The lateral muscles of the lumbar spine include the quadratus lumborum and psoas. The quadratus lumborum originates from the iliac crest and inserts at the twelfth rib and transverse process of the lower four lumbar vertebrae. The quadratus lumborum produces lateral bending of the lumbar spine with unilateral contraction and stabilizes the trunk with bilateral contraction. The psoas major muscle originates from the anterior surfaces of the transverse pricesses of all the lumbar vertebrae and inserts at the lesser trochanter of the femur. The psoas major flexes the trunk and the hip.
The spine and trunk muscles exist in pairs, one on each side of the body.In general, bilateral contraction results in movement in saggittal plane. The anterior muscles flex the spine, while the posterior muscles extend the spine. Unilateral contractions results in lateral bend or axial rotation.
Thank You for interesting questions,Miroslava.