Psoas Major

The psoas major is a long, fusiform muscle extending from the lumbar region of the vertebral column through the pelvis to the proximal femur. It forms an important muscular connection between the axial skeleton and the lower limb. In the pelvis, the psoas major joins the iliacus, and the two muscles pass beneath the inguinal ligament as the iliopsoas before inserting on the lesser trochanter of the femur.^[1][2]

The psoas major is one of the most important flexors of the hip. When the femur is fixed, it can also act on the lumbar vertebral column and pelvis. Its anatomical position gives it additional importance because the lumbar plexus forms within its substance and several major nerves emerge from its borders.

Although commonly studied with the muscles of the hip, much of the psoas major lies within the posterior abdominal wall. It therefore has important relationships with abdominal, retroperitoneal, pelvic, neural, and vascular structures.

Location

The psoas major lies along the sides of the lumbar vertebral column. It begins in the posterior abdominal wall, descends across the pelvic brim, and enters the thigh beneath the inguinal ligament.

Superiorly, the muscle lies adjacent to the lumbar vertebral bodies and transverse processes. Inferiorly, it passes anterior to the hip joint and combines with iliacus before reaching the lesser trochanter.

The muscle is enclosed by the psoas fascia, a fascial layer that surrounds it and is continuous with neighboring fascial structures of the posterior abdominal wall.

General Anatomy

Feature	Anatomy
Muscle	Psoas major
Location	Posterior abdominal wall, pelvis and proximal thigh
Origin	Bodies, intervertebral discs and transverse processes of lumbar vertebral region
Insertion	Lesser trochanter of femur through iliopsoas tendon
Innervation	Anterior rami of lumbar spinal nerves
Principal action	Hip flexion
Functional group	Iliopsoas
Major neural relationship	Lumbar plexus forms within the muscle

Origin

The psoas major has an extensive origin from the lumbar vertebral column. Its attachments include the sides of the vertebral bodies and intervening intervertebral discs in the lower thoracic and lumbar region, together with the transverse processes of the lumbar vertebrae.^[1][3]

The muscle is commonly described as arising from the sides of the bodies of T12 and the lumbar vertebrae, the intervening intervertebral discs, and the transverse processes of the lumbar vertebrae.

The vertebral body attachments and transverse process attachments give rise to fascicles that unite as the muscle descends. The lumbar spinal nerves and components of the lumbar plexus have important relationships with the spaces between and within these fascicles.

Insertion

As the psoas major descends into the pelvis, it approaches the iliacus, which occupies the iliac fossa. The two muscles become closely associated and pass beneath the inguinal ligament into the anterior thigh.

The psoas major contributes strongly to the iliopsoas tendon, which inserts onto the lesser trochanter of the femur. Iliacus fibers also insert into the tendon and adjacent femoral region.

The insertion on the lesser trochanter places the iliopsoas anterior and medial to the axis of the hip joint, giving it a strong mechanical advantage for hip flexion.

Iliopsoas

The term iliopsoas refers primarily to the functional combination of psoas major and iliacus. Although the muscles have separate origins and different nerve supplies, they converge distally and share a common functional role at the hip.

Psoas major originates from the vertebral column, whereas iliacus arises primarily from the iliac fossa. Their fibers converge as they approach the inguinal region and continue toward the lesser trochanter.

The iliopsoas is generally considered the most powerful flexor of the hip joint. Its location also allows it to contribute to stabilization and positioning of the hip during standing, walking, running, and movements of the trunk.

Course

From its vertebral origin, the psoas major descends through the posterior abdominal wall. The muscle becomes progressively narrower as it approaches the pelvis.

It crosses the pelvic brim and passes along the anterior aspect of the hip region. Together with iliacus, it travels beneath the inguinal ligament through the muscular compartment of the subinguinal region.

The iliopsoas then passes anterior to the hip joint capsule before turning toward its insertion on the lesser trochanter.

Psoas Fascia

The psoas major is enclosed by a strong fascial covering known as the psoas fascia. This fascia is part of the fascial organization of the posterior abdominal wall and blends with neighboring fascial layers.

Superiorly, fascial relationships connect the psoas region with structures near the diaphragm. Laterally, the psoas fascia is related to the fascia covering quadratus lumborum and iliacus.

The fascial compartment surrounding the psoas is clinically important because fluid, infection, or hemorrhage can track along the muscle from the retroperitoneal region toward the pelvis and proximal thigh.

