Musculus Psoas Major: Structure, Function & Diseases

The psoas major muscle is a skeletal muscle of the hip muscles, also known as the great lumbar flexor. The hip muscle is involved in flexion and 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 encircle the hip joint and attach to the proximal portion of the femur. Topographically and functionally, the hip muscles are subdivided into an internal, an external and a deep portion. The psoas major muscle corresponds to a skeletal muscle of the internal hip musculature. 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 subdivided into a superficial layer and a deeper layer, which have different origins. The superficial layer originates from the twelfth thoracic vertebral body, the first lumbar vertebral bodies and their associated intervertebral discs. The deep layer originates from the transverse processes of the lumbar vertebrae. In German literature, the musculus psoas major is also referred to as the great lumbar muscle.

Anatomy and structure

The psoas major muscle lies in close proximity to the lumbar plexus. Motor innervation of the hip muscle is provided by the femoral nerve in addition to direct branches from L1 to L4 of the lumbar plexus. The psoas major muscle, together with the iliacus muscle, is attached to the lesser trochanter of the femur. The superficial part and the deep part of the large lumbar muscle unite in the course with the iliac muscle. Together, the muscles are encased in fascia and in this unit are referred to as the lumbar iliac muscle. The lumbar iliac muscle passes through the so-called muscular gate (lacuna musculorum), where it reaches the small rolling mound (trochanter minor) of the femur. On top of the psoas major muscle usually lies the psoas minor muscle. In the human body, this muscle is extremely variably formed and is sometimes replaced by fascia. One of the important features of the psoas major muscle is considered to be the visibility of the tissue on X-ray images. Signals from the central nervous system reach the psoas major muscle through its motor endplate.

Function and tasks

The psoas major muscle is a muscle that is close to joints. Joints correspond to mobile bone connections and allow different types of movement depending on their anatomical location. Muscles that are close to the joint and have their insertion at one of the meeting bones are responsible for these movements. The attachment of the Musculus psoas major proximal part of the femur, which meets the pelvic bone in the hip joint. The hip joint is a nut joint. In the acetabulum of the joint lies the femoral head, which corresponds to a roughly spherical process of the femur. Since 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 musculature move the hip when the pelvis is fixed or move 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. Flexion is also known as flexion in technical language. 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 muscle and the iliacus muscle is responsible not only for flexion in the hip joint, but also for straightening the trunk area from the supine position. This straightening, in fact, requires flexion in the hip joint. In addition, the two muscles roll the thigh outward and are thus involved in the rotational movement in the joint. The psoas major muscle also participates in lateral tilt (lateral flexion) and inclination of the lumbar spine.

Diseases

Paralysis of the psoas major muscle occurs in the setting of damage to the supplying femoral nerve. This peripheral nerve supplies a variety of muscles. Femoral nerve palsy therefore manifests as severe movement disorders.In the case of complete paresis of the nerve structure, there is total failure of the psoas group. The hip joint can no longer be actively bent. In addition, the affected person can no longer or hardly straighten up from a lying position. In addition, the patient’s knee remains in a flexed position and can no longer be extended under its own power. When stepping, the affected person’s leg buckles. In the reflex examination, the neurologist notices a failure or weakening of the patellar tendon reflex. Femoral paralysis can be caused by hematomas, for example, especially bruises in the iliac and psoas muscles. Postoperatively, the nerve can be paralyzed because of stretching or pressure damage in the appendix area, as well as by prostatectomies, kidney transplants, childbirth, or more severe hip surgery. Traumatically induced paralysis of the nerve is usually due to uncontrolled and sudden hyperextension. In isolated cases, tumors or aneurysms compress the nerve structure, which can also lead to paralysis of the psoas major muscle. Compression by hyperplastic femurs is also conceivable. In individual cases, paralysis also occurs after a herpes simplex infection. The right greater lumbar muscle may also suggest inflammation of the appendix. When the muscle is strongly contracted in the sense of a thigh flexion, inflammation of the appendix causes pain in the right lower abdominal region.