How to model human shoulders in OpenSim Creator?

[adamkewley]

AK: question ported from an email. Some information was redacted/edited to fit the public discussion format.

I have been working on the arm26.osim model. It allows for shoulder flexion/extension and elbow flexion/extension which I have been studying carefully. Now I wanted to add a third degree of freedom to analyse the movements and muscle forces further. The degree of freedom I wanted to add is the abduction/adduction one in the shoulder (please see image below):

I also wanted to include the main muscles involved in this movement which are the supraspinatus, the deltoid and the trapezius. This should be simulated in a similar way to the muscles in the model presented above, as a single bar with the correct insertion points and properties. Thus what I wanted to achieve is:

• An additional DOF for shoulder abduction/adduction
• 3 additional muscles (supraspinatus, deltoid and trapezius)
• All this without losing the previous muscles (triceps long/medial/short, Biceps long/short & brachialis) nor the degrees of freedom´

[adamkewley]

I think what you're asking is actually more complicated than it first appears: 3 DoF shoulder models are (from what I have heard) a point of debate in the wider biomechanical community because there is some disagreement over how each degree of freedom should be stated, how the motion should be modelled at extreme angles, and what kind of joint should be used to describe the shoulder. So keep this in mind while you write your thesis.

E.g. my direct supervisor, Ajay Seth, worked on an OpenSim model of the shoulder that uses a custom-made ScaluloThoracicJoint component (OpenSim Creator directly supports models that contain this):

https://simtk.org/projects/scapulothoracic
cite: Seth A, Matias R, Veloso AP and Delp SL. A biomechanical model of the scapulothoracic joint to accurately capture scapular kinematics during shoulder movements. PLOS ONE. (2015) 

Which is probably the most modern interpretation of how to do it. But there's many other models on SimTK:

https://simtk.org/search/search.php?srch=shoulder&Search=Search&type_of_search=soft

That demonstrate other ways of modelling the shoulder. Most data on SimTK is expressed as OpenSim models, so many of those models probably open in OpenSim and OpenSim Creator.

For what you actually need to do:

• Figure out which joint(s) you actually want to put into your model such that you end up with 3 DoFs (e.g. ScapulothoracicJoint comes with 3 already, but other models use a combination of several joints for various technical reasons)
• Add a joint into the model that attaches your arm to the necessary bodys
• (probably) you may need to move things around in your model if the frames are rotated differently from your original model
• (this part takes a large amount of time, because placing/moving things around in 3D accurately is very tough and OpenSim/OpenSim Creator only provide some of the tooling: work in progress)

For the muscles:

• Hope to god that you can find an existing model with the necessary muscles and copy over the muscle data, if possible, manually in the osim representation
• Or add new muscles into the model that roughly represent the behavior you'd expect. OpenSim Creator's support for adding new muscles is a bit touch-and-go, but 0.4.0 has some basic support for visually moving muscle paths and wrapping surfaces around
• (this part also takes a large amount of time to do right, because creating a muscle path is an art form and usually involves things like looking at dissection data, moment arm curves, etc. - sorry again)

Ideally, though, your best bet is to try and find an existing model that has all the degrees of freedom and muscles you need and to then augment that existing model with whichever modifications you want, though. Some of these OpenSim models take months to develop because (e.g.) joint placement and (e.g.2) muscle placement are very very hard to do well: OpenSim Creator is gradually grinding towards making it easier, but it's still quite hard.