Can Tight Hip Flexors Cause SI Joint Pain?

Tight hip flexors can contribute to sacroiliac (SI) joint pain, though the relationship is indirect. The hip flexors, particularly the iliopsoas, attach directly to the pelvis and lumbar spine, and when they become chronically shortened, they alter pelvic position, inhibit the gluteal muscles that stabilize the SI joint, and shift mechanical loads in ways that can irritate SI joint structures. Research supports each link in that chain, even if no single study draws a straight line from “tight iliopsoas” to “SI joint dysfunction” as a simple cause-and-effect.

Why the Hip Flexors Matter to a Joint They Do Not Touch

The SI joint sits where the sacrum meets the two ilia at the back of the pelvis. It is not a highly mobile joint. It moves only a few degrees, and its stability depends on two complementary systems. The first is the shape of the joint surfaces themselves, which include interlocking ridges and grooves that resist slipping. The second is the compression generated by muscles, ligaments, and fascia pulling across the joint during movement. Researchers refer to these as “form closure” and “force closure.” Ligaments binding the SI joint are among the strongest in the body, but they are not enough on their own. Muscles that generate compressive force across the joint, especially the gluteus maximus and deep core muscles, are essential for keeping it stable under load.¹

The iliopsoas, the primary hip flexor, does not cross the SI joint directly. But it originates from the lumbar vertebrae and the inside of the ilium, descends over the pelvic brim, and inserts on the femur. Its attachment to the pelvic brim is so substantial that some anatomists have described it as functioning like a ligament of the innominate bone.² A chronically shortened or overactive iliopsoas tugs the pelvis into an anterior tilt, increasing the curve of the lower back and changing the angle at which force passes through the SI joint. That positional shift is where the trouble starts.

Gluteal Inhibition and Why It Matters

The most well-supported mechanism linking tight hip flexors to SI joint problems runs through the gluteus maximus. When the hip flexors are chronically shortened, the gluteus maximus on the same side tends to fire less effectively. A study of college-aged female soccer players found that those with restricted hip flexor length showed significantly less gluteus maximus activation and a lower ratio of gluteus maximus to hamstring activity during hip extension tasks.³ The underlying idea is that a tight muscle sends constant low-level neural signals that dampen the activity of its opposing muscle, a concept physical therapists call reciprocal inhibition.

This matters for the SI joint because the gluteus maximus is one of its primary stabilizers. The deep fibers of the glute max pull across the SI joint and compress it, contributing directly to force closure. When glute output drops, the joint loses some of that compressive stability, and the ligaments and joint surfaces have to absorb forces they were not designed to handle alone. Research on people with existing SI joint dysfunction bears this out: one study found significant gluteus maximus weakness on the affected side, and after a targeted strengthening program, subjects saw both measurable strength gains and meaningful decreases in pain.⁴

So the sequence looks something like this: tight hip flexors suppress gluteal activation, weaker glutes reduce the compressive stability of the SI joint, and a less stable SI joint becomes vulnerable to pain with everyday loading. The hip flexors are not acting on the SI joint directly. They are weakening the muscle that protects it.

Posture, Pelvic Tilt, and Compensatory Patterns

Tight hip flexors do not just inhibit the glutes. They also change how you stand and move, which reshapes the forces passing through the pelvis. When the iliopsoas is chronically shortened, it pulls the pelvis forward into an anterior tilt. This deepens the lumbar curve and shifts the body’s center of gravity. Some people compensate by leaning their upper body backward, which can create a “sway back” posture associated with both hip flexion changes and low back pain.⁵

An anterior pelvic tilt changes the sacrum’s position relative to the ilia. The sacrum is wedge-shaped, and its fit between the two halves of the pelvis depends partly on alignment. When the pelvis tilts forward, the sacrum’s orientation shifts, altering how weight transfers through the SI joint during standing and walking. The ligaments on the back of the joint may be placed under more sustained tension, and the joint surfaces may not interlock as effectively. Over time, this can produce the kind of dull, achy pain around the SI region that people often mistake for a low back problem.

