints (I think) exist mostly to roughly emulate the effect of collisions/wrapping along a path and may even predate wrapping altogether.

For wrapping, this has been previously asked and I have written an answer here:

• How can I make paths wrap over wrap objects? #905
• Related: re-expressing their attachment frame: How can I re-express a wrapping surface or contact geometries in a different frame? #868

But tl;dr is that you need to add the WrapGeometry and then separately state which geometries interact with which paths. The most common issue with adding WrapGeometry is selecting a suitable quadrant for the path to wrap over. I'd strongly recommend looking at other models that use wrapping (e.g. arm26.osim, Rajagopal2015.osim) for inspiration about how they achieved that.

If you're having issues where your path isn't moving when you adjust a joint, and it isn't related to wrapping or the MovingPathPoint's motion then that's usually because the points of the path are attached to the same underlying base frame (body) and you need to ensure that (parts of the) path cross joints and are attached to other bodies. You can re-socket the points if that edit is necessary.

[JPi2323]

Hi Adam,

Thank you kindly for the response and insight. I will tinker and try to see if I can get it to work. Thank you very much for your work on this.

[JPi2323]

@adamkewley I do not see the option to Add - WrapGeometry only contact geometry. I do not think these are the same when looking at the Arm26 model. Is there a setting that needs to be turned on first? Please advice. Sorry for all the questions

[JPi2323]

Or is it adding a new body and then adding the geometry to it - joined to the existing body of choice (aka adding a new cylinder "body" to the elbow joint joined to the parent body of the humerus?

[adamkewley]

You can add a wrap object to any Frame in the model (Bodys are Frames), so you can either Add > Wrap Object to an existing body or add it to an existing PhysicalOffsetFrame or similar

[JPi2323]

I got it. Thank you so much for your assistance. :)

[JPi2323]

Hello Again Adam,

Just wanted to confirm, when adding a new muscle, is it necessary to add in Components(i.e., PathPoints or MovingPathPoints) prior to adding the force (i.e.Thelen2003Muscle)? I have been adding muscles this way but this adds new tabs under the model tab which i do not see in other examples i.e., Arm26). Or can I add new muscles just using the existing bodies in the model? Just want to ensure that I am approaching this in the most efficient way possible. Thank you.

[adamkewley]

Hey @JPi2323

The way that OpenSim currently works is that a muscle owns (has) a set of points that define the muscle. Therefore, when creating a muscle, at least two points must be specified at creation time. It's a chicken-and-egg issue (i.e. you need to make a muscle, which has points, but you can't create a muscle with no points - fun 😉) and therefore OSC's UI for it is sub-optimal - for now.

The way I'd usually do it is either edit the .osim XML (hard-mode but, fundamentally, the XML always supports all features whereas the UI only supports a subset). Or use OSC in the following way:

• Add at least two temporary stations/markers into the model that are attached to relevant frames/bodies. This is done via right-click -> add -> component -> Station/Marker.
• Add the muscle you'd like to the model via right-click -> add -> force -> MillardEquilibriumMuscle (for example). Select the temporary stations/markers as the "Path Points" (dialog at the bottom) - this is required to create a muscle in OpenSim.
• Delete the temporary stations/markers. You will now have a muscle with its own path points that were copied from them.
• You can then add additional path points, edit frames, locations, etc. of those path points by clicking the muscle, going to its path property in the Properties panel, and then clicking the little edit button.
• Additionally, you can add path points by right-clicking a muscle and clicking the relevant "Add Path Point" option or similar, and you can change the frame of each path point by right-clicking the path point and using the Sockets menu to change the parent_frame of that path point.

You can't handle MovingPathPoint etc. via this flow, though, but the official OpenSim GUI does have a relevant dialog/flow for that, so once you have a basic muscle with normal path points you can edit the model in OpenSim GUI to use the fancier path points - OSC is capable of reading/using/rendering any models from OpenSim GUI (it just can't perform all of the same edits because that requires UI development)

The official documentation also outlines adding muscles to a model, here: https://docs.opensimcreator.com/manual/en/latest/the-mesh-importer-advanced.html#add-muscle-paths

[JPi2323]

Hey @adamkewley

Thanks for that very detailed response. I think I got about 90% of that correct in my attempt, minus adding in the initial stations to the bodies.

I know I am asking a lot of questions, but really enjoy the learning process of OSC and looking forward to the enhancements in future versions. I will ask 2 more, so please bare with me

1. Is there documentation on the differences between the Muscles / Force options we can selected - i.e. does MillardEquilibriumMuscle differ from Thelen2003 Muscle - depending on the model or objective?

2. Does adding in the stations matter if they are then deleted? Currently, I am selecting the origin and insertion of each muscle to add in a rudimentary muscles and then using the path icon in the properties of this new muscle to add in additional points between the 2 ends that I can edit the coordinates for. Would this approach work as well, or is there a specific reason to adding in the stations first?

My current goal is to add in the upper body muscles to the existing Rajagopal2015 model, having them move within the anatomical DOF (as close as possible).

Again, thank you very much for your support and help as well as work on OSC.

[adamkewley]

Is there documentation on the differences between the Muscles / Force options we can selected - i.e. does MillardEquilibriumMuscle differ from Thelen2003 Muscle - depending on the model or objective?

They do differ, but I'm not personally sure which is better for what purpose (I don't have a biomechanical engineering background). My impression is that the Millard muscle model is "more modern", but you'll have to read the actual literature to discern whether it's an important difference or not. Relevant links that you can read:

• https://simtk.org/api_docs/opensim/api_docs40/classOpenSim_1_1Millard2012EquilibriumMuscle.html#details
• https://simtk.org/api_docs/opensim/api_docs40/classOpenSim_1_1Thelen2003Muscle.html#details

They've been around for 13 years now, so it's also likely that LLMs like ChatGPT etc. have ingested the literature surrounding the various muscle models if you'd like to have a chat with AI about it.

Does adding in the stations matter if they are then deleted?

Doesn't matter at all. The stations are a hack to break the aforementioned chicken-and-egg issue around OpenSim's path implementation. I.e. you can't create a muscle without a path, a path must have two points, those two points must be part of the path, but you don't have a path yet, so you have nowhere to put the points (on and on this goes, in a circle). Stations have no such dependency--they can be added into the model one-at-a-time--so the stations you're adding are purely to "seed" the initial creation of the muscle. Once it's created, you can delete the stations and play around with the resulting muscle path as much as you want (although, again, you can't go below two points).

[JPi2323]

Hi @adamkewley

THanks again for your time and your insight, I will take a read through those resources about the various muscles and their applications. I will continue to work on the model with all the information you have provided and will ask questions sparingly as I continue. Thank you again and hope you have a wonderful day

[JPi2323]

Hi @adamkewley. I was wondering if there was any documentation or model for the shoulder. Hoping to get some insight into how to wrap the various muscles (teres mj/min, infra and supraspinatus. etc.) to wrap in all ROMS (arm flex, add/abduction etc). I saw a discussion about it in the forum but was wondering if there was any documentation or an already existing model that I could use as a template. Thanks again