MUD Mired in time...

If the foot can't do it then the smaller ground irregularities inertial force drives the hip upward pivoting around the center of mass of the body. In normal walking on level ground one hip rises and the other lowers alternately.

The undulating mountains and valleys that the hips travel thus average the center of mass through a tunnel in those rising falling paths. This pelvic tilt as seen from the front is often confused with Trendelenburg gait which it isn't. High heel shoes, by preempting the full forefoot wheel arc, cause a great increase in hip hiking stimulating the boys on the corner to whistle. (mud?)