Sacroiliac Joint Pain — What's Actually Going On
If you've been told your pain is coming from your sacroiliac (SI) joint, it's worth understanding both what that joint actually does, and why the diagnosis might not be quite as straightforward as the location suggests.
What is the SI joint, and what does it do?
The sacroiliac joints sit either side of the base of your spine, connecting the sacrum (the triangular bone at the bottom of your spine) to the ilium bones of your pelvis. Unlike the facet joints further up your spine, which exist to enable spinal movement, the SI joints have a different job entirely: transferring load between your upper body and your legs. They're strong, weight-bearing joints, capable of only a small amount of movement — a few degrees at most — which makes sense given how much force passes through them with every step you take.
Because of that load-transferring role, the SI joints are particularly loaded by anything asymmetrical: walking, running, climbing stairs, getting in and out of a car, or standing on one leg to put your trousers on. Pain here often sits lower and more to one side than typical lower back pain, and can refer into the buttock or upper thigh.
Here's the important bit: location doesn't confirm the source
In my experience, pain felt at the SI joint is very often disc-referred rather than driven by the SI joint itself. This surprises a lot of people, but it makes sense once you understand how load actually travels through the pelvis — the disc can be the structure doing most of the work, even when what you consciously feel is squarely over the SI joint. It's common for the SI joint to be a secondary driver of pain rather than the primary one — but true SI joint dysfunction, without disc involvement, is a genuinely distinct presentation, not just a milder version of the same thing.
If you assume "it hurts here, so this joint must be the problem" and focus all your attention (or treatment) on the SI joint itself, you can miss the actual driver — which means the pain keeps coming back, because the real source was never addressed.
How can you tell the difference?
Honestly — not reliably by yourself, which is exactly why assessment matters here more than with some other causes. That said, a few things point one way or the other:
Sitting — disc-driven pain is often provoked by prolonged sitting itself. True SI joint pain tends to be less bothered by sitting, but more provoked by getting up from sitting, or by uneven weight-bearing (sitting with your wallet in your back pocket, slouching to one side).
Leg symptoms — if pain travels down the leg following a clear nerve pathway, especially past the knee, that points more toward disc involvement. SI-referred leg pain, when it happens, tends to stay higher — buttock or upper thigh.
Single-leg loading, without bending — standing upright on one leg (rather than bending down on one leg, which loads the disc too) is a cleaner test — think standing to pull trousers on while upright, or simply standing on one leg and noticing if that alone reproduces pain. Everyday single-leg tasks like putting shoes on, getting out of a car, or rolling over in bed usually combine asymmetry with some bending or flexion, so they tend to provoke both disc and SI pain and aren't reliable as a distinguishing sign on their own.
None of these are definitive on their own, which is exactly the point — this is genuinely one of the harder distinctions to make without a proper assessment.
Pregnancy and the SI joint
It's worth mentioning that the SI joint is also the joint involved in pregnancy-related pelvic girdle pain (PGP/SPD), where hormonal changes loosen the ligaments supporting it, coupled with changing mechanics and loading related to pregnancy. That's a genuinely different mechanism to non-pregnancy SI-related pain — worth reading about separately if that applies to you.
How treatment helps
Once we've established what's actually driving your pain — whether that's the SI joint itself, the disc referring pain there, or a combination — treatment can be targeted properly, rather than working on the joint you happen to feel it in. Hands-on treatment can address genuine SI joint restriction directly, and where the disc turns out to be the primary driver, treatment shifts to address that instead, with SI joint symptoms settling as a result.
Not sure what's driving your pain? Book an initial appointment.