The muscle originates on the inner surface of the ilium and the lower borders of the transverse processes (l1-l5), sides of the bodies of the last thoracic vertebra (t12), lumbar vertebrae (l1-l5), intervertebral fibrocartilages, and base of the sacrum.

The iliopsoas muscle originates on the inner surface of the ilium and the lower borders of the transverse processes (L1-L5), sides of the bodies of the last thoracic vertebra (T12), lumbar vertebrae (L1-L5), intervertebral fibrocartilages, and base of the sacrum.

Explanation:

The iliopsoas is a muscle that acts in very important functions and is often the cause of low back pain, being a muscle that is not very accessible. It originates on the inner surface of the ilium, base of the sacrum and sides of the bodies of the last dorsal vertebra (known thoracic vertebra as well) (T12) and of all the lumbar vertebrae.

Anatomically the iliopsoas is formed by two muscles: the iliac and the psoas, which in turn we could divide into psoas major and psoas minor. The psoas major is inserted proximally in the body of the 12th dorsal vertebra, in all the lumbar vertebrae and through thinner fascicles, in the lumbar transverse processes. On the other hand, we have the psoas minor that is absent in 41-50% of people. Its path includes from the anterolateral face of the 12th dorsal vertebra and the first two lumbar, until reaching the pectineal line.

Finally, the iliac muscle, which is the second muscular component, originates in the upper two thirds of the internal surface of the iliac fossa, until it reaches the lesser trochanter. It's here that the majority of the fibers join the psoas major forming the tendon of the iliopsoas.