The human body in rough forms is relatively easy to reproduce / draw, but
as long as you dont understand what's happening under the skin and what the muscles do,
that long you wont be able to draw with confidence.
At least that's how I feel about it.

So I started to check George B Bridgeman's "The human machine".
I'ts a rather sketchy book for artists that depicts the human body as mechanical joints that follow easy rules of pulling or rotating, etc.
As useful as it may be, I always need another source (internet) to re-check anatomical issues I dont understand, since
Bridgeman doesnt explain anatomy in detail, but the mechanics.
Some sketches might as well be plainly wrong.

It's not my goal to explicitly learn every single muscle (although that would be quite cool) but to
improve my overall understanding of the human body and to transfer the sketches into a
three-dimensional imagination.
The object like thinking will help later to point out where highlights or shadows on a body should be.

credits:
Many sketches are very close to how they appear in the book, others are done using other references, like anatomical plastics, drawings, photos or 3d models.