Pain low in the back and buttock, worse rolling in bed or on stairs? How SI joint pain and sacroiliitis differ, the warning signs, and what helps.
The pain sits low — below the belt line, just off to one side of the spine, sometimes spreading into the buttock or the back of the thigh. It catches when you roll over in bed, climb stairs, get out of the car or stand on one leg. You may have been told it is your "SI joint", or that you have sacroiliitis.
These terms are often used interchangeably, but they can mean quite different things. Most SI joint pain is mechanical and responds well to exercise and load management. But in a smaller group of people, especially younger adults with long-standing back pain and morning stiffness, sacroiliitis is a sign of an inflammatory condition called axial spondyloarthritis, which needs a doctor's diagnosis and specific treatment. This guide explains the difference, how SI joint pain is assessed, and what helps.
At a glance
- The SI joints: where the base of the spine (sacrum) meets the pelvis, one on each side
- SI joint pain: a recognised source of low back and buttock pain, often mechanical
- Sacroiliitis: inflammation of the SI joint, most importantly as part of axial spondyloarthritis
- Red flag pattern: back pain starting before 45, lasting over 3 months, worse with rest, better with exercise, with long morning stiffness
- Mechanical SI pain: responds to strengthening, movement and load management
- Inflammatory sacroiliitis: needs GP and rheumatologist care — plus exercise, which is a core part of treatment
What is the sacroiliac joint?
The sacroiliac (SI) joints connect the sacrum — the triangular bone at the base of the spine — to the two large pelvic bones (ilia). They are strong, stable joints held by some of the toughest ligaments in the body, and they move only a few millimetres. Their job is to transfer load between the spine and the legs, which is why they can become painful when that load changes.
A common myth is that SI joints "slip out of place" and need to be "put back in". The joint is far too stable for that. What people feel as "out of place" is usually pain and protective muscle tension, which can ease with hands-on treatment — but the joint itself has not moved out of position.
Two different problems
Mechanical SI joint pain
This is pain arising from the SI joint and its ligaments without an inflammatory disease. A comprehensive review estimated that the SI joint is the main source of pain in a meaningful minority of people with chronic low back pain, although estimates vary widely depending on how it is measured. It is more common:
- During and after pregnancy, when hormones and changing posture increase load on the pelvis (often called pelvic girdle pain)
- After a fall onto the buttock, a jarring step or a car accident
- With leg length differences, a change in walking pattern, or after lumbar fusion surgery
- In runners and athletes who load one leg repeatedly
Inflammatory sacroiliitis: axial spondyloarthritis
Axial spondyloarthritis (axSpA), which includes ankylosing spondylitis, is a long-term inflammatory condition that typically starts in the SI joints and spine. A review in The Lancet describes it as often beginning in young adults, frequently before the age of 30, and commonly being diagnosed years after symptoms start. Recognising it early matters, because effective medical treatments are available.
Is my back pain inflammatory? The warning pattern
Consider inflammatory back pain, and see your GP, if you have back or buttock pain for more than three months that started before the age of about 45, along with several of these:
- Gradual onset rather than after an injury
- Morning stiffness lasting more than 30 minutes
- Improves with exercise and movement, but not with rest
- Waking in the second half of the night with pain, easing once you get up and move
- Alternating buttock pain, switching sides
- Other features: inflamed eyes (uveitis), psoriasis, inflammatory bowel disease, heel pain from the Achilles or plantar fascia, swollen joints, or a family history of spondyloarthritis
International classification criteria developed by the Assessment of SpondyloArthritis international Society (ASAS) combine features like these with MRI evidence of sacroiliitis and blood tests such as HLA-B27. A GP can arrange blood tests and imaging, and refer to a rheumatologist.
“Young adults with months of back pain that is worse in the morning and better with exercise should be checked for inflammatory arthritis. Physio is part of the treatment, but a diagnosis from a rheumatologist comes first.”
— UpPlus Physiotherapy team
Symptoms of SI joint pain
- Pain low in the back, just to one side, below the belt line
- Pain into the buttock and sometimes the back of the thigh, usually not below the knee
- Pain rolling over in bed, getting in and out of a car, climbing stairs, or standing on one leg
- Pain sitting on the affected side or after prolonged standing
- Tenderness over the joint, near the "dimples" at the base of the back
How it is assessed
No single test reliably identifies SI joint pain. Research by Laslett and colleagues found that a cluster of pain provocation tests — for example pressing the pelvis apart or together, or pushing through the thigh — is more useful than any single test: when several are positive, the SI joint is more likely to be involved, and when none reproduce your pain, it is less likely. A physiotherapist also examines the lower back and hip, because problems in either can feel very similar.
Imaging is not usually needed for mechanical SI pain. If inflammatory sacroiliitis is suspected, MRI of the SI joints is the most sensitive test.
Your physio will also ask about your general health, other joints, your eyes, skin and bowels, and any family history, because those details help separate mechanical SI pain from an inflammatory cause. It can feel like a lot of questions about a sore back, but they matter.
Treatment: mechanical SI joint pain
1. Load management
- Avoid prolonged single-leg standing, sitting on one buttock, and standing with weight on one hip.
