The posterior view will show anterior rotation of the right ilium (A/S), with posterior rotation of the left ilium (P/I). The direction of the pelvis is considered to be the primary movement but, within the pelvis, the sacrum moves counter to the innominates and, therefore, counter to the primary movement. Therefore, the pelvis will rotate to the left, and carry the entire trunk to the left, on a global basis but, relative to the pelvis, the spine will rotate and laterally flex to the right, with two vertebrae as exceptions; L4 and L5 will rotate and laterally flex to the left due to their ties to the ilium by the iliolumbar ligaments, and the action of some fibers from parts of the psoas, multifidus, and lumbar parts of the longissimus and iliocostalis [1-3]. Although the sacrum laterally flexes to the left to form the bottom part of the lateral C – curve, it rotates to the right. The sacral base will move posteriorly and superiorly on the right, and the sacral apex will move to the right. In this manner, counter-rotation occurs at L5-S1 and L3-L4.
Coupled motion dictates that right spinal rotation will combine with right lateral lumbar flexion. To demonstrate this on yourself, rotate your pelvis to the left and then try bending forward and to the right; the movement is simple to do so you can easily bring your left shoulder toward your right hip. But, while maintaining left rotation of your pelvis, try bending and rotating your shoulders to the left; it will be extremely difficult because normal spinal dynamics follows the patterns expressed in nutation and counternutation. (For more, see Coupled Motion).
1. Santaguida, P.L. and S.M. McGill, The psoas major muscle: a three-dimensional geometric study. Journal of Biomechanics, 1995. 28(3): p. 339-45.
2. Bogduk, N., Clinical Anatomy of the Lumbar Spine and Sacrum. 2005: Elsevier Churchill Livingstone.
3. Adams, M.A., et al., The Biomechanics of Back Pain. 2002: Churchill Livingstone.