Fig. 1

Illustration demonstration the effect of loading on the pelvis during gait. During weight-bearing the reactive normal force vector, directed cranially, results in nutation of the ipsilateral pelvis/sacrum. The sacral base (promontory) tilts anteriorly and inferiorly and the distal sacrum tilts posteriorly relative to the ilium (nutation, or “to nod”). The ipsilateral hemipelvis shifts slightly in the y-axis through the pubic symphysis. The opposite motion occurs in the contralateral pelvis (counternutation). The motion is exaggerated in the images for demonstration. This alternating motion during gait is reciprocal unilateral motion