Here is a similar problem. The back is bent and the shoulders are pulled forward because of the nature of the gesture. Try this pose. You will feel your elbows jut way out and your shoulders follow suit. The elbows seem to fold up while the knees spread apart. Why? Because the top of the chair back is very narrow so the elbows have to squeeze together to fi t. Contrariwise, the bottom of the chair is wider, forcing the knees apart:

4 Sketcher The cartoonist, when he sketches is going through a process of study. He concentrates upon the model, plumbs its movement, bulk, and outline. Then he sets it down, remembering that he wants only the spirit — the “guts” of the thing he’s after. He puts into his drawing (even though it may be as big as your thumbnail) all his experience. He simplifi es. He plays with his line. He experiments. He isn’t concerned with anatomy, chiaroscuro, or the symmetry of “fl owing line. ” There’s nothing highbrow about his approach to the sketchpad. He is drawing because he likes to draw!