Biomechanics

The pelvic girdle functions as an integrated unit with all three bones (the sacrum and the two hip bones) and three joints (the symphysis pubis and the two sacroiliac joints) influenced by the lower extremities below and the vertebral column and trunk above. We will limit our discussion to the following motions:

1. Normal, adaptive, and nonadaptive biomechanics of the lumbosacral junction.
2. Innomiate shear at the symphysis pubis.
3. Innomiate shear at the sacroiliac joints.
4. Anterior and posterior rotation of one innominate with the contralateral one about a transverse axis through the symphysis pubis.
5. Anterior and posterior rotation of the sacrum about a transverse axis through the posterior part of the sacrum (bilateral sacral flexion and extension).
6. Side-bending of the sacrum about a coronal axis and/or rotation of the sacrum about a vertical axis (when sidebending and rotation both occur, the resulting motion is equivalent to rotation about some unknown oblique axis).
7. Rotation of the sacrum about the right and left oblique axes.

It should be understood that these motions seldom occur in isolation, but are intimately coupled to one another.