One-sided, low, and worst when you put weight through one leg — that pattern is very recognisable.
Also called: sacroiliac joint pain · SIJ pain · sacroiliac dysfunction
The sacroiliac joints are where the triangular bone at the base of your spine meets the two halves of your pelvis. There is one on each side, roughly under the dimples in your lower back. They barely move by design — their job is transferring load between your legs and your spine.
Because they are load-transfer joints, they get irritated when that transfer is uneven or poorly controlled. That is why symptoms cluster around anything asymmetrical: standing on one leg, rolling over, climbing stairs, getting out of a car.
It is common in pregnancy and postpartum, when tissue laxity and load both change quickly. But it happens plenty outside that too, particularly after a period of one-sided habit — an injury you limped on, or months of carrying a child on the same hip.
The pattern usually looks like:
The first job is confirming it is actually the SI joint. Low back, hip and SI pain overlap considerably and get mistaken for each other constantly, which is a common reason treatment misses.
Manual therapy can settle an irritated joint and the muscles guarding around it, which often gives quick relief. That relief tends not to last on its own, so it buys room for the part that does.
That part is control — building the strength and coordination to transfer load evenly through single-leg positions. Combined with evening out the asymmetrical habits driving it, that is what stops it recurring.
Almost certainly not in the way that phrase suggests. These joints are extremely stable and move very little. What people describe as things going out is usually irritation and protective muscle guarding, which is good news — it responds to treatment rather than requiring repeated realignment.
Because the joints are loaded independently. Anything asymmetrical — how you stand, which hip you carry a child on, a limp you had months ago — loads one more than the other, and the joint taking more eventually objects.
It helps some people considerably and does nothing for others. It is worth trying properly, positioned correctly, rather than dismissing it. It works best as short-term support while you build control, not as a permanent solution.
It overlaps heavily. Pregnancy-related pelvic girdle pain often involves the SI joints, the pubic symphysis at the front, or both. If you are pregnant, the pregnancy pelvic pain page covers the specifics of that situation.