Iliopsoas Muscle Anatomy

Location & Overview

Iliopsoas most commonly refers to the combined hip-flexor unit formed by psoas major and iliacus. Some broader descriptions use the term iliopsoas muscle complex or iliopsoas musculotendinous unit to include psoas minor as well, but psoas minor is an inconstant muscle and has a different distal attachment from psoas major and iliacus. Psoas major arises beside the lumbar vertebral column, iliacus occupies the iliac fossa on the internal surface of the pelvis, and their fibers pass deep to the inguinal ligament before inserting together on the lesser trochanter of the femur.^[1,2]

Psoas minor is closely related to this region, but it is not always present and it does not insert onto the femur. When present, it lies anterior to psoas major and remains a lumbar-to-pelvic muscle rather than a direct femoral hip flexor.^[1,2]

The accepted anatomical terminology includes iliopsoas, iliacus, and psoas major. Hyrtl’s muscle is an older alternative name associated with iliopsoas, but it is no longer commonly used. The standard anatomical names are clearer and more precise.^[3]

Psoas major lies on each side of the lumbar vertebral column and pelvic brim. The lumbar plexus is embedded in the posterior part of psoas major, and the femoral nerve descends first through psoas major fibers and then in the groove between psoas major and iliacus. This close relationship makes iliopsoas important anatomy for the posterior abdominal wall, pelvis, anterior thigh, and hip.^[1,2]

Iliacus is a broad triangular sheet in the iliac fossa. Distally, it converges with the psoas major tendon and passes in front of the hip capsule, separated from it by a bursa. Psoas minor is much smaller and more variable, being present in about 40-60% of individuals, so it should be understood as an inconstant associated muscle rather than a universal part of femoral iliopsoas function.^[1,2]

Origin & Insertion

As a group, iliopsoas links the lumbar spine and internal pelvis to the lesser trochanter of the femur through psoas major and iliacus. Psoas minor follows a different pattern because it attaches to the pubic and fascial region rather than the femur. This is why psoas minor is best described as an associated muscle of the iliopsoas region rather than a direct hip-flexing component of the femoral iliopsoas tendon.^[1,2]

Psoas Major Origin & Insertion

Psoas major arises from the transverse processes of L1-L5 and from the bodies of T12-L5 with the adjacent intervertebral discs. Its fibers pass inferiorly through the posterior abdominal wall and across the pelvic brim to insert on the lesser trochanter of the femur.^[1,2]

Iliacus Origin & Insertion

Iliacus arises mainly from the superior two-thirds of the iliac fossa. It also attaches to the inner lip of the iliac crest and the lateral sacrum. The muscle runs inferiorly and laterally, merges with psoas major near the hip, and reaches the lesser trochanter through the common iliopsoas tendon.^[1,2]

Psoas Minor Origin & Insertion

Psoas minor, when present, arises from the bodies of T12 and L1 and the intervening intervertebral disc. It descends as a small muscle with a long tendon to insert on the iliopectineal eminence and pectineal line of the pubis. Because this attachment remains on the pelvis rather than the femur, psoas minor does not cross the hip joint.^[1,2]

Iliopsoas tendon anatomy can vary. Philippon et al. (2014) reported single, double, and triple iliopsoas tendons in cadaveric hemipelvis specimens, and Lin et al. (2022) also described multiple distal iliopsoas tendons in cadaveric work.^[4,5] Crompton et al. (2014) found bifid distal iliopsoas tendons in a pediatric MRI sample, so this evidence should be interpreted as pediatric imaging evidence rather than a general adult prevalence estimate.^[6]

Action & Function

The main action of psoas major and iliacus together is hip flexion, meaning flexion of the thigh at the hip joint. This is the key movement when the thigh moves forward relative to the pelvis, as in the swing phase of gait or when lifting the knee.^[1,2]

Iliopsoas also helps stabilize the hip joint during standing. Stabilization means that the muscle group contributes to joint control and position, not only visible movement. Iliopsoas is an especially powerful hip flexor with a long working range.^[2]

Psoas major has additional context-dependent actions. It can weakly assist lateral flexion of the trunk, can weakly assist external or lateral rotation of the femur, and may contribute to dynamic stability of the hip and femur. These secondary actions are not equivalent to its primary hip-flexion role because psoas major mechanics depend on body position, fixation, and the relationship between the trunk, pelvis, and femur.^[1,2]

When the femur or lower limb is fixed, bilateral iliopsoas activity can help flex the trunk and pelvis forward at the hip. This reverses the usual description of the action: instead of moving the thigh toward the trunk, the muscle group can help move the trunk toward the thigh.^[1,2]

Psoas minor should be kept separate from the femoral hip-flexion role of psoas major and iliacus. It may weakly flex the trunk, but it does not cross the hip and should not be described as a direct hip flexor. Neumann and Garceau (2015) proposed a possible stabilizing role through the iliac fascia, based on cadaveric dissection, but that remains a hypothesis rather than a standard action.^[1,7]

Research on psoas major mechanics supports this more nuanced view. Yoshio et al. (2002) described angle-dependent passive behavior of psoas major in a cadaveric functional study, while Bogduk, Pearcy and Hadfield (1992) modeled psoas major biomechanics and cautioned that its spinal moments are relatively small compared with its hip action.^[8,9]

Innervation

The psoas major, iliacus, and psoas minor have different nerve-supply patterns. Psoas major is supplied by direct branches from lumbar anterior rami, iliacus is supplied by the femoral nerve, and psoas minor receives a lumbar anterior-ramus branch when present.^[1,2]

Psoas major is supplied by direct branches from the lumbar anterior rami. These branches arise from the upper lumbar plexus, and root-level descriptions vary between sources, with L1-L3 and L2-L4 both reported. Anatomically, psoas major receives direct lumbar branches rather than femoral nerve supply.^[1,2,10]

Iliacus is supplied by the femoral nerve, commonly described with L2-L4 roots. Psoas minor, when present, receives a branch from the upper lumbar anterior rami and is most commonly described here as L1. Root-level ranges can be reported more broadly, but the named pattern remains femoral nerve supply to iliacus and an anterior-ramus branch to psoas minor.^[1,2]

Blood Supply

Psoas major has a broad arterial supply rather than one isolated artery. Its supply includes lumbar branches, including the lumbar branch of the iliolumbar artery, with additional contributions described from obturator, external iliac, and femoral arterial branches.^[1,2]

Iliacus receives supply from the iliolumbar artery, deep circumflex iliac artery, obturator artery, and femoral artery. This matches its position in the iliac fossa and anterior hip region, where the internal pelvic and external iliac/femoral arterial systems are closely related.^[1,2]

Psoas minor is supplied mainly by lumbar arteries. Because psoas minor is small and inconstant, its arterial supply is described separately from psoas major and iliacus.^[1]