Anatomy & Organs

Musculus psoas major – structure, function & diseases

Musculus psoas major

The psoas major muscle is a skeletal muscle of the hip muscles, also known as the large lumbar flexor. The hip muscle is involved in flexion, internal and external rotation in the hip joint and plays a role in lateral flexion and inclination of the lumbar spine . Damage to the femoral nerve paralyzes the psoas major muscle.

What is the psoas major muscle?

The hip muscles are a group of skeletal muscles in the area of ​​the hip joint. The hip muscles surround the hip joint and attach to the proximal portion of the femur. The hip musculature is topographically and functionally subdivided into an inner, an outer and a deep part.

The psoas major muscle corresponds to a skeletal muscle of the inner hip muscles. Together with the iliacus muscle, the hip muscle forms the functional unit of the iliopsoas muscle, which is located in the retroperitoneal space. The psoas major muscle can be divided into a superficial and a deeper layer, which have different origins. The superficial layer arises from the twelfth thoracic vertebrae , the first lumbar vertebrae and their associated intervertebral discs. The deep layer originates from the transverse processes of the lumbar vertebrae. In the German specialist literature, the psoas major muscle is also referred to as the large lumbar muscle .

Anatomy & Structure

The psoas major muscle is in close proximity to the lumbar plexus. The motor innervation of the hip muscle takes over the femoral nerve in addition to the direct branches from L1 to L4 of the lumbar plexus. The psoas major muscle, together with the iliacus muscle, attaches to the lesser trochanter of the thigh bone (femur).

The superficial part and the deep part of the large psoas muscle unite in the course with the iliac muscle. Collectively, the muscles are wrapped in fascia and referred to in this unit as the lumbar-iliac muscle. The lumbar iliac muscle passes through the so-called muscle portal (lacuna musculorum), where it reaches the lesser trochanter (trochanter minor) of the femur. The psoas minor muscle normally lies on top of the psoas major muscle. In the human body, this muscle is extremely variable and is sometimes replaced by fascia . One of the important characteristics of the psoas major muscle is the visibility of the tissue on X-ray images. Signals from the central nervous system reach the psoas major muscle via itsmotor endplate .

Function & Tasks

The psoas major muscle is a joint-related muscle. Joints correspond to movable bone connections and enable different types of movement depending on their anatomical position. Muscles close to the joint are responsible for these movements , which have their attachment to one of the bones that meet . The insertion of the psoas major muscle Proximal part of the femur that meets the pelvic bone in the hip joint. The hip joint is a nut joint.

The femoral head, which corresponds to an approximately spherical extension of the femur , lies in the hip socket of the joint . Because the large lumbar flexor is directly connected to the femur, its contraction moves the femoral head in the hip joint. Depending on the initial position, all the muscles of the hip muscles move the hips when the pelvis is fixed or the pelvis when the thigh is fixed. 

The contractions of the hip muscles thus enable everyday forms of movement such as standing and walking. Together with the iliacus muscle, the psoas major muscle forms the iliopsoas muscle, which is considered the strongest flexor of the hip joint. Bending is also referred to as inflection in technical jargon. Extension or stretching in the hip joint is the opposite form of movement initiated by the extensors of the hip muscles.

The functional unit consisting of the psoas major and iliacus muscles is not only responsible for flexing the hip joint, but also for straightening the trunk area from the supine position. This straightening requires a flexion in the hip joint. In addition, the two muscles roll the thigh outwards and are thus involved in the rotational movement in the joint. The psoas major muscle is also involved in side bending (lateral flexion) and inclination of the lumbar spine.


Paralysis of the psoas major muscle occurs as part of damage to the supplying femoral nerve. This peripheral nerve supplies a variety of muscles. Femoral paralysis therefore manifests itself in the form of severe movement disorders. A complete paresis of the nerve structure leads to a total failure of the psoas group.

The hip joint can no longer be actively bent. In addition, those affected cannot or hardly get up from lying down. In addition, the patient’s knee remains in a flexed position and can no longer be straightened under its own power. When they step, the affected person’s leg buckles. In the reflex examination, the neurologist notices a failure or a weakening of the patellar tendon reflex. Paralysis of the femur can be caused, for example, by hematomas , especially bruises in the iliacus and psoas muscles. Postoperatively, the nerve can be paralyzed due to stretching or pressure damage in the appendix area, as well as due to prostatectomies , kidney transplants, childbirth or major hip surgery.

Paralysis of the nerve caused by trauma is usually based on uncontrolled and sudden overstretching. In individual cases, tumors or aneurysms compress the nerve structure, which can also lead to paralysis of the psoas major muscle. Compressions from hyperplastic femurs are also conceivable. In individual cases, paralysis also occurs after a herpes simplex infection. The right large lumbar muscle can also indicate inflammation of the appendix . With a strong contraction of the muscle in the sense of a thigh flexion, inflammation of the appendix causes pain in the right lower abdominal region.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.