If side bending of the lumbar spine is introduced when the
lumbosacral spine is in either flexion or extension, non-neutral
coupling occurs. The sacrum responds to non-neutral lumbar
spine coupling by moving around an oblique axis, resulting in sacral
torsion. By convention, the left oblique axis runs from the upper
extremity of the left sacroiliac joint to the lower end of the right
sacroiliac joint.
