I think practical knowledge of the human body is somewhat necissary. I mean, about how the body-structure interacts with itself. When one leg is lifted, rarely do you not see the waist bend in the opposite direction, and perhaps the arm move out stighly to give balance to the weight that is the person standing on the one leg. If you simply raised the leg, it would not look natural and that might be what your 'minds-eye' is catching. They don't seem right because something is out of place enough. The laws of gravity and physics do pertain to some degree, in terms of realism I mean.