Normally, with each step the pelvis drops a few degrees to the side of the non–weight-bearing (or swinging) leg, but contractions of the hip abductors of the opposite (and weight-bearing) side usually
minimize the extent of this fall.
Yet, if these muscles are weak, they cannot hold up the contralateral pelvis, thus letting it sag toward the unsupported side. Hence, the “swinging” limb becomes too low to clear the ground. In an effort to raise the leg, patients learn to lean away from the unsupported side, with a compensatory “lateral lurch” toward the side of the weakened abductors. When both sides are weak, this “lateral lurch” becomes bilateral, producing a typical “waddling (or anserine) gait.” Note that patients may also compensate by stepping very high on the unsupported side, thus allowing the “swinging” leg to clear the ground. This is known as “(high) steppage gait.”