Relations

The long course of the psoas major gives it numerous anatomical relationships. These differ at the abdominal, pelvic, and proximal femoral levels.

Posterior Relations

The psoas major lies directly along the lateral surfaces of the lumbar vertebral bodies and is attached to their transverse processes. The vertebral column therefore forms the principal skeletal relationship behind the muscle.

The lumbar spinal nerves emerge from the intervertebral foramina and enter the region of the muscle, where their anterior rami contribute to formation of the lumbar plexus.

Medial Relations

Medially, the psoas major lies close to the lumbar vertebral bodies and major vessels of the posterior abdominal region. The sympathetic trunks also descend along the anterolateral surfaces of the vertebral bodies medial to the psoas muscles.

Lateral Relations

The quadratus lumborum lies posterolateral to the psoas major. More inferiorly, the iliacus occupies the iliac fossa lateral to the psoas and joins it as the two muscles approach the thigh.

Anterior Relations

Several retroperitoneal structures lie anterior to the psoas major. These relationships differ on the right and left sides and include portions of the kidneys, ureters, colon, and major vessels.

The ureter descends on the anterior surface of the psoas major, making the muscle an important posterior landmark when following the ureter through the abdomen.

Relationship to the Lumbar Plexus

The lumbar plexus forms within the posterior portion of the psoas major from the anterior rami of the upper lumbar spinal nerves, with contributions varying according to the nerve and individual anatomy.

Several major nerves emerge from different borders or surfaces of the muscle. This makes the psoas major one of the most important muscular landmarks for understanding lumbar plexus anatomy.

Nerve	Relationship to Psoas Major
Femoral nerve	Emerges from lateral border
Lateral femoral cutaneous nerve	Emerges from lateral border
Iliohypogastric nerve	Emerges from lateral border superiorly
Ilioinguinal nerve	Emerges from lateral border superiorly
Genitofemoral nerve	Pierces the muscle and descends on its anterior surface
Obturator nerve	Emerges from medial border

Relationship to the Femoral Nerve

The femoral nerve forms within the psoas major and emerges from its lateral border. It then descends in the groove between psoas major and iliacus.

The nerve passes beneath the inguinal ligament into the thigh, lateral to the femoral vessels. Its position between the iliacus and psoas major makes the iliopsoas region an important anatomical landmark for locating the femoral nerve.

The femoral nerve supplies the anterior compartment of the thigh and provides sensory branches to the anterior thigh and medial leg through its terminal branches.

Relationship to the Obturator Nerve

The obturator nerve forms within the lumbar plexus and emerges from the medial border of the psoas major. It then descends toward the lateral wall of the pelvis and passes through the obturator canal.

This course contrasts with that of the femoral nerve, which emerges laterally. The psoas major therefore provides a useful reference for understanding the divergence of major lumbar plexus branches toward different compartments of the lower limb.

Relationship to the Genitofemoral Nerve

The genitofemoral nerve is distinctive because it typically pierces the psoas major and emerges onto its anterior surface.

It descends on the muscle before dividing into genital and femoral branches. This anterior relationship makes the genitofemoral nerve an important structure encountered on the surface of the psoas during retroperitoneal dissection.

Relationship to the Ureter

The ureter descends retroperitoneally on the anterior surface of the psoas major. This relationship is especially useful in anatomical dissection and cross-sectional imaging.

As the ureter descends toward the pelvis, it crosses structures near the pelvic brim and continues toward the urinary bladder. The psoas major provides a consistent muscular landmark along part of this abdominal course.

Relationship to the Kidney

The kidneys occupy the posterior abdominal wall at a higher level and lie anterior to muscles that include psoas major and quadratus lumborum.

The psoas major therefore contributes to the muscular bed of the kidney, particularly medially. Fascial and retroperitoneal tissues separate the renal structures from the muscle itself.

Relationship to the Diaphragm

The superior psoas region is related to the diaphragm. The medial arcuate ligament of the diaphragm forms a fascial arch over the upper portion of the psoas major.

This relationship creates continuity between the posterior abdominal wall and the inferior thoracic region and is an important landmark in the attachments of the diaphragm.

Relationship to the Hip Joint

After passing beneath the inguinal ligament, the iliopsoas lies anterior to the hip joint capsule. Its tendon continues toward the lesser trochanter.

This close relationship explains why movement of the iliopsoas tendon occurs directly adjacent to the anterior hip during flexion and extension.

