Prevents collapse of the vertebral column in the frontal plane (i.e. scoliosis) Reserve mover actions: anterior pelvic tilt. Contralateral lateral pelvic rotation. Clinical significance The quadratus lumborum muscles can be the source of back pain when overused, or in association with scoliosis or weak gluteal muscles. Mechanism The quadratus lumborum is a common source of unilateral or bilateral lower back pain, including localized pain and tenderness over the wing of the ilium. Because quadratus lumborum connects the pelvis to the spine and is therefore capable of extending the lower back when contracting bilaterally, the two quadratus lumborum muscles pick up the slack, as it were, when the lower fibers of the erector spinae are weak or inhibited (as they often are in the case of habitual seated computer use and/or the use of a lower back support in a chair). Given their comparable mechanical disadvantage, constant contraction while seated can overuse the quadratus lumborum, resulting in muscle fatigue. A constantly contracted quadratus lumborum, like any other muscle, will experience decreased blood flow, and, in time, adhesions in the muscle and fascia may develop, the end point of which is muscle spasm. Association with kyphosis This chain of events can be and often is accelerated by kyphosis, which is invariably accompanied by rounded shoulders, both of which place greater stress on the quadratus lumborum by shifting body weight forward, forcing the erector spinae, quadratus lumborum, multifidi, and especially the levator scapulae to work harder in both seated and standing positions to maintain an erect torso and neck. The experience of "productive pain" or pleasure by a patient upon palpation of the quadratus lumborum is indicative of such a condition. Association with weak gluteal muscles Hip abduction is performed primarily by the hip abductors (gluteus medius and minimus). When the gluteus medius/minimus are weak or inhibited, the tensor fasciae latae or quadratus lumborum will compensate by becoming the prime mover. The most impaired movement pattern of hip abduction is when the quadratus lumborum initiates the movement, which results in hip hiking during swing phase of gait. Hip hiking places excessive side-bending compressive stresses on the lumbar segments. Thus, a tight quadratus lumborum may be another hidden cause of low back pain (Janda 1987). When the hip adductors are tight or hypertonic, their antagonist (gluteus medius) may experience reciprocal inhibition. The gluteus medius will become weak and inhibited. This in turn may cause