The iliopsoas is not a single muscle but a composite of two distinct muscles, the psoas major and the iliacus, that merge near the hip and work together as the body’s most powerful hip flexor. It runs from the lower spine and the inner bowl of the pelvis down to the upper thighbone, and it plays a central role in walking, running, sitting upright, and stabilizing the lower back. Despite its importance, most people never hear of it until something goes wrong, usually in the form of hip pain, a snapping sensation, or an aching lower back that resists conventional explanations.
Two Muscles Sharing One Job
The psoas major originates from the spine. Its superficial head arises from the intervertebral discs between the lowest thoracic vertebra and the fourth lumbar vertebra, along with the adjacent rims of those vertebral bodies. A deeper head comes from bony projections on all five lumbar vertebrae and the twelfth rib. The iliacus, by contrast, has nothing to do with the spine. It originates from the iliac fossa, the broad, concave inner surface of the pelvic bone. Both muscles converge as they descend toward the hip and attach to the lesser trochanter, a small bony bump on the inner side of the upper femur.1PubMed Central. Action and Contribution of the Iliopsoas and Rectus Femoris as Hip Flexor Agonists Examined with Anatomical Analysis
For decades, anatomy textbooks described the two muscles as sharing a single “conjoint tendon” at that attachment site. Recent dissection work has challenged this. Researchers found that the psoas tendon inserts along a crest running from the top to the front of the lesser trochanter, while the iliacus inserts separately, attaching with fleshy fibers into the front of the lesser trochanter and a ridge extending downward from it. When this separate-insertion anatomy was modeled in three dimensions, it showed that the traditional conjoint-tendon model underestimates the iliacus’s capacity to generate hip flexion.2PubMed. Revision of hip flexor anatomy and function in modern humans, and implications for the evolution of hominin bipedalism The distinction matters clinically, because a surgeon or therapist treating one component of the iliopsoas complex may be affecting only part of the hip-flexion apparatus.
A third, much smaller muscle sometimes enters the picture. The psoas minor sits in front of the psoas major, but it is not even present in everyone. Cadaver studies across different populations have found it in anywhere from about a quarter to two-thirds of people, and its size varies considerably. In one Bulgarian cohort, the mean length was under 20 centimeters, while Indian and Brazilian populations tended to have longer specimens.3PubMed Central. Psoas Minor Muscle: A Cadaveric Morphometric Study Because it inserts on the pelvic fascia rather than the femur, the psoas minor contributes little to hip flexion and is largely a vestigial structure.4PubMed Central. Action and Contribution of the Iliopsoas and Rectus Femoris as Hip Flexor Agonists Examined with Anatomical Analysis
What the Iliopsoas Actually Does
The most obvious function is hip flexion, the movement of lifting your knee toward your chest. Every step you take uses the iliopsoas to swing the leg forward. Intramuscular EMG recordings in people walking and running on a treadmill showed that the iliacus and psoas both increase their electrical activity markedly as speed rises, with a particularly sharp jump when a person transitions from walking to running at speeds above about 2 meters per second (roughly a brisk jog). The basic firing pattern stayed consistent regardless of speed, suggesting a relatively fixed neural program governs the muscle during locomotion, with only minor timing adjustments needed as you speed up.5Acta Physiologica Scandinavica. Intramuscular EMG from the Hip Flexor Muscles during Human Locomotion
But reducing the iliopsoas to “the hip flexor” sells it short. Because the psoas major originates on the lumbar spine, it also acts as a spinal stabilizer. EMG research has demonstrated that the psoas and iliacus have distinct, task-specific activation patterns. The iliacus can be selectively activated to stabilize the pelvis when the opposite leg extends during standing. The psoas, meanwhile, kicks in selectively when you sit with a straight back and during activities that require stabilizing the spine against side-to-side forces.6PubMed. The role of the psoas and iliacus muscles for stability and movement of the lumbar spine, pelvis and hip So although these two muscles get lumped under one name, they can and do act independently depending on what the body needs at any given moment.
The Iliopsoas, Posture, and Low Back Pain
A tight or shortened iliopsoas is one of the most commonly cited culprits in excessive lumbar lordosis, the exaggerated inward curve of the lower back. A cross-sectional study of desk workers found a moderate positive correlation between iliopsoas muscle length and the degree of lumbar lordosis, meaning that shorter muscles were associated with a flatter or differently curved lumbar spine, and length differences tracked with postural changes.7PubMed Central. A Cross-sectional Study on Association of Iliopsoas Muscle Length with Lumbar Lordosis Among Desk Job Workers People who sit for long hours tend to keep the iliopsoas in a shortened position, which can lead to adaptive tightening over time.