A large bursa, the iliopsoas bursa, commonly intervenes between the iliopsoas tendon and the anterior hip joint region.

Iliopsoas Bursa

The iliopsoas bursa, also called the iliopectineal bursa, is a large synovial bursa located between the iliopsoas and structures of the anterior hip, including the joint capsule.

Its function is to reduce friction as the iliopsoas tendon moves over the anterior aspect of the hip during flexion and extension.

The bursa may communicate with the hip joint cavity in some individuals. Its size and anatomical relationships can therefore become important in conditions involving distension or inflammation.

Innervation

The psoas major receives direct muscular branches from the anterior rami of lumbar spinal nerves, commonly described principally from L1 to L3, with variation in the exact segmental contribution.

This differs from the iliacus, which is supplied primarily by the femoral nerve. Despite functioning together as the iliopsoas, the two muscles therefore retain distinct innervation reflecting their different developmental and anatomical origins.

Blood Supply

The psoas major receives blood from several arteries along its extensive course. Important contributions arise from the lumbar arteries and the iliolumbar artery, with additional regional contributions possible from neighboring vessels.

The lumbar arteries arise from the posterior aspect of the abdominal aorta and supply muscles and other structures of the posterior abdominal wall.

The iliolumbar artery, usually arising from the posterior division of the internal iliac artery, contributes to the vascular supply of the lower portion of the psoas and iliacus region.

Actions

Hip Flexion

The principal action of the psoas major is flexion of the thigh at the hip joint. Acting together with iliacus, it forms the powerful iliopsoas muscle group.

Hip flexion brings the thigh toward the trunk and is required during walking, running, climbing stairs, and movements in which the lower limb is advanced forward.

Flexion of the Trunk

When the femur is fixed, bilateral contraction of the psoas major can contribute to flexion of the trunk at the hip. The mechanical effect depends on body position and the activity of other muscles controlling the spine and pelvis.

The iliopsoas is therefore active not only during movement of the thigh but also during movements in which the trunk moves relative to a stabilized lower limb.

Lateral Flexion

Unilateral contraction of the psoas major can assist lateral flexion of the lumbar vertebral column toward the same side.

Because the muscle attaches directly to multiple lumbar vertebrae, its action differs from that of iliacus, which has no direct vertebral attachment.

Postural and Lumbar Stabilization

The psoas major can contribute to stabilization of the lumbar spine during movements involving the trunk and lower limbs. Its exact mechanical role varies with posture, spinal position, hip angle, and the activity of abdominal and posterior spinal muscles.

Rather than functioning simply as a hip flexor, the psoas major acts within a coordinated system that controls the relationship between the lumbar spine, pelvis, and femur.

Role During Walking

During gait, the iliopsoas contributes to advancing the lower limb by flexing the hip. Its activity becomes particularly relevant as the limb transitions from the later portion of stance toward the swing phase.

Hip flexion helps move the thigh forward so that the foot can clear the ground and the limb can be positioned for the next step.

The psoas major also participates in controlling the relationship between the pelvis and lumbar spine as forces are transferred between the trunk and lower limbs.

Role During Running

Running requires more rapid and forceful hip flexion than ordinary walking. The iliopsoas therefore becomes an important contributor to acceleration of the thigh during the recovery and swing phases.

The psoas major works with other hip flexors, including rectus femoris, sartorius, and tensor fasciae latae, although their mechanical roles and relative contributions differ according to joint position and activity.

Comparison of Psoas Major and Iliacus

Feature	Psoas Major	Iliacus
Origin	Lumbar vertebral region	Iliac fossa and adjacent iliac structures
Insertion	Lesser trochanter through iliopsoas tendon	Iliopsoas tendon and adjacent proximal femur
Innervation	Direct branches of lumbar anterior rami	Femoral nerve
Hip flexion	Yes	Yes
Direct action on lumbar spine	Possible because of vertebral attachments	No direct vertebral attachment
Lumbar plexus relationship	Plexus forms within muscle	Femoral nerve lies between iliacus and psoas

Relationship to Psoas Minor

The psoas minor is a slender muscle that may lie anterior to the psoas major when present. Unlike psoas major, it does not extend to the femur.

Psoas minor typically arises from the lower thoracic and upper lumbar vertebral region and inserts into fascial and bony structures near the pelvic brim. It is absent in a substantial proportion of individuals.

