The psoas major ( or ; from ) is a long

fusiform muscle Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle's mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force pro ...

located in the lateral lumbar region between the

vertebral column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

and the brim of the

lesser pelvis The pelvic cavity is a body cavity that is bounded by the bones of the pelvis. Its oblique roof is the pelvic inlet (the superior opening of the pelvis). Its lower boundary is the pelvic floor. The pelvic cavity primarily contains the reproduc ...

. It joins the iliacus muscle to form the

iliopsoas The iliopsoas muscle (; ) refers to the joined psoas major and the iliacus muscles. The two muscles are separate in the abdomen, but usually merge in the thigh. They are usually given the common name ''iliopsoas''. The iliopsoas muscle joins t ...

. In other animals, this muscle is equivalent to the tenderloin.

Structure

The psoas major is divided into a superficial and a deep part. The deep part originates from the transverse processes of

lumbar vertebrae The lumbar vertebrae are located between the thoracic vertebrae and pelvis. They form the lower part of the back in humans, and the tail end of the back in quadrupeds. In humans, there are five lumbar vertebrae. The term is used to describe t ...

L1–L5. The superficial part originates from the lateral surfaces of the last

thoracic vertebra In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...

, lumbar vertebrae L1–L4, and the neighboring

intervertebral disc An intervertebral disc (British English), also spelled intervertebral disk (American English), lies between adjacent vertebrae in the vertebral column. Each disc forms a fibrocartilaginous joint (a symphysis), to allow slight movement of the ver ...

s. The

lumbar plexus The lumbar plexus is a web of nerves (a nerve plexus) in the lumbar region of the body which forms part of the larger lumbosacral plexus. It is formed by the divisions of the first four lumbar nerves (L1–L4) and from contributions of the su ...

lies between the two layers. Together, the iliacus muscle and the psoas major form the iliopsoas, which is surrounded by the

iliac fascia The iliac fascia (or Abernethy's fascia) is the fascia overlying the iliacus muscle. Superiorly and laterally, the iliac fascia is attached to the inner aspect of the iliac crest; inferiorly and laterally, it extends into the thigh to unite with ...

. The iliopsoas runs across the iliopubic eminence through the muscular lacuna to its insertion on the

lesser trochanter In human anatomy, the lesser trochanter is a conical, posteromedial, bony projection from the shaft of the femur. It serves as the principal insertion site of the iliopsoas muscle. Structure The lesser trochanter is a conical posteromedial p ...

of the femur. The iliopectineal bursa separates the tendon of the iliopsoas muscle from the external surface of the hip-joint capsule at the level of the iliopubic eminence. The iliac subtendinous bursa lies between the lesser trochanter and the attachment of the iliopsoas.

Nerve supply

Innervation of the psoas major is through the anterior rami of L1 to L3 nerves.

Variation

In fewer than 50 percent of human subjects,Platzer (2004), p 234 the psoas major is accompanied by the psoas minor muscle. One study using autopsy data found that the psoas major muscle is substantially thicker in men of African descent than in Caucasian men, and that the occurrence of the psoas minor is also ethnically variant, being present in most of the white subjects and absent in most of the black subjects. In mice, it is mostly a fast-twitching, type II muscle, while in humans it combines slow- and fast-twitching fibers.

Function

The psoas major joins the upper body and the lower body, the axial to the appendicular skeleton, the inside to the outside, and the back to the front. As part of the iliopsoas, psoas major contributes to

flexion Motion, the process of movement, is described using specific anatomical terminology, anatomical terms. Motion includes movement of Organ (anatomy), organs, joints, Limb (anatomy), limbs, and specific sections of the body. The terminology used de ...

in the hip joint. On the lumbar spine, unilateral contraction bends the trunk laterally, while bilateral contraction raises the trunk from its

supine position The supine position () means lying horizontally, with the face and torso facing up, as opposed to the prone position, which is face down. When used in surgical procedures, it grants access to the peritoneal, thoracic, and pericardium, pericardial ...

.''Thieme Atlas of Anatomy'' (2006), p 422 In addition, attachment to the lesser trochanter, located on the aspect of the

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

, causes lateral rotation and weak adduction of the hip. It forms part of a group of muscles called the hip flexors, whose action is primarily to lift the upper leg towards the body when the body is fixed or to pull the body towards the leg when the leg is fixed. For example, when one performs a sit-up that brings the torso (including the lower back) away from the ground and towards the front of the leg, the hip flexors (including the iliopsoas) will flex the spine upon the pelvis. Owing to the frontal attachment on the vertebrae, rotation of the spine will stretch the psoas.

Clinical significance

Tightness of the psoas can result in spasms or lower-back pain by compressing the lumbar discs. A hypertonic and inflamed psoas can lead to irritation and entrapment of the ilioinguinal and the iliohypogastric nerves, resulting in a sensation of heat or water running down the front of the thigh. Psoas can be palpated with active flexion of the hip. A positive psoas contracture test and pain with palpation reported by the patient indicate clinical significance. Care should be taken around the abdominal organs, especially the colon when palpating deeply. The appearance of a protruding belly can visually indicate a hypertonic psoas, which pulls the spine forward while pushing the abdominal contents outward. The psoas lies posterolateral to the lumbar sympathetic ganglia, and the needle tip will often pass through the psoas major during a lumbar sympathetic block. The

genitofemoral nerve The genitofemoral nerve is a mixed branch of the lumbar plexus derived from anterior rami of lumbar nerves L1–L2. It splits into a genital branch and a femoral branch. It provides sensory innervation to the upper anterior thigh, as well as t ...

is formed in the midsection of the psoas muscle by the union of branches from the anterior rami of L1 and L2 nerve roots. The nerve then courses inferiorly within the psoas muscle and finally "pierces" the muscle and emerges on the anterior surface of the psoas distally. The nerve then traverses the

retroperitoneum The retroperitoneal space (retroperitoneum) is the anatomical space (sometimes a potential space) behind (''retro'') the peritoneum. It has no specific delineating anatomical structures. Organs are retroperitoneal if they have peritoneum on their ...

, descending over the anterior surface of the psoas.

Notes

References

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Additional images

File:Psoas major muscle - animation05.gif, Position of psoas major muscle. Animation.

Hip bone The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the Ilium (bone) ...

s are shown in semi-transparent. File:Gray1038.png, Horizontal disposition of the

peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mesotheli ...

in the lower part of the

abdomen The abdomen (colloquially called the gut, belly, tummy, midriff, tucky, or stomach) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates. The area occupied by the abdomen is called the abdominal ...

. Psoas major labeled at bottom left. File:Gray388.png, Diagram of a transverse section of the posterior abdominal wall, to show the disposition of the lumbodorsal fascia. File:Gray430.png, Muscles of the iliac and anterior femoral regions. File:Gray823.png, The lumbar plexus and its branches. File:Slide11pp.JPG, Psoas major muscle File:Slide5ppp.JPG, Psoas major muscle

External links

{{Authority control Hip flexors Hip muscles Spine flexors Iliopsoas muscles Muscles of the lower limb