Whether the size of the psoas major directly causes low back pain is less clear than popular accounts suggest. An MRI-based study found that people with low back pain actually had larger psoas cross-sectional areas at certain lumbar levels compared to pain-free controls, though patients whose spines showed degenerative changes had smaller psoas muscles than those without such changes.8PubMed Central. MRI features of the psoas major muscle in patients with low back pain A separate study using spine MRI found that increasing psoas area was associated with lower pain scores, suggesting that a bigger, presumably stronger, psoas may be somewhat protective.9PubMed Central. The importance of psoas muscle on low back pain: a single-center study on lumbar spine MRI Yet another study in patients with chronic nonspecific low back pain found no meaningful relationship between psoas morphology and either pain scores or disability.10PubMed. The relationship between the psoas major muscle morphology characteristics with disability index and pain in patients with chronic nonspecific low back pain
The takeaway is that psoas size alone is a poor predictor of whether someone has low back pain. The relationship between the iliopsoas and the lower back is real but involves more than just muscle bulk. Tightness, activation patterns, and how the muscle interacts with surrounding structures all seem to matter, and no single MRI measurement captures the full picture.
Snapping Hip Syndrome
If you have ever felt or heard a popping or clicking sensation at the front of your hip during certain movements, the iliopsoas tendon is a likely suspect. Internal snapping hip syndrome occurs when the iliopsoas tendon catches on a bony prominence as it slides from a lateral to a medial position during hip extension and internal rotation. The femoral head appears to be the most common obstruction, though the iliopectineal eminence of the pelvis, the iliopsoas bursa, and even a bony outgrowth of the lesser trochanter can all contribute.11PubMed Central. Snapping Hip Syndrome: A Comprehensive Update The iliopsoas bursa, which sits between the tendon and the bony pelvis, is the largest synovial bursa in the body, and when it is inflamed it can become a major mechanical obstacle to smooth tendon gliding.
Internal snapping hip often produces an audible pop and can be painful, though in many people it is painless and merely annoying. Ultrasound imaging can capture the tendon’s abnormal movement in real time and help distinguish internal snapping from problems inside the joint itself.12PubMed Central. Understanding and Treating the Snapping Hip Treatment typically starts with anti-inflammatory medication, stretching, and avoiding the activities that trigger the snap. Persistent cases that do not respond to conservative care may need surgery to lengthen the iliopsoas tendon.
Iliopsoas Problems After Hip Replacement
An underappreciated cause of groin pain after total hip arthroplasty is irritation of the iliopsoas tendon by the artificial joint’s components. This typically happens when the front edge of the acetabular cup (the socket portion of the implant) overhangs slightly, or when retained cement, protruding screws, or other hardware rubs against the tendon as it passes over the front of the hip.13PubMed Central. Iliopsoas Impingement After Total Hip Arthroplasty: A Review of Diagnosis and Management The result is persistent anterior hip or groin pain, often worst with activities that involve lifting the knee, like climbing stairs or getting out of a car.
A systematic review of iliopsoas injections for this kind of impingement found that cortisone or anesthetic injections into the area around the tendon substantially reduced pain scores and improved hip function. However, roughly 29 percent of patients who received injections eventually went on to need surgery, with tendon release (tenotomy) being the most common procedure.14PubMed. Iliopsoas Injections: A Systematic Review of Patient Outcomes and Progression to Surgery When endoscopic tenotomy was studied specifically in post-arthroplasty patients, over 93 percent reported resolution of pain at follow-up, with significant improvements in daily-activity and sports-function scores and only one minor complication reported.15PubMed. Evaluation of Endoscopic Iliopsoas Tenotomy for Treatment of Iliopsoas Impingement After Total Hip Arthroplasty
Testing Iliopsoas Tightness
The modified Thomas test is the standard clinical method for assessing iliopsoas length. You lie on the edge of a table, pull one knee to your chest to flatten your lower back, and let the other leg hang off the end. If that hanging thigh cannot reach the horizontal or stays above it, the iliopsoas on that side is considered tight. The test is simple, but its reliability depends on technique. Multiple studies have examined this and generally found high intrarater and interrater reliability when the pelvis and lower back are properly stabilized.16PubMed Central. Reliability of the modified Thomas test using a lumbo-plevic stabilization17PubMed Central. Reliability of Goniometric Techniques for Measuring Hip Flexor Length Using the Modified Thomas Test Without that stabilization, compensatory pelvic tilting can mask tightness or create false positives, which is why a clinician who does not control for pelvic movement may get inconsistent results.
Rehabilitation and Conservative Treatment
Most iliopsoas-related problems respond to non-surgical treatment. For iliopsoas tendinopathy, a condition common in runners and athletes who repeatedly flex the hip under load, progressive loading programs have shown good outcomes. A case report followed a runner treated with an eccentric-biased exercise program combined with kinetic-chain exercises and progressive tendon loading. After 12 weeks the runner returned to unrestricted running, and at five-year follow-up she had maintained her pre-injury mileage with continued improvement on pain and function measures.18PubMed Central. The Rehabilitation of a Runner with Iliopsoas Tendinopathy Using an Eccentric-Biased Exercise – A Case Report
For broader iliopsoas syndrome, which can involve pain in the groin, anterior thigh, or lower back triggered by hip flexion, a hip-rotation strengthening program has produced positive results. In a retrospective case series, seven of nine patients improved, and all but two returned to full activity at follow-up averaging over a year after diagnosis.19PubMed. Treatment of iliopsoas syndrome with a hip rotation strengthening program: a retrospective case series The emphasis on hip rotator strength, rather than just stretching the iliopsoas, reflects the idea that surrounding muscles need to share the workload rather than leaving the iliopsoas to handle forces it is not designed to bear alone.
