Sacroiliac joint pain comes from the two sacroiliac joints, where the sacrum at the base of your spine meets the pelvis on either side. They pass the weight of your upper body down to your legs. When one or both are strained or inflamed, the ache usually sits low in the back and buttock and can spread down the back of the thigh. It is an often overlooked cause of lower back pain.
Stairs and long spells in one position tend to aggravate the ache
Climbing stairs, and sitting or standing in one position for a long time, are the usual triggers. Turning or twisting at the hips can catch SI joint pain as well. Many people find they cannot lie on the sore side, which ends up deciding their sleeping position for them.
Pregnancy and falls are common starting points for the pain
In pregnancy, hormones loosen the ligaments around the pelvis while extra weight and a changing posture load the joint. Giving birth can strain it too. A fall onto the buttock can injure the joint, and so can a car crash.
In other people it builds up from repeated uneven strain, such as lifting with a twist day after day, or a sport like golf that turns hard through the pelvis. Doctors also look for a difference in leg length, and for an earlier operation that fused vertebrae in the lower back.
A disc or a buttock muscle can cause the same pain down the thigh
With sciatica, a disc pressing on a nerve root in the lower back is the most common cause. With piriformis syndrome, a muscle deep in the buttock irritates the sciatic nerve itself. Doctors weigh both, along with the hip, whenever this joint is suspected.
No single examination test is reliable on its own, so a doctor combines several movements that stress the joint. To confirm it as the source, a doctor may inject local anaesthetic into the joint under imaging guidance and see whether the familiar pain fades while the numbing lasts.
Sacroiliitis is inflammation of the joint, and it belongs with a doctor
Arthritis is the main cause of sacroiliitis. Often that means ankylosing spondylitis, which usually starts in early adulthood and is one of a family of inflammatory conditions called spondyloarthritis. Inflammatory back pain has its own pattern. The stiffness is worst first thing, and rest does little to ease it.
NICE, the UK body that sets clinical guidelines, advises a specialist assessment when long-standing back pain that began before the age of 45 comes with several features together. They include buttock pain, waking in the second half of the night, pain that improves with movement, and spondyloarthritis in a parent, sibling or child. A doctor can arrange blood tests, imaging and a referral to a rheumatologist.
SpinalSync readings show how your back is moving, and they are not a diagnosis
SpinalSync tests each segment of the spine on its own, with the readings on a screen as they are taken. Adaptive biosensors then deliver any adjustment at a controlled force, angle and depth, which suits people who prefer not to be adjusted by hand. MyoSync works with fascia and soft tissue, and repeat measurement shows what has changed as we support comfort and movement. We do not diagnose, prescribe medication, inject, operate, or provide or read imaging, so if your pain looks inflammatory, a Sync specialist will say so early and point you to a doctor. Our page on back pain care explains how the assessment runs.
Common questions about sacroiliac joint pain
Yes. It often reaches the buttock and the back of the thigh, and sometimes goes below the knee. An irritated nerve root in the lower back can produce a very similar pattern, so leg pain with numbness or weakness should be checked by a doctor.
A scan usually cannot confirm a strained joint, so imaging is mainly used to rule other causes out. Inflammation is different, because MRI can pick up early sacroiliitis that a plain X-ray misses. Ordering and reading scans is a doctor's job, and The Sync does neither.
For a strained joint, the usual first step is an exercise programme guided by a physiotherapist, and a pelvic support belt is sometimes used in pregnancy. Injections, and rarely surgery, are options a specialist may consider if that does not help. Sacroiliitis caused by inflammatory arthritis is a matter for a rheumatologist.
No. Naming the joint as the source is a doctor's call, made from several examination tests and sometimes a numbing injection. A Sync specialist assesses and measures how your lower back and hips move, monitors change and supports comfort and movement, and says early when something needs a doctor.
Written by the Aphyx Content team, reviewed by the clinical team at The Sync. Published 27 October 2026. Last updated 27 October 2026.