McGill37 conceptualizes lumbar spine stability as a
fishing rod placed upright and vertical with tensioned
guy wires attached at different levels along its length and
those guy wires being attached to the ground in a circular
pattern. Here the rod represents the lumbar vertebrae and
the guy wires are the various muscles attaching to the
lumbar spine. Reducing the tension on one of the muscles
(wires) will allow the spinal segment (rod) to buckle and
allow spinal injury to occur. Juker et al.12 showed that the
psoas major counteracts the action of iliacus during hip
flexion. They believe that the iliacus would torque the
pelvis into anterior pelvic tilt and that the psoas major
works against these forces, adding to the stiffness within
the pelvis and the lumbar spine. An activated and stiffened
psoas major will contribute some shear stiffness to
the lumbar motion segment.38,39