You can usually put one finger on it. It sits low and off to one side, right about where that little dimple is in your back, and it flares when you roll over in bed or push up a flight of stairs. Somewhere along the way, someone probably told you that your SI joint "goes out," and that you need it put back in. I want to offer you a different explanation, because the one you were given is doing you a disservice.
Not in the way the phrase suggests. Your sacroiliac joints are where the sacrum, the triangular bone at the base of your spine, meets the two halves of your pelvis. They are built for stability rather than motion, wrapped in some of the thickest ligaments in the body, and the movement available to them is measured in millimeters. A joint like that does not slip out and sit crooked until someone shoves it back.
What can happen is that the joint gets irritated and starts reporting pain, usually after a specific event: a fall, a car accident, a pregnancy, or months of repetitive one-sided loading [1]. That last one matters, because it means the pain often has less to do with the joint itself and more to do with how well the muscles around it are managing load. The clicks and pops you feel when you stretch are not the joint relocating. They are just a joint doing what joints do.
Most people with SI joint pain point to a spot just below and inside the bony bump at the top of the pelvis, and that pointing test is one of the more useful things we have [1]. From there it tends to travel. Deep buttock pain is common, pain down the back of the thigh is common, and one review found eighteen different referral patterns, with about half of people feeling it in the posterior or lateral thigh and a smaller group feeling it below the knee or even into the foot [1].
That is exactly why this gets mislabeled as sciatica so often. The leg pain looks the same from the outside.
| Where it usually sits | What sets it off | The tell | |
|---|---|---|---|
| SI joint | Low, one-sided, near the dimple; buttock and back of thigh | Rolling in bed, stairs, standing on one leg, getting out of a car | You can point to it with one finger |
| Disc or nerve root | Back, then a defined stripe down the leg | Sitting, bending forward, coughing or sneezing | Numbness, tingling, or true weakness in the foot |
| Hip joint | Groin, sometimes deep in the front of the thigh | Deep squatting, pivoting, crossing your legs | Pain in the groin, and stiffness turning the leg in |
None of this is a substitute for an exam, but if your pain lives in the groin, the hip deserves a look, and I have written more about that in groin pain in women.
Common enough that it surprises people. Up to a quarter of low back pain appears to involve the SI joint [1], and one study that specifically screened the sacrum instead of only the lumbar spine found joint involvement in more than half of the low back pain cases it looked at [2]. The authors' explanation is the part I think about most: clinicians often do not screen outside the immediate area of the complaint, so a sacrum that is contributing to the problem never gets examined [2].
If your pain started during pregnancy or in the year after a baby, this joint deserves an even closer look. It is one of the first places the pelvis complains when tissue is more mobile and demands are higher.
Here is the piece that usually gets skipped. Your pelvic floor is part of the muscular system that compresses the two halves of the pelvis toward each other, which is what gives the SI joints their working stability. In cadaver research that simulated pelvic floor tension, that tension increased stiffness of the sacroiliac joints by about 8.5% in female specimens and rotated the sacrum backward [4]. It is a small number on paper, and it tells you the pelvic floor is on the team.
The clinical picture is more striking. In a study of women who came to outpatient orthopedic clinics with lumbopelvic pain, 95.3% had some form of pelvic floor dysfunction, with 71% showing pelvic floor muscle tenderness and 66% showing weakness [3]. Nearly all of them. These were women who came in for their backs and hips, not for pelvic symptoms.
In my practice, when someone has had SI joint pain for months and has already done a round of glute strengthening without much to show for it, an internal assessment is often what finally explains the plateau. A tender, guarded pelvic floor on one side does not usually announce itself as pelvic pain. It shows up as a back that keeps giving out on the same side, and that is a very fixable problem once you know it is there.
Imaging is honestly not the deciding factor here. Scans do a good job of ruling out inflammatory disease and other structural problems, but they do not reliably confirm or exclude a painful SI joint on their own [1]. So the exam carries the weight.
I use a group of provocation tests that load the joint in different directions. When three or more out of six of them reproduce your familiar pain, the cluster identifies the SI joint with about 94% sensitivity and 78% specificity, and just as usefully, when none of them reproduce your pain, we can reasonably take the joint off the list [5]. That second half saves people a lot of time chasing the wrong structure.
From there I look at how you load: single-leg stance, step-up, the way your ribs and pelvis stack when you stand. And if the picture points that way, I will talk with you about an internal pelvic floor assessment. That is always your call, and we can learn a good deal from external work if you would rather start there.
Does SI joint pain need an injection to get better? Usually not as a first step. Conservative care with a stabilization and stretching program is the recommended starting point, and injections sit further down the list for people who do not respond [1].
Is it safe to exercise when it hurts? In most cases yes, with the right exercises. Pain in this joint is generally a load-management problem rather than a sign of damage, so the goal is finding the version of a movement you tolerate and building from there.
Does an SI belt help? It can. A pelvic support belt is a recognized part of conservative care [1], and it works best as short-term help while you build the strength that eventually does the same job.
Why do stairs and rolling over in bed set it off? Both put uneven load through one side of your pelvis. Single-leg positions and transitional movements are the classic provokers, which is also why they show up in the tests I use.
Could it be my tailbone instead? They sit close together and get confused often. If your pain is right at the very bottom, especially with sitting, tailbone pain is a more likely culprit, and if you are not sure where your pain is coming from at all, start with what's actually causing your pelvic pain.
If you have been told your pelvis keeps going out, I hope this reframes it a little. Nothing is slipping around in there. Your pelvis is asking for better support from the muscles that surround it, and that is something we can genuinely change.
If your SI joint pain keeps circling back, let's take a proper look at it, pelvic floor included. You can reach Protea Physical Therapy at (909) 265-3584, and I see patients here in Anaheim Hills, serving Orange County.
This article is for educational purposes and is not medical advice. Please consult a healthcare provider about your own symptoms.
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