If the body of a armor stand is rotated (around x or z) the leg stay at the point where they would be if the body would be straight down.

The following command makes the legs stay on ground and let the upper body float:
/summon ArmorStand ~ ~ ~ {Pose:{Body:[90.0f,0.0f,0.0f]}}

The legs should not be at a fixed position, but instead allways begin at the end of the body.