How to simulate/use resulting models?

[adamkewley]

AK: Question asked via email. Researcher's details/specifics were removed from the question. I've reposted it here in a more general form so that other (prospective) users have something to search through longer-term.

I have successfully created a muscle model using OpenSim Creator and would now like advice on how to simulate it. What kind of approaches/techniques are out there that you know of, generally speaking?

[adamkewley]

I am not a researcher (IANAR 😉), but I'm guessing that, in terms of how to simulate a model, that's strongly dependent on your research question.

First, here's some external resources that I would recommend researchers read through:

• The Official OpenSim Documentation (https://opensimconfluence.atlassian.net/wiki/spaces/OpenSim/overview). It contains an "Examples and Tutorials" section that outlines, concretely, how to use some of the tools/ideas I'll write below.
• SimTK.org (https://simtk.org/). Contains many models + studies from the wider OpenSim ecosystem. It's usually a good source of inspiration to browse through it and look at what other researchers in the area tend to do with their models.
• The most recent OpenSim paper (https://doi.org/10.1371/journal.pcbi.1006223 and ). Contain a higher-level explanation of the platform.

In terms of common uses for models created via OpenSim Creator, here's a few (list is work in progress):

• Model Verification. If you have just created the model--for example, by placing the meshes, bodies, and muscles--and now you would like to verify it, then some example verification steps that I've seen in the wild are:

  • Muscle moment arm verification: Using either the moment arm plotter in the official OpenSim GUI, or the live moment arm plotter in OpenSim Creator, a common verification step is to take moment arm/strain measurements from (e.g.) cadavers or other experimental measurement techniques and compare them with the quantities that OpenSim generates.
  • Motion/IK verification: If the model contains markers, and you have an external measurement technique that's considered to be reliable (e.g. marker tracking), then it's common to use inverse kinematics (IK) to verify that the model is capable of moving in the same ways that are observed in a lab. If your joint constraints are off (e.g. too constrained), or your virtual markers are off, then the IK result will have a high error. This is usually done with the IK tooling in OpenSim GUI.
  • Metabolic verification: Once the model is able to move via something like IK, and the various muscle redundancy/CMC algorithms are used to figure out which muscles are emitting forces, another verification step is to check if the metabolic cost of transport from a model is comparable (at least, in terms of trends) to the metabolic cost measured via (e.g.) breathing apparatus. This is usually done with help from the official OpenSim GUI's plotting/analysis tools.

• Optimal control studies: From what I understand, these usually use/modify an existing, validated, model and give it some kind of objective. An example objective might be "Get to the other side of this room, while using the least energy". OpenSim Creator can produce the models that are input into these studies, but cannot run those studies itself. For that, I'd recommend reading into OpenSim MoCo documentation, which is included in the OpenSim API. There's also SCONE, which provides a full desktop experience for performing those kinds of studies.

• Modification analysis: Once a model is validated, a common study is to (e.g.) test the effect of modifying it in some way to perform a "before vs. after" style study. OpenSim's official documentation + tutorials (listed above) outline using OpenSim to perform studies like "what happens if this muscle insertion point is moved, or a muscle is weakened", etc. OpenSim Creator can help with this by providing the ability to plot muscle outputs vs. a coordinate, or view an output while editing the model. The official OpenSim GUI comes with a variety of analysis tools for helping with those types of studies. I'd recommend reading the official OpenSim documentation if that's what you're looking for.

• Prosthetics: Once a model is validated, another common study is to see what the effect of prostheses or exos are on a model. Those studies typically involve modifying the model by adding new mechanical components to them (e.g. bodies attached by sliders/springs and so on), followed by measuring relevant outputs/effects from that modification. OpenSim Creator mostly helps with building/modifying the model,, and its output plotting capabilities are handy for getting a general idea of the effect. The official OpenSim GUI, or the OpenSim API, is usually where final analyses are performed.