The iliopsoas is not just a hip flexor. It is the primary hip flexor in your body, responsible for pulling your thigh upward toward your torso every time you walk, climb stairs, or lift your knee. It’s actually two muscles that merge into one: the psoas major, which originates along your lower spine, and the iliacus, which lines the inside of your pelvic bone. Together they attach to a bony bump near the top of your thighbone called the lesser trochanter.
Two Muscles, One Unit
The psoas major is a long, spindle-shaped muscle that starts from the sides of your lowest thoracic vertebra and first four lumbar vertebrae. It runs downward through the pelvis. The iliacus is fan-shaped and originates from the upper two-thirds of the inner surface of your hip bone and part of the sacrum. The two muscles merge as they pass beneath the inguinal ligament (the crease at the front of your hip) and share a thick tendon that anchors to the femur.
Because they fuse together and pull in the same direction, clinicians and anatomy texts treat them as a single functional unit. When someone refers to “the iliopsoas,” they mean both muscles working together to flex the hip.
How It Flexes the Hip
Hip flexion isn’t one simple motion. The iliopsoas plays different roles depending on how far your leg has already moved. In the first 15 degrees of movement, it acts mainly as a stabilizer, keeping the head of your thighbone seated properly in the hip socket. Between 15 and 45 degrees it helps maintain an upright posture, and from 45 to 60 degrees it becomes a powerful flexor pulling the femur upward. Beyond hip flexion, it also helps rotate your thigh outward and draw it slightly toward the midline of your body.
Interestingly, the iliopsoas is not the dominant force through the entire range of hip flexion. A 2024 anatomical analysis published in Juntendo Medical Journal found that the rectus femoris, one of the quadriceps muscles, actually contributes about two-thirds of the flexion torque in mild flexion up to 60 degrees. The iliopsoas contribution increases sharply beyond 60 degrees and becomes the dominant force in deep flexion at 80 to 90 degrees. So while the iliopsoas is rightly called the primary hip flexor, its peak contribution kicks in when your thigh is already well above parallel.
Other Muscles That Assist Hip Flexion
The iliopsoas does not work alone. Several other muscles contribute to pulling your thigh forward:
- Rectus femoris: The only quadriceps muscle that crosses both the hip and the knee. It dominates in the early range of hip flexion and is especially active during leg raises from 0 to 45 degrees.
- Sartorius: The longest muscle in the body, running diagonally from the front of the hip to the inner knee. It assists with flexion, especially when you cross your legs.
- Tensor fasciae latae: Sits on the outer hip and contributes to flexion while also stabilizing the pelvis.
- Pectineus and adductors: These inner-thigh muscles provide a small amount of flexion force, particularly when the hip is in a neutral or slightly extended position.
Despite this team effort, the iliopsoas remains the muscle with the greatest overall capacity to flex the hip, particularly in deep ranges of motion that the other muscles cannot reach as effectively.
Its Role in Spinal Stability
The psoas major portion of the iliopsoas does double duty. Because it originates on the lumbar spine, it directly influences your lower back posture. The muscle’s individual fiber bundles adjust their contraction levels to match the current degree of spinal curvature, essentially acting as a tension cable that stabilizes your lower back’s natural inward curve during standing. This stabilization happens passively with minimal effort, which is why the psoas is considered a postural muscle rather than purely a movement muscle.
This dual role helps explain why problems with the iliopsoas often show up as lower back pain rather than (or in addition to) hip pain.
Why Sitting Tightens the Iliopsoas
When you sit, your hips stay flexed for hours. The iliopsoas spends that entire time in a shortened position. As a postural muscle, it has an inherent tendency to adapt to that shortened length over time. Research on desk workers confirms that prolonged sitting causes the iliopsoas to shorten, which tips the pelvis forward into an anterior tilt. That forward tilt deepens the curve of your lower back, a combination that is one of the most common mechanical causes of low back pain.
The relationship is straightforward: a shorter, tighter muscle generates less force and is more vulnerable to injury. If you sit for most of the day, your iliopsoas is likely tighter than it would be otherwise, and that tightness pulls on both the hip and the spine simultaneously.
Testing for Iliopsoas Tightness
The standard clinical test is the Thomas test. You lie on your back at the edge of an exam table, pull one knee to your chest to flatten your lower back, and let the other leg hang freely. If the hanging thigh cannot rest flat on the table while the knee stays bent beyond 80 degrees, the iliopsoas specifically is tight. This distinguishes it from tightness in two-joint muscles like the rectus femoris, where the thigh rests on the table but the knee straightens out instead.
A common mistake during this test is pulling the opposite knee too aggressively, which tilts the pelvis backward and makes tight hip flexors appear normal. Proper stabilization of the pelvis is critical for an accurate result.
When the Iliopsoas Causes Problems
Iliopsoas dysfunction is frequently underdiagnosed because its symptoms mimic other hip and groin conditions. MRI studies show iliopsoas problems in roughly 21% of athletes with groin pain. After total hip replacement surgery, iliopsoas tendonitis appears in up to 24% of patients, making it a significant source of post-surgical discomfort.
Common symptoms include deep groin pain that worsens when lifting the knee, pain when getting up from a chair, and a snapping or clicking sensation in the front of the hip. Because the psoas attaches to the lumbar spine, irritation can also radiate into the lower back, making it easy to mistake for a spinal problem.
Stretching and Strengthening
A tight iliopsoas benefits from a combination of lengthening and controlled strengthening. The classic half-kneeling hip flexor stretch, where you kneel on one knee and gently press your hips forward while keeping your torso upright, targets the iliopsoas directly. The key is maintaining a neutral pelvis throughout the stretch rather than arching the lower back, which shifts the stretch away from the muscle.
For strengthening, particularly after tendon irritation, eccentric exercises have shown good results. One approach used in physical therapy involves a side-lying hip flexion exercise with a resistance band, focusing on the lowering (eccentric) phase. This limits the demand on core control while progressively loading the tendon. Supporting exercises typically address hip abductor and extensor weakness, since these imbalances often accompany iliopsoas problems and contribute to excessive loading of the hip flexors during activities like running.
Addressing both length and strength matters. Stretching alone may temporarily relieve tightness, but without building the muscle’s capacity to handle load through its full range, the tightness tends to return.