It is surely the most important muscle of the human body, for at least three reasons:

First must with its anatomical positioning. It goes from the depth towards surface, it connects deep fabrics to surface fabrics.
Second is lièe with the pathology which it can involve: Lumbago, one of the most ignored pathologies most current and one of.
The most important third and is its faculty to express the health of one individual at the same time to determine a precise diagnosis, but especially to play a preventive part in a significant number of pathologies.

It is a long and spindle-shaped muscle, located along the lumbar column, on the basis of the twelfth dorsal vertebra and of the five lumbar vertebrae to the lesser trochanter.

Insertions:

On the lower part and side of the twelfth dorsal vertebra.
By tendineux beams on the side part of the vertebral discs and the contiguous part of the vertebral body of the vertebra located at the top and the lower part of each disc concerned. The fibrous beams are connected between them by arcades, delimiting with the vertebral body corresponding, an opening of passage of the lumbar vessels and communicating branches of the sympathetic nerve.
By fleshy strips, on the former face of the transverse apophyses of the lumbar vertebrae.

The beams which are born from the vertebral bodies and those which stick on the transverse apophyses are separate, on the level of the vertebral bodies, by a cellulous interstice in which the branches of the lumbar plexus walk on giving the nerves: fémoro-cutaneous, crural, génito-crural, small and large abdomino-genital.

The various fascicule of muscle fibreses gather in a body fleshy and bulky which descent along the lumbar column, crosses the iliaque pit interns and penetrates in the thigh while passing behind from the arcade crurale. It moves then in bottom and behind, passes in front of the articulation of the hip and comes to stick on the top of the lesser trochanter by a strong tendon.

Action:

The psoas bends the thigh on the basin. Taking its fixed point on the femur, it bends the trunk and a rotational movement prints to him which carries its face former on the opposite side.