Players' feet cannot change on the Y axis, meaning, they have to only stay flat on the ground, and can only rotate like you can in the game, on the X axis. There was a good picture describing this a while ago.
Wait, try this: The way your player can tilt to the front and back (His body, head, arms, hands.) is the Y axis. The feet cannot turn on this axis, only on the X axis, which is the way you can turn in a circle without actually moving forward or backward.