The major and minor psoas muscles and the iliacus muscle make up the iliopsoas musculotendinous unit (IPMU). Commonly called iliopsoas muscle. It is essential for correct standing or sitting lumbar posture, stabilizing the coxofemoral joint, and is crucial during walking and running. The iliopsoas musculotendinous unit (IPMU) is the primary flexor of the thigh with the ability to add and extra-rotate the coxofemoral joint. The iliopsoas musculotendinous unit is part of the inner muscles of the hip and forms part of the posterior abdominal wall, lying posteriorly at the retroperitoneum level.

  • The psoas major has a fusiform shape and originates from the transverse processes and the lateral surfaces of the bodies of the first four lumbar vertebrae; it involves the transverse process of the last thoracic vertebra and the vertebral body. The path also involves intervertebral discs. The muscle bundles are directed downwards, parallel to the lumbar vertebrae, reaching the iliac fossa where there are the bundles of the iliac muscle. The bundles of the psoas major muscle and the iliac muscle join together, passing under the inguinal ligament. With a robust tendon, they insert onto the small trochanter of the femur. Generally, the right muscle size is greater than the left muscle.
  • The psoas minor muscle is located in front of the major psoas, originating from the last thoracic vertebra and the first lumbar; it is present in 60% to 65% of the population. Distally, it converges with the iliac fascia and the psoas major tendon to insert onto the iliopectineal eminence (for 90% of the population). This muscle should help the action of the iliopsoas muscle.
  • The iliacus muscle has a fan shape and originates from the upper two-thirds of the iliac fossa and the lateral parts of the sacral bone wing. Its bundles (together with the major psoas muscle bundles) pass under the inguinal ligament and in front of the hip joint. The muscle bundles of the iliac muscle merge into the large psoas muscle tendon and the lesser trochanter. Small fibers that make the iliac muscle, known as the infratrochanteric muscle, lie lateral to the iliac muscle, widening its contact surface to the iliac bone


The iliopsoas muscle runs near different viscera; some pathologies affecting the muscle can simulate a disease of the internal organs, in fact has also been considered by some yoga teacher as the soul muscle with a lack of scientific evidence for it. In the presence of a lesion of the right muscle, the pain is felt in the lower right quadrant, mimicking appendicitis. A few research have reported that psoas dysfunction contribute to low back pain, and the relationship between low back pain and psoas spasticity remains hypothesized (1). The iliopsoas tendon directly overlies the femoral head. When a tight iliopsoas tendon impinges on the femoral head and anterior hip capsule repetitively, could result in a chronic injury to the iliopsoas tendon, namely iliopsoas tendinopathy. Disorders of hip and lumbar have overlapping presentations and symptoms.

Some authors have found that positive hip physical examination is prevalent in patients with low back pain, and positive hip examination findings have more pain and worse function compared with patients without hip examination findings. It is estimated that about 60.4% patients with hip osteoarthritis also complain of low back pain, and after total hip arthroplasty, significant improvement of low back pain can be observed 1-year postoperative. The iliopsoas musculotendinous unit is a powerful hip flexor which has important function in femoral external rotation and with lateral bending, flexion, and stabilizing the spine in the frontal plane. We hypothesize that the pathology of the iliopsoas musculotendinous unit may contribute to low back pain caused by hip disorders.

These observations prompted the inference that the psoas major may function as a lumbar spine stabilizer. Others have since proposed and found evidence for various roles that the psoas major may play with respect to lumbar spine stability and movement. These roles include psoas major being a flexor of the lumbar spine on the pelvis, a lateral flexor of the lumbar spine, a stabilizer of the lumbar spine, stabilizer of the hip, power source for bipedal walking and running, and controller of the lumbar lordosis when supporting difficult lumbar loads. Dangeria and Naesh conducted a clinical prospective cohort study examining the cross-sectional area of the psoas major in healthy volunteers and subjects with unilateral sciatica caused by a disc herniation. These authors demonstrated that in most patients with a lumbar disc herniation there was a significant reduction in the cross-sectional area of the psoas major on the affected side only and most prominently at the level of the disc herniation (2). Physiotherapy could be an helpful conservative treatment to improve the flexibility of IPMU with manual therapy and exercises therapy to improve core strength, balance and posture.


  1. Marrè-Brunenghi G, Camoriano R, Valle M, Boero S. The psoas muscle as cause of low back pain in infantile cerebral palsy. J Orthop Traumatol. 2008;9(1):43–47.
  2. Dangaria TR, Naesh O. Changes in cross-sectional area of psoas major muscle in unilateral sciatica caused by disc herniation. Spine (Phila Pa 1976). 1998 Apr 15;23(8):928-31.

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