The psoas major is a lengthy fusiform muscle that can be found on the side of the lumbar region of the vertebral column. It also is found at the brim of the lesser pelvis area. It connects the iliacus muscle to the iliopsosas. In some (but not all) humans, this muscle is assisted by the psoas minor. This muscle is divided into two parts;.it has both a superficial part and a deep part. The deep part starts at the transverse process of the I-V lumbar vertebrae. The superficial part starts at the lateral surface of the end of the thoracic vertebrae, the I-V lumbar vertebrae, and nearby invertebral discs. The lumbar plexus is what separates the superficial part and the deep part. The psoas major assists with the flexion and external rotation of the hip joint. It is also considered part of the group of muscles that are known as the hip flexors. If this muscle tightens, the action may result in lower back pain.
Written and medically reviewed by the Healthline Editorial Team
In Depth: Psoas major