The psoas major ( or ; from ) is a long fusiform muscle located in the lateral lumbar region between the vertebral column and the brim of the lesser pelvis. It joins the iliacus muscle to form the iliopsoas. In animals, this muscle is equivalent to the tenderloin. Structure The psoas major is divided into a superficial and a deep part. The deep part originates from the transverse processes of lumbar vertebrae L1–L5. The superficial part originates from the lateral surfaces of the last thoracic vertebra, lumbar vertebrae L1–L4, and the neighboring intervertebral discs. The lumbar plexus lies between the two layers. Together, the iliacus muscle and the psoas major form the iliopsoas, which is surrounded by the iliac fascia. The iliopsoas runs across the iliopubic eminence through the muscular lacuna to its insertion on the lesser trochanter of the femur. The iliopectineal bursa separates the tendon of the iliopsoas muscle from the external surface of the hip-joint capsule at the level of the iliopubic eminence. The iliac subtendinous bursa lies between the lesser trochanter and the attachment of the iliopsoas. Nerve supply Innervation of the psoas major is through the anterior rami of L1 to L3 nerves. Variation In fewer than 50 percent of human subjects, the psoas major is accompanied by the psoas minor muscle. One study using autopsy data found that the psoas major muscle is substantially thicker in men of African descent than in Caucasian men, and that the occurrence of the psoas minor is also ethnically variant, being present in most of the white subjects and absent in most of the black subjects. In mice, it is mostly a fast-twitching, type II muscle, while in humans it combines slow- and fast-twitching fibers. Function The psoas major joins the upper body and the lower body, the axial to the appendicular skeleton, the inside to the outside, and the back to the front. As part of the iliopsoas, psoas major contributes to flexion in the hip joint. On the lumbar spine, unilateral contraction bends the trunk laterally, while bilateral contraction raises the trunk from its supine position. In addition, attachment to the lesser trochanter, located on the aspect of the femur, causes lateral rotation and weak adduction of the hip. It forms part of a group of muscles called the hip flexors, whose action is primarily to lift the upper leg towards the body when the body