A common model of lumbar stability shows the musculature surrounding the spinal vertebrae forming a cylinder. The top of the cylinder is the diaphragm, the bottom is the pelvic floor, and the wall is formed by segmentally attaching abdominal and posterior spinal musculature, specifically the transversus abdominus and the segmental fibers of lumbar multifidus.33 There is growing evidence that demonstrates how these muscles coordinate their activity to stabilize the spine. For example, transversus abdominis has been shown to co-contract with: the diaphragm;34 the pelvic floor;35 and the deep fibres of lumbar multifidus.36 According to this model, the psoas major is ideally located to assist in a stabilizing role. Psoas major has intimate anatomical attachments to the diaphragm and the pelvic floor. This unique anatomical location allows the psoas major to act as a link between these structures and may help in maintaining the stability of the lumbar cylinder mechanism. This can be thought of conceptually as a supporting rod in the middle of the cylinder. Early biomechanical literature suggested that the psoas major might aid in the stabilization of the lumbar spine through its large potential to generate compressive forces, which would result in increased spinal stiffness.30
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