What's the psoas muscle?

Please see below: Proximally, the psoas major muscle attaches to the 12th thoracic and to all of the lumbar vertebral bodies and the corresponding intervertebral discs and the transverse processes of the lumbar vertebrae. The iliacus attaches proximally to iliac fossa, the sacrum and the anterior sacroiliac ligaments. Distally, the two tendons merge forming the iliopsoas tendon to attach onto the lesser trochanter.