The psoas major ( or ; from grc, ψόᾱ, psóā, muscles of the loins) is a long fusiform muscle located in the lateral lumbar region between the

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

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 reprodu ...

. It joins the iliacus muscle to form the iliopsoas. In 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 The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

of lumbar vertebrae L1–L5. The superficial part originates from the lateral surfaces of the last thoracic vertebra, lumbar vertebrae L1–L4, and the neighboring

intervertebral disc An intervertebral disc (or intervertebral fibrocartilage) lies between adjacent vertebrae in the vertebral column. Each disc forms a fibrocartilaginous joint (a symphysis), to allow slight movement of the vertebrae, to act as a ligament to h ...

s. The

lumbar plexus The lumbar plexus is a web of nerves (a nervous plexus) in the lumbar region of the body which forms part of the larger lumbosacral plexus. It is formed by the Ventral ramus of spinal nerve, divisions of the first four lumbar nerves (L1-L4) and ...

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 a fascia in the region of the ilium of the pelvis. It has the following connections: * ''laterally'', to the whole length of the inner lip of the iliac crest. * ''medially'', to the linea terminalis of ...

. The iliopsoas runs across the iliopubic eminence through the

muscular lacuna The muscular lacuna (Latin: ''lacuna musculorum'') is the lateral compartment of the thigh inferior to the inguinal ligament, for the passage of the iliopsoas muscle, the femoral nerve and the lateral cutaneous nerve of the thigh The lateral ...

to its insertion on the

lesser trochanter The lesser trochanter is a conical posteromedial bony projection of the femoral shaft. it serves as the principal insertion site of the iliopsoas muscle. Structure The lesser trochanter is a conical posteromedial projection of the shaft of the fe ...

of the femur. The

iliopectineal bursa The iliopectineal bursa or the iliopsoas bursa is a large synovial bursa that separates the external surface of the hip joint capsule from the normally just the tendon of the iliopsoas muscle. The most proximal of part the iliopectineal bursa li ...

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 Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...

, 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 terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

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 ( or ) 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, pericardi ...

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

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates ...

, 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 The sit-up (or curl-up) is an abdominal endurance training exercise to strengthen, tighten and tone the abdominal muscles. It is similar to a crunch (crunches target the rectus abdominis and also work the external and internal obliques), but s ...

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 The ilioinguinal nerve is a branch of the first lumbar nerve (L1). It separates from the first lumbar nerve along with the larger iliohypogastric nerve. It emerges from the lateral border of the psoas major just inferior to the iliohypogastric, a ...

and the

iliohypogastric nerve The iliohypogastric nerve is a nerve that originates from the lumbar plexus that supplies sensation to skin over the lateral gluteal and hypogastric regions and motor to the internal oblique muscles and transverse abdominal muscles. Struct ...

s, 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 The sympathetic ganglia, or paravertebral ganglia are autonomic ganglia, of the sympathetic nervous system. Ganglia are 20,000 to 30,000 Afferent nerve fiber, afferent and Efferent nerve fiber, efferent nerve cell bodies that run along on either s ...

, and the needle tip will often pass through the psoas major during a lumbar sympathetic block. The

genitofemoral nerve The genitofemoral nerve refers to a nerve that is found in the abdomen. Its branches, the genital branch and femoral branch supply sensation to the upper anterior thigh, as well as the skin of the anterior scrotum in males and mons pubis in fema ...

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, isch ...

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 mes ...

in the lower part of the

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...

. 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