The two muscles should therefore not be considered equivalent structures despite their similar names and neighboring locations.

Anatomical Variations

The psoas major may show variation in the size and arrangement of its fascicles, vertebral attachments, and relationship to branches of the lumbar plexus.

Accessory muscular slips may occur, and nerves of the lumbar plexus can have variable relationships with these fascicles. Such variations are particularly relevant during retroperitoneal and lateral approaches to the lumbar spine.

The configuration of the distal iliopsoas tendon can also vary, including differences in the relative contributions and separation of psoas and iliacus fibers.

Clinical Significance

Psoas Sign

The psoas sign refers to pain produced by maneuvers that stretch or contract the iliopsoas in the presence of irritation near the muscle. It is classically associated with inflammatory processes adjacent to the psoas, including certain positions of an inflamed appendix.

The anatomical basis of the sign is the close relationship between the psoas major and retroperitoneal or abdominal structures. The finding is not specific to a single disease.

Psoas Abscess

A psoas abscess is a collection of infected material within or around the psoas muscle. The long retroperitoneal course of the muscle and its fascial compartment allow infection to extend along the posterior abdominal wall and potentially toward the pelvis or proximal thigh.

Its close relationship to the vertebral column also provides an anatomical pathway by which spinal infection can involve the psoas region.

Psoas Hematoma

Bleeding into the psoas or iliopsoas compartment can produce a psoas hematoma. Expansion within this region may affect neighboring neural structures, particularly branches of the lumbar plexus.

The femoral nerve is especially relevant because it emerges from the lateral border of psoas major and descends between psoas and iliacus. Compression or irritation can therefore produce neurological findings in the femoral nerve distribution.

Iliopsoas Bursitis

Inflammation or enlargement of the iliopsoas bursa can produce symptoms in the anterior hip or groin. The close relationship between the bursa, iliopsoas tendon, and hip capsule explains why abnormalities in this region may mimic or accompany hip joint pathology.

An enlarged bursa may also affect neighboring structures depending on its extent.

Internal Snapping Hip

The iliopsoas tendon can contribute to internal snapping hip, in which movement of the tendon or associated structures produces an audible or palpable snapping sensation in the anterior hip.

The anatomical basis involves movement of the iliopsoas across structures of the anterior hip as the joint moves through flexion and extension.

Hip Flexor Strain

The psoas major can be involved in injuries collectively described as hip flexor strains. Such injuries may occur during rapid acceleration, kicking, sprinting, or other activities requiring forceful hip flexion.

Because several muscles contribute to hip flexion, pain in the anterior hip or groin does not necessarily identify the psoas major as the isolated injured structure.

Lumbar Plexus Procedures

The position of the lumbar plexus within the psoas major is highly important during regional anesthesia and surgical approaches involving the posterior abdominal wall and lumbar spine.

Procedures that traverse or retract the psoas must account for the femoral, obturator, genitofemoral, and other lumbar plexus branches associated with the muscle.

Imaging

The psoas major is readily visible on CT and MRI as a large paired muscle adjacent to the lumbar vertebral column. Its size, symmetry, internal structure, and surrounding fascial planes can be assessed on cross-sectional imaging.

Imaging can demonstrate abnormalities such as collections, hemorrhage, muscular injury, masses, or changes associated with neighboring vertebral and retroperitoneal disease.

The muscle also serves as a useful anatomical landmark for identifying the lumbar vertebrae, ureters, lumbar plexus region, and retroperitoneal structures.

Surgical Anatomy

The psoas major is encountered in several approaches to the lumbar spine, retroperitoneum, pelvis, and hip. Its intimate relationship with the lumbar plexus is particularly important during procedures that involve splitting, retracting, or passing adjacent to the muscle.

The genitofemoral nerve on its anterior surface and the femoral and obturator nerves emerging from its lateral and medial borders are important landmarks. Injury to these structures can produce sensory or motor deficits in the lower limb.

The psoas major is therefore anatomically important far beyond its role as a hip flexor. It forms a direct muscular connection between the lumbar vertebral column and femur, houses the lumbar plexus, lies adjacent to major retroperitoneal structures, and joins iliacus to form the powerful iliopsoas muscle group.

Its extensive vertebral origin and distal attachment to the lesser trochanter allow it to act on either the femur or the trunk depending on which segment is stabilized. These relationships make the psoas major central to the anatomy and biomechanics of the lumbar spine, pelvis, and hip.