My physical therapist had determined that some trauma to my foot had caused the joint between my ankle and heel, the sub-talus, to seize up. That joint allows the heal to slip around, provides shock absorption with each heel-strike, and allows the foot to flatten mid-stride, so that we launch off our big toes at the end of the stride.
At the same time our foot flattens, our tibia rotates inward so that the knee is pretty much right over our big toes at that point. The femur and the hips roll in at the same time.
A whole lotta shakin' goin' on!