If I’ve neglected anything in learning to draw human beings, it’s the back. I have only a basic memory of the muscles in the back, which are many and overlapping, for the support all the things we do with our arms. Really, if you look at the human body from an engineer’s perspective, you can memorize where everything stops and starts far easier, because you’d need the tricep to back up the bicep here and here, for exampel. We might not be a perfect design, but it’s a perfect way to remember it.
The photo I was drawing had an incredible interplay of light and shadow on the back, and here’s my three-value attempt at copying it.