Trying to find the best approach to creating all the muscles, I first had to make sure silicone would stick to the skeleton and stretch without breaking. So I had to create some silicone muscles. I took the clay sculpts and made an injection-type mold from them. Injection meaning that I literally inject the silicone into the mold rather than pouring it.
And it worked. I used some super glue just to test things out, and the silicone muscles stuck pretty good and stretched beautifully when the arm was bent and straightened.
So now that I had proof-of-concept, I had to decide how to proceed. My first thought was to sculpt every muscle and create injection molds. But then I thought making the molds on the computer would be more accurate and easier to create right and left sides. I actually spent two days creating a complex injection mold on the computer that had all the upper arm muscles and scapula muscles in one mold. However after getting feedback from my 3D printing company (Xometry), they convinced me it was not the best approach. So instead I decided to have all the muscles 3D printed and then I will create my own injection molds using those 3D printed parts. It will be a long and expensive process, but I think it is the best approach for now.