- Take smaller steps on stairs and use the rail; get in and out of the car with both legs together.
- Break up long periods of sitting or standing.
2. Strengthening
The muscles around the pelvis — gluteals, deep abdominals, back and hip muscles — help transfer load across the SI joints. A progressive programme might include:
- Bridges and single-leg bridges
- Side planks and dead bugs
- Clams and side steps with a band
- Squats, step-ups and lunges with good control
- Deadlift patterns as strength improves
3. Hands-on treatment
Mobilisation, soft-tissue work and muscle energy techniques can reduce pain and protective tension in the short term, making exercise easier.
4. Pregnancy-related pelvic girdle pain
European guidelines for pelvic girdle pain recommend individualised exercise in pregnancy, advice on daily activities, and, for some women, a pelvic support belt for short-term relief. Most pregnancy-related pelvic pain improves after birth, though some women need ongoing rehab.
5. Medical options
For persistent mechanical SI pain, a doctor may consider image-guided injections or, rarely, other procedures. A systematic evaluation of SI joint interventions found the evidence for them varies, so they are usually reserved for people who have not improved with active treatment.
Exercises to start with
These are common starting points for mechanical SI joint pain. They should feel like work, not sharp pain.
- Bridge: lying on your back with knees bent, squeeze your buttocks and lift your hips level. Hold 3 seconds. 3 sets of 10.
- Dead bug: lying on your back with knees and hips bent to 90 degrees, slowly lower one heel to the floor and back, keeping your lower back steady. 8 each side.
- Side plank from the knees: hold 15–30 seconds each side, 3 times.
- Clams: lying on your side with knees bent, lift the top knee while keeping the feet together. 3 sets of 12.
- Hip flexor stretch: half-kneeling, gently move the hips forward. Hold 30 seconds each side.
- Walking: regular, comfortable walks, building gradually; shorter steps can feel easier at first.
Sleeping, sitting and moving
- Rolling in bed: bend both knees, squeeze them together and roll as one unit.
- Side-lying: a pillow between the knees keeps the pelvis level; in pregnancy, a long body pillow helps.
- Sitting: sit evenly on both buttocks with feet flat; avoid crossing your legs or sitting on a wallet.
- Dressing: sit down to put on socks, shoes and pants rather than balancing on one leg.
Treatment: inflammatory sacroiliitis
Axial spondyloarthritis is managed by a rheumatologist, usually starting with anti-inflammatory medicines, and, if needed, biological medicines that are very effective for many people. Exercise is a core part of treatment: a Cochrane review found that individual or supervised exercise programmes improved pain, spinal movement and function in people with ankylosing spondylitis, with supervised group programmes doing better than home exercise alone.
Physiotherapy for axSpA typically includes:
- Spinal and hip mobility exercises, including extension and rotation
- Strengthening and postural exercises
- Breathing and chest expansion exercises
- Aerobic fitness — swimming, cycling and walking are often well suited
- A long-term routine to maintain movement
Myths about the SI joint
- "My SI joint is out of place." SI joints are very stable and do not slip out of position.
- "I need it cracked back in every week." Hands-on treatment can ease pain, but strength and movement give lasting change.
- "Sacroiliitis just means a sore SI joint." True sacroiliitis can be part of an inflammatory condition that needs medical care.
- "Rest is best." For both mechanical and inflammatory SI pain, staying active helps.
When to see a physio or doctor
- Low back or buttock pain lasting more than a couple of weeks
- Pain during or after pregnancy that limits walking, stairs or sleep
- The inflammatory pattern above — see your GP
Seek urgent medical help for back pain with numbness in the groin or saddle area, new bladder or bowel problems, leg weakness, fever, or unexplained weight loss.
How UpPlus Physiotherapy helps
- Assessment of the SI joints, lower back and hips, and screening for inflammatory features
- Referral to your GP for blood tests, imaging and rheumatology when needed
- A progressive strengthening programme, in the clinic, our gym-based rehab or hydrotherapy
- Pregnancy and postnatal pelvic pain care
- Long-term exercise programmes for axial spondyloarthritis alongside your rheumatologist
Read more about sacroiliac joint pain, lower back pain and our back pain physiotherapy.
Finding a physio near me for SI joint pain
When searching for the best physio for SI joint pain or sacroiliitis, look for someone who screens for inflammatory back pain, examines the back and hips as well as the pelvis, and builds a strengthening plan rather than relying on repeated manipulation.
Our clinic is at 177 Trimmer Parade, Seaton, a short drive from Woodville, Findon, West Lakes, Henley Beach and Lockleys. No referral is needed, and you can claim on the spot with private health cover.
Physio take-home message
- SI joint pain is real, but the joint does not "slip out of place".
- Mechanical SI pain improves with strengthening and load management.
- Inflammatory back pain — young onset, morning stiffness, better with exercise — needs a GP review.
- Exercise helps both mechanical SI pain and axial spondyloarthritis.
- Pregnancy-related pelvic pain is common and treatable.
If low back or buttock pain is holding you back, our physiotherapists can assess it and plan your recovery. Book online or call 0403 907 587.
This article is general information, not a diagnosis or a substitute for an individual assessment. If you are unsure about your symptoms, see a health professional.
