It's not difficult for an experienced artist to pose and render a character like that. The more you study anatomy and gesture, the easier it is to draw an arbitrary character in any pose you want.
You refine your initial sketch until it looks as anatomically correct as you can make it, then proceed to render and polish it.

Here are some examples:

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And this: http://helgesonart.blogspot.de/2014/08/a-formula-step-by-step.html