SI Joint Pain: What the Sacroiliac Joint Does and Why It Hurts

The sacroiliac joint does not get much attention until it hurts, at which point it becomes the center of your existence. It sits where the spine meets the pelvis, a large, strong joint designed for stability rather than mobility. When it becomes a pain source, the location is distinctive: below the belt line, usually on one side, right over the bony dimple of the lower back. The pain often radiates into the buttock and the back of the thigh, and it tends to be worst during transitions: sitting to standing, rolling over in bed, climbing stairs.

SI joint pain is frequently confused with lumbar disk pain or sciatica, and the confusion goes in both directions. Some people are told their SI joint is the problem when it is not. Others spend months treating a supposed disk issue when the SI joint was the actual source all along. The diagnostic overlap is real, but the treatments are different enough that getting the source right matters.

What is happening

The SI joint is an articulation between the sacrum (the triangular bone at the base of the spine) and the ilium (the large pelvic bone). It transmits the weight of the upper body to the legs. Unlike the hip or knee, the SI joint has minimal movement, typically 2 to 4 degrees of rotation [1]. This is not a joint that moves through a large range of motion. It is a joint that resists movement while transferring load, and that distinction defines everything about how it fails and how it is treated.

The joint relies on two mechanisms for stability:

Form closure: The irregular, interlocking surfaces of the joint provide inherent stability, similar to two puzzle pieces fitting together. The joint surfaces are not smooth; they have ridges and grooves that interlock as the joint matures. These features increase through adulthood, which is why SI joint problems are more common in younger adults whose joint surfaces have not yet fully developed their interlocking geometry, and in older adults where degenerative changes alter the surface contours.

Force closure: The surrounding muscles and ligaments (gluteus maximus, piriformis, pelvic floor, thoracolumbar fascia) actively compress the joint surfaces together. When these muscles are weak, imbalanced, or inhibited, force closure is insufficient and the joint becomes unstable under load [1]. The posterior ligaments of the SI joint are among the strongest in the body, but they cannot compensate for inadequate muscular compression. A joint that relies on muscle force for stability will become painful when that muscle force is inadequate.

SI joint pain is estimated to be the primary source of pain in 15 to 30% of patients with chronic low back pain [3]. It is more common in women, particularly during and after pregnancy, when hormonal changes increase ligamentous laxity [5]. Relaxin, the hormone that loosens pelvic ligaments to allow passage of the baby during delivery, also loosens the SI joint. In most women, the joint returns to its pre-pregnancy stability within months. In some, the laxity persists, and the SI joint becomes a chronic pain source.

How it is diagnosed

No single test reliably identifies the SI joint as a pain source. The gold standard is a diagnostic injection (anesthetic injected into the joint under imaging guidance), but this is invasive and not routine.

The clinical approach uses a composite of provocation tests. Laslett and colleagues validated five tests against diagnostic injection and found that three or more positive tests provided the best balance of sensitivity and specificity [2]:

1. Distraction test: Pressure applied to the front of the pelvic bones, separating the SI joints
2. Compression test: Pressure applied to the side of the pelvis, compressing the SI joints
3. Thigh thrust: A posteriorly directed force through the femur into the SI joint
4. Sacral thrust: Pressure applied centrally over the sacrum
5. Gaenslen’s test: One hip flexed, the other extended off the edge of the table

If three or more of these reproduce your familiar pain, the SI joint is the likely source. Two or fewer positive tests make the SI joint less likely and suggest looking elsewhere, typically the lumbar spine or hip. A clinician trained in these tests can perform the full set in a few minutes, and the composite approach is substantially more reliable than any individual test.

What to try

Pelvic stability exercises

Because SI joint pain often stems from insufficient force closure, exercises that strengthen the muscles responsible for compressing the joint are the foundation of treatment. A systematic review found that exercise and manual therapy were effective for reducing SI joint pain [4]. The exercises target the muscles that cross or support the SI joint: the gluteals, the deep hip rotators, the transversus abdominis, and the pelvic floor.