Stretching still has its place, especially for people who sit for extended periods. A half-kneeling lunge with the back knee on the ground and the pelvis tucked under targets the iliopsoas directly. The key is to avoid arching the lower back during the stretch, because that lumbar extension takes tension off the psoas and loads the spine instead. Strengthening the glutes and abdominals at the same time helps maintain the length gains from stretching, because a weak posterior chain and a weak core leave the iliopsoas chronically overworked.
Nerve Relationships and Referred Symptoms
The femoral nerve, one of the body’s major lower-limb nerves, passes directly through or alongside the iliopsoas complex on its way to the front of the thigh. Anatomical studies have found that the nerve varies in about 35 percent of cases, sometimes splitting into two or three branches with the psoas muscle threading between the slips.20Folia Morphologica. Lumbar plexus — review This intimate relationship means that a swollen, spasming, or enlarged psoas can potentially compress the femoral nerve, producing pain, numbness, or weakness down the front of the thigh and into the knee. Clinicians sometimes attribute vague anterior thigh pain to the knee when the real source is iliopsoas-related femoral nerve irritation higher up.
The iliopsoas also sits next to retroperitoneal structures, including the kidneys, ureters, and parts of the colon. Diseases in those organs can masquerade as musculoskeletal hip-flexor problems. Diverticular disease in the colon, for instance, can cause inflammation that spreads into the retroperitoneum and irritates the psoas, producing hip and back symptoms that mimic a primary muscle problem.21PubMed Central. Conservative management of complex diverticular disease causing a retroperitoneal perforation Iliopsoas abscesses are another example. Primary abscesses tend to occur in immunocompromised individuals and are most often caused by Staphylococcus aureus, while secondary abscesses arise from nearby infections in the spine, intestine, or urinary tract and tend to harbor gut bacteria.22JAMA Surgery. Primary vs Secondary Iliopsoas Abscess: Presentation, Microbiology, and Treatment Because the muscle lies deep in the body, imaging with CT or MRI is usually required to spot these collections.
The Iliopsoas in Human Evolution
The iliopsoas played a pivotal role in the transition from quadrupedal movement to upright bipedal walking. In four-legged primates, the hip rarely extends fully, so the iliopsoas functions mainly during the swing phase to pull the leg forward. In humans, the hip goes through a “double extension” during the gait cycle, fully extending behind the body during push-off and then fully flexing forward during swing. This required significant changes in the iliopsoas’s moment arm, the leverage it exerts around the hip joint, along with shifts in the relative size and function of the gluteal muscles and hamstrings.23PubMed Central. Evolution of the human hip. Part 2: muscling the double extension
The separate-insertion anatomy described earlier may have evolutionary significance. A biomechanical model using separate psoas and iliacus insertions produced more realistic predictions of hip-flexion capacity than one using a shared tendon.24PubMed. Revision of hip flexor anatomy and function in modern humans, and implications for the evolution of hominin bipedalism The implication is that as the pelvis widened and the lumbar spine curved to support upright posture, having two independently inserting muscles gave finer-grained control over hip flexion than a single fused tendon would have allowed. In a sense, the iliopsoas is a muscle built by committee, two components cooperating with just enough independence to meet the unusual demands of walking on two legs.
Adolescent Avulsion Fractures of the Lesser Trochanter
In teenagers, the attachment point of the iliopsoas on the lesser trochanter has a growth plate that has not yet fused to the rest of the femur. A sudden, forceful contraction of the iliopsoas, typically during sprinting, kicking, or rapid direction changes in sport, can yank that bony fragment away from the thighbone entirely. This avulsion fracture is uncommon but almost exclusive to adolescents, because in adults the growth plate has fused and the bone is far stronger than the tendon. The classic presentation is sudden, sharp groin pain during an explosive athletic movement, with pain on any attempt to flex the hip against resistance. Most of these fractures heal with rest and activity modification, though the displaced fragment remains visible on X-rays and occasionally causes confusion on later imaging if clinicians are unaware of the old injury.
Imaging the Iliopsoas
Because the iliopsoas lies deep in the abdomen and pelvis, you cannot see or easily palpate most of it. Ultrasound is useful for dynamic assessment, particularly in snapping hip syndrome, where a clinician can watch the tendon in real time as you move your hip through the arc that triggers the pop. For suspected tears, MRI is the preferred tool. Iliopsoas tendon tears are uncommon and occur mostly in older adults. Case series have noted that the diagnosis can be significantly delayed because the clinical presentation is misleading and the anatomy is complex enough that subtle tears are easy to miss on imaging.25Canadian Association of Radiologists Journal (Elsevier). Iliopsoas tendon tear: clinical and imaging findings in 4 elderly patients CT scanning is the standard when an iliopsoas abscess is suspected, as it shows fluid collections and can guide needle drainage.