Here’s my Ecorche final for Life Drawing III. It was challenging to work from a photograph and then imagine all the muscles underneath.
I found it helpful working with an Ecorche app on my iPad. Another helpful app is Skelly for skeletal work. If you frequently rotate the model in the app it helps get a 3D sense of where the muscles are in space.[youtube https://www.youtube.com/watch?v=yipZDR8pF70&w=560&h=315]