When the spasm starts at the pelvis
Pain in one dimple of the low back or deep in the buttock, worse on stairs, standing on one leg, and rolling over in bed.
The sacroiliac joints are where the sacrum — the triangular bone at the base of your spine — meets the pelvis, one on each side. They move very little, but they transfer everything: the entire load of your upper body passes through them into your legs.
When one becomes irritated, the muscles around the pelvis and up into the low back take over the job of holding it still. That's the spasm.
Why the spasm ends up in your back
The SI joint sits at a junction. Glutes, deep hip rotators, the pelvic floor, the abdominal wall and the low back muscles all attach to or cross the pelvis. When the joint sends a threat signal, the guarding response recruits the whole neighbourhood, and the loudest part of it is usually felt higher up, in the low back.
This is a common reason people treat their low back for weeks without much change. The back is doing the shouting. The pelvis is doing the talking.
What it usually feels like
- Pain on one side, often pointed to with a single finger just below the dimple of the low back
- Deep buttock pain that can refer into the groin or the back of the thigh, usually not past the knee
- Worse going up stairs, standing on one leg, getting dressed, getting out of a car
- Rolling over in bed is a classic provocation
- Often follows pregnancy, a fall onto one buttock, a misjudged step off a kerb, or a long period of asymmetric loading
During the acute phase
Start with the three immediate steps, then:
Keep the pelvis symmetrical. Side-lying with a pillow between the knees keeps the two sides level and is usually the most comfortable position available. Avoid sleeping with one leg hitched up across the body.
Move both legs together. Getting in and out of bed, swing both legs at once rather than one at a time. Sit down to put trousers on. Anything that loads one leg independently tends to provoke it.
A firm support belt worn around the pelvis — low, over the bony points at the front, not around the waist — gives many people noticeable relief in the acute phase. It's a short-term aid, not a long-term fix.
What tends to make it worse
- Standing with weight parked on one hip
- Stairs, lunges, stepping up into a van
- Crossing the legs when sitting
- Aggressive glute or piriformis stretching, which loads the joint it's meant to relieve
Get assessed if
The usual warning signs apply first.
Otherwise: if one-sided pelvic pain persists beyond a few weeks, if it followed a fall or pregnancy and hasn't resolved, or if it keeps returning on the same side. SI joint problems respond well to specific stabilisation work, but the work has to be specific — general core exercise often doesn't touch it.
Get the emergency protocol before the next one
The full 21-page guide: what to do in the first 48 hours, the day-by-day recovery, and how to come back without re-triggering it. Free.
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Other patterns
- Sciatica and nerve irritation Pain that travels into the buttock or down the leg, with the low back locking to protect the nerve root.
- Disc bulge or herniation Pain that spikes with bending, sitting and coughing, and a back that locks the moment you move the wrong way.
- Spinal stenosis Legs that ache or go heavy when you stand and walk, and ease the moment you sit or lean forward.
- Facet joint irritation A sharp local catch on arching, twisting or standing up, with pain that stays close to the spine.
- Overload, fatigue and stress The episode with no dramatic cause — a sneeze, a sock, the third bad night of sleep in a stressful month.