• Clamshells: Side-lying, knees bent to about 90 degrees, feet together. Open the top knee while keeping the feet in contact. The rotation should come from the hip, not the pelvis. If the pelvis rolls backward during the movement, the range is too large. 3 sets of 15. Progress by adding a resistance band around the knees.
• Bridges: Supine with knees bent, feet hip-width apart. Lift the hips by squeezing the glutes, hold at the top for 2 to 3 seconds. The lumbar spine should not arch excessively at the top. 3 sets of 12. Progress to single-leg bridges when the bilateral version is comfortable and controlled.
• Bird-dog: On all fours, hands under shoulders, knees under hips. Extend opposite arm and leg simultaneously, keeping the hips level (a dowel rod across the hips should not tilt). Hold 5 seconds. 10 repetitions each side. The challenge is not the limb movement; it is maintaining pelvic stability while the limbs move.
• Side plank (modified): From the knees initially, forearm on the floor, body in a straight line from knees to shoulders. Hold 15 to 30 seconds. 3 repetitions each side. This loads the lateral stabilizers of the pelvis, including gluteus medius, which is frequently weak in people with SI joint pain.

Hip mobility work

A stiff hip transfers rotational and asymmetric loads to the SI joint. If the hip cannot internally rotate adequately, the pelvis compensates during walking and the SI joint absorbs forces it was not designed to handle. Gentle hip internal rotation stretches (seated with feet wider than knees, letting the knees drop inward) and hip flexor stretches (half-kneeling, keeping the pelvis level) can reduce the demand on the SI joint by restoring motion where it belongs.

SI joint belt

A pelvic belt worn just below the bony prominences at the front of the pelvis (ASIS) can augment force closure externally. It is most useful during aggravating activities (walking, standing, transitional movements) and should be viewed as a temporary support while building muscular stability. The belt does not fix the problem; it provides external compression that mimics the force closure the muscles should be providing. As strength improves, reliance on the belt should decrease.

Activity modification

Reduce asymmetric loading: avoid standing on one leg for prolonged periods, take stairs one step at a time during flares, and avoid single-leg exercises until symptoms settle. Sleep with a pillow between the knees to reduce rotational stress on the pelvis. When sitting, distribute weight evenly on both sit bones rather than leaning to one side. These modifications are not permanent restrictions; they are acute-phase adjustments that reduce irritation while the strengthening program takes effect.

Red flags: when to see a doctor

SI joint symptoms should not include numbness below the knee, progressive weakness, or bladder and bowel changes. These suggest a spinal cause, not an SI joint problem, and require a different evaluation pathway. Morning stiffness lasting more than 30 minutes, particularly in younger adults under 40, may indicate inflammatory sacroiliitis (as in ankylosing spondylitis), which requires blood tests (HLA-B27, inflammatory markers) and imaging. Inflammatory sacroiliitis is a different condition from mechanical SI joint pain, and the distinction matters because the treatment is medical rather than exercise-based.

What the evidence does not support

“SI joint misalignment” is a common explanation offered in manual therapy settings, but the SI joint moves so little that measurable misalignment detectable by palpation is not supported by the evidence [1]. The therapist cannot feel a 2-degree rotational difference through layers of tissue, muscle and fascia. The pain is real; the biomechanical explanation may not be. This is not a criticism of manual therapy itself; it is a criticism of the explanatory model. Hands-on treatment can reduce pain through other mechanisms (pain gating, muscle relaxation, reduced guarding) without the alignment explanation being accurate.

Repeated forceful manipulation of the SI joint without accompanying exercise for pelvic stability addresses symptoms without addressing the cause. Manual therapy has a role as part of a program, not as a standalone treatment. A manipulation that reduces pain for a day or two, followed by the same provocation pattern that caused the pain in the first place, is a cycle, not a treatment plan.

Imaging of the SI joint in the absence of red flags for inflammatory disease is generally not useful for diagnosing mechanical SI joint pain. The joint looks abnormal on imaging in many people who have no pain, and can look normal in people who do. The provocation tests are more diagnostically useful than an MRI for this specific condition.

When to see a doctor. If your back pain is severe, getting worse, or accompanied by bladder or bowel changes, leg weakness, or numbness in the groin or inner thighs, seek medical care promptly. A GP or physical therapist can assess whether imaging or specialist referral is appropriate.