Right now, the two best resources to learn human anatomy are "Anatomy for Sculptor" and "Anatomy of Facial Expression". It's a very modern and unbiased approche of anatomy contrary to Andrew Loomis' work that I find way too stylized. I always have these two books on my desk by the way. Have also a look at Rafael…
It would also help to study anatomy and proportions. We have a ton of great resources, cached here: Anatomy Reference For example, PoseManiacs has excellent references https://www.posemaniacs.com/tools/viewer/poses/01H6RBJ01K4AJGPMCEYW5FKYFZ
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