Prolonged sitting makes all of this worse. Hours in a chair hold the hip flexors in a shortened position, and the glutes are essentially turned off. For someone who sits most of the day and then goes for a run or lifts weights, the pelvis enters the activity in a compromised state: the hip flexors are tight and pulling the pelvis forward, the glutes are sluggish and not compressing the SI joint effectively, and the joint is asked to absorb high forces without the muscular support it needs.

How to Tell If Your SI Joint Is Actually the Problem

One of the biggest challenges with SI joint pain is figuring out where the pain is actually coming from. The lower back, the SI joint, and the hip joint all refer pain to overlapping regions around the buttock and posterior pelvis.⁶ A person with tight hip flexors and pain near the SI joint could be dealing with SI joint dysfunction, lumbar disc or facet problems, hip joint pathology, or even iliopsoas tendinopathy itself.

Clinicians use a cluster of hands-on provocation tests to assess the SI joint. These involve applying pressure or stress to the joint in different directions and seeing which maneuvers reproduce the person’s familiar pain. No single test is reliable enough on its own. The current clinical standard considers the SI joint a possible pain source when at least three provocative maneuvers are positive.⁷ Among the commonly used tests, the FABER test (where the hip is flexed, abducted, and externally rotated) tends to show the highest sensitivity for detecting sacroiliitis, while lateral compression may be more specific.⁸

When physical examination is inconclusive, diagnostic SI joint injections remain the gold standard. A numbing agent is injected directly into the joint under imaging guidance. If the pain disappears temporarily, the SI joint is confirmed as the source. This step is especially important because imaging alone often cannot identify SI joint dysfunction. You can have a normal-looking SI joint on MRI and still have significant pain originating from it.

If you suspect tight hip flexors are driving your SI pain, a clinician should also assess your hip flexor length and gluteal function. The modified Thomas test, where you lie on the edge of a table and let one leg hang, reveals whether the hip flexor is too short to allow full hip extension. Connecting the dots between limited hip flexor length and positive SI joint provocation tests gives a much clearer treatment target than addressing either finding in isolation.

When the Two Problems Overlap and Hide Each Other

A published case report illustrates how intertwined these issues can become. A patient presented with chronic pelvic girdle pain that resisted initial treatment. After extended assessment, the diagnosis turned out to be a combination of iliopsoas tendinopathy and SI joint dysfunction occurring simultaneously. The overlapping nature of these two conditions made it difficult to distinguish between them and created a significant challenge in developing an effective treatment plan. Ultimately, treatment focused on improving joint mobility and lengthening the iliopsoas resolved her pain completely.⁹

This case is not unusual in concept. The iliopsoas and the SI joint share a neighborhood, and problems in one often provoke or mimic problems in the other. A tight, irritated iliopsoas can cause anterior hip and groin pain on its own while simultaneously destabilizing the SI joint through the mechanisms described above. When a clinician chases only the SI joint or only the hip flexor, treatment tends to stall. Addressing both is frequently the key to lasting improvement.

Treatment That Targets Both Sides of the Problem

Because the connection between tight hip flexors and SI joint pain runs through both muscle tightness and muscle weakness, effective treatment typically has to address both. Stretching the hip flexors alone is unlikely to fix SI joint pain if the glutes remain weak, and strengthening the glutes alone may not work if the hip flexors stay too tight to let them activate properly.

On the stretching side, the goal is restoring normal hip flexor length so the pelvis can return to a more neutral position. Half-kneeling hip flexor stretches, where you lunge forward with the back knee on the ground and gently shift your weight forward, are the most commonly prescribed approach. Holding for at least 30 seconds and repeating several times per day tends to produce better results than aggressive one-time stretching sessions. Some clinicians add a posterior pelvic tilt cue during the stretch, asking you to tuck your tailbone slightly, to ensure the stretch targets the iliopsoas rather than just pushing into lumbar extension.

On the strengthening side, gluteus maximus exercises that also challenge SI joint stability are ideal. Bridges, clamshells, and hip thrusts are common starting points. The key is making sure the glutes are actually doing the work rather than the hamstrings or lower back muscles picking up the slack. Research on SI joint dysfunction shows that a structured glute strengthening program over roughly ten sessions produced significant increases in strength, improvements in function, and decreases in pain.⁴

Core stability work adds another layer of benefit. The deep core muscles, including the transversus abdominis and pelvic floor muscles, also contribute to SI joint force closure. A study comparing core stability exercises combined with manual therapy mobilization found that the combination reduced pain, disability, and functional limitations in people with SI joint dysfunction more than either approach alone.¹⁰ Separately, research on combining core strengthening with a traditional pulling exercise similarly found greater pain reduction and functional improvement compared to exercise alone.¹¹

Manual therapy, including SI joint mobilization, soft tissue work on the iliopsoas, and manipulation, can also help in the short term by reducing pain enough that the person can tolerate the exercise program. But manual therapy without the exercise component tends to produce temporary relief at best. The exercises are what create lasting change by rebuilding the muscular support system around the joint.

What Runners Should Know

Running places repetitive, high-impact loads through the SI joint with every stride. For runners with tight hip flexors, the combination of gluteal inhibition and altered pelvic alignment may make the SI joint particularly vulnerable. Research on female runners with SI joint pain found several gait differences compared to healthy controls, including less knee flexion at initial contact, a greater tibial overstride angle, and more contralateral pelvic drop when standing on their symptomatic leg.¹²

The contralateral pelvic drop is especially telling. When the glute on the stance leg is not firing well, the opposite side of the pelvis drops, and the SI joint on the stance side absorbs shearing forces it is not designed to handle repeatedly. A runner who stretches their hip flexors before a run but does not address the underlying glute weakness is dealing with the symptom rather than the cause. A pre-run routine that includes a brief hip flexor stretch followed by glute activation exercises like banded lateral walks or single-leg glute bridges is more likely to protect the SI joint than stretching alone.

Pregnancy, Hormones, and an Important Distinction

SI joint pain is extremely common during and after pregnancy, which sometimes leads people to assume that the same mechanisms at play in the general population also explain pregnancy-related pelvic pain. Tight hip flexors can certainly contribute during pregnancy, since the growing abdomen pulls the pelvis into an anterior tilt and the hip flexors shorten accordingly. However, the hormonal component is more complicated than many people assume. Relaxin, a hormone produced during pregnancy, has long been blamed for loosening the SI joint ligaments and causing instability. A systematic review of high-quality studies found that the evidence for an association between relaxin levels and pregnancy-related pelvic girdle pain is actually low, with three well-designed studies finding no link between the two.¹³

This does not mean pregnancy-related SI joint pain is not real. It is very real and can be debilitating. But the driver may be biomechanical, including the anterior tilt, glute inhibition, and load changes from a shifting center of gravity, rather than purely hormonal. For pregnant and postpartum individuals, addressing hip flexor tightness and gluteal weakness remains a sensible approach, since those are modifiable factors regardless of what relaxin is or is not doing.

Imaging the Iliopsoas Directly

When the clinical picture is unclear and a clinician suspects the iliopsoas itself is part of the problem, musculoskeletal ultrasound offers a way to evaluate it in real time. Unlike MRI, which captures a static image, ultrasound can visualize the iliopsoas during active movement. This makes it useful for identifying thickening, tendinopathy, or the snapping that occurs when the tendon catches over a bony prominence during hip flexion.¹⁴ It is also relatively inexpensive and accessible compared to MRI, which makes it practical for use during a routine physical therapy or sports medicine visit.

For someone whose SI joint pain has not responded to standard treatment, an ultrasound evaluation of the iliopsoas can help determine whether the muscle or tendon itself is structurally compromised, not just tight. Tendinopathy of the iliopsoas can mimic or coexist with SI joint dysfunction, and knowing whether you are dealing with one, the other, or both changes how aggressively and in what direction treatment needs to go.