The latest version of the the rig can be found here.
I did a quick experiment in Blender with an “arm rig”. The problem is that - when an arm is bent - it usually distorts the elbow and the inner arm too much to be really usable, as shown here (it tends to look to much like a rag doll! - which is why Toy Story’s characters look the way they do!):
There are ways to work around that, e.g. lattices, that are used to “distort” the shape back to a more normal elbow.
I decided to rather use bones to accomplish that. While I was busy, I also decided to add a few muscles. Here is an animation without showing the bones…
And here is one that shows the bones. As you can see - apart form the “muscle bones” - I also added an elbow bone that scales based on the rotation of the forearm. This is by no means perfect, but it is a lot better with a relative simple rig: