Quick answer: the sacroiliac joints sit where the base of the spine meets the pelvis, one on each side, roughly under the dimples in your lower back. Pain from them is routinely mistaken for a disc problem or sciatica because it refers into the buttock and back of the thigh. Three features usually separate it: you can often point to it with one finger, it rarely travels below the knee, and it is provoked by single-leg loading and rolling over in bed rather than by coughing or straining.
When to Get Checked First
Most sacroiliac pain is mechanical and not dangerous. These presentations are different and need assessment before you start exercising.
- Inflammatory pattern. Pain that began before age 45, morning stiffness lasting more than 30 minutes, pain in the second half of the night that wakes you, and symptoms that improve with movement and worsen with rest. That combination suggests axial spondyloarthritis, including ankylosing spondylitis, which involves the sacroiliac joints directly and needs rheumatology assessment rather than physiotherapy alone. It is frequently missed for years.
- Fever, severe one-sided pain and feeling unwell. Septic sacroiliitis is rare but is an infection of the joint and needs urgent medical care.
- Numbness around the groin, inner thighs or buttocks, bladder or bowel changes, or worsening leg weakness. These indicate possible cauda equina syndrome and need same-day emergency assessment.
- Pain after a fall or accident, or sudden severe pain in anyone with osteoporosis or on long-term corticosteroids, where a sacral or pelvic fracture is possible.
- A history of cancer, unexplained weight loss, or unrelenting night pain.
- Pelvic girdle pain in pregnancy that stops you walking or turning in bed. This is treatable and you should not be told to put up with it. Ask for a referral to a physiotherapist experienced in pelvic and women’s musculoskeletal care.
Is It the SI Joint or Your Disc? Four Discriminators
This is the practical question, since the two are constantly confused in both directions, and it is where most articles are vague.
| More like sacroiliac joint | More like disc or nerve root | |
|---|---|---|
| Where you point | One finger, just below and inside the dimple of the lower back on one side | A broad area across the lower back, or a stripe down the leg |
| How far it travels | Buttock, groin, back of thigh. Rarely below the knee | Frequently below the knee, into the calf or foot |
| What provokes it | Standing on one leg, stairs, uneven ground, rolling over in bed, getting up from sitting | Coughing, sneezing, straining, prolonged sitting, bending forward |
| Numbness and tingling | Usually absent, or vague and non-specific | Often present in a defined band matching one nerve root |
The one-finger point is known as the Fortin finger test, and while it is far from conclusive on its own, it is a genuinely useful thing to be able to demonstrate at an appointment. If coughing and straining reliably reproduce your pain, that points away from the sacroiliac joint and towards the spinal canal. Our guide to back pain when coughing covers that pattern, and sciatica exercises covers pain travelling below the knee, including the three-movement check for which nerve root is involved.
Two other things it gets confused with
- The hip joint. Hip osteoarthritis refers into the groin and buttock and is often mislabelled as sacroiliac. The distinguishing question is whether putting on socks and shoes, or getting in and out of a car, is difficult. That points to the hip.
- The coccyx. Pain that is worst specifically when sitting on a hard chair, and worst again in the moment of standing up, sits lower and in the midline. See coccydynia and tailbone pain.
What You Can Check at Home
None of this diagnoses anything. It gives you specific, describable observations to bring to an appointment, which is worth considerably more than saying your back hurts.
- Point with one finger. Ask yourself whether you can put a fingertip on the worst spot. If the answer is a fingertip just below and inside the dimple on one side, note it. If the answer is a spread palm across the whole lower back, note that instead.
- Stand on one leg for 30 seconds, holding a wall for balance. Compare sides. Reproducing your familiar pain on the affected side is a meaningful observation, as is being noticeably less steady on that side.
- Climb a flight of stairs and note whether the pain is worse going up, going down, or neither.
- Cough firmly twice. If that reproduces the pain, the sacroiliac joint becomes less likely and the spinal canal more likely.
- Note the bed test. Rolling from your back onto your side is a classic sacroiliac provocation, and it is one of the more specific things you can report.
Anatomy: What the Joint Actually Does
The sacroiliac joint connects the sacrum, the triangular bone at the base of the spine, to the ilium of the pelvis on each side. It is a synovial joint with a very limited range of movement, on the order of a few degrees of rotation and a millimetre or two of translation.
It is stabilised by some of the strongest ligaments in the body, including the posterior sacroiliac and sacrotuberous ligaments, and indirectly by the gluteal muscles, piriformis, erector spinae and the deep abdominal wall. Despite barely moving, it transmits the entire load of the upper body into the legs during walking, running, lifting and any single-leg activity. That combination of high load and minimal movement is why it becomes symptomatic.
Clinicians describe its stability in two parts: form closure, the bony and ligamentous fit, and force closure, the compression the surrounding muscles add. You cannot change form closure. Force closure is trainable, and it is what rehabilitation targets.
One consequence worth knowing: because the joint barely moves, claims that it has slipped out of place and needs putting back are not anatomically plausible. What changes with treatment is how well the surrounding muscles control load through it, not the position of the bones.
Causes
- Pregnancy and the postpartum period. Hormonal changes increase ligament laxity, and the growing load changes how the pelvis is stressed. Sacroiliac pain is a major component of pelvic girdle pain and can persist after delivery.
- Asymmetric loading. Leg length difference, an altered gait after another injury, sitting on a wallet, or habitually standing on one hip.
- Trauma. Falls onto the buttock, road accidents, and sports impacts.
- Inflammatory arthritis. Axial spondyloarthritis causes sacroiliitis, true inflammation of the joint, which behaves differently from mechanical pain and is treated differently.
- Generalised joint hypermobility, which reduces the passive stability the ligaments provide.
- Previous lumbar fusion, which increases load transfer through the sacroiliac joints below the fused segment.
How It Is Diagnosed, and Why That Is Harder Than It Sounds
Estimates of how much chronic lower back pain originates at the sacroiliac joint range from roughly 10 to 25 percent, depending on the population studied and the criteria used. That spread is itself a clue about how difficult this diagnosis is.
No single physical test is conclusive. Clinicians use a cluster of pain provocation tests, most commonly the distraction, thigh thrust, compression and sacral thrust tests, sometimes with Gaenslen’s test and FABER. In the original validation work, three or more positive tests out of six gave a sensitivity around 94 percent and a specificity around 78 percent against diagnostic injection.
Here is the part usually left out. A later systematic review with meta-analysis found that a positive cluster identified sacroiliac joint pain correctly only around 35 percent of the time. In plain terms: a negative cluster is fairly good at ruling the joint out, but a positive cluster is much weaker at ruling it in. That is exactly why misdiagnosis runs in both directions, with disc problems labelled as sacroiliac and vice versa, and it is a reason to be sceptical of any practitioner who diagnoses your sacroiliac joint confidently from a single test.
Imaging is used mainly to exclude other causes rather than to confirm this one, though MRI is important where an inflammatory cause is suspected. An image-guided anaesthetic injection into the joint remains the reference standard for confirming the source, and is generally reserved for cases where the diagnosis genuinely changes management.
Treatment: Building Force Closure
Rehabilitation aims to increase the compressive support the muscles give the joint under load. The key contributors are the gluteus maximus, gluteus medius, multifidus, the hamstrings and the deep abdominal wall.
Build up gradually and use the same rule that applies to all back rehabilitation: mild soreness is fine, but judge by the next morning, and stop any movement that pushes symptoms further down the leg.
Glute bridge
The foundation. Lie on your back, knees bent, feet flat and hip-width apart. Press through the heels and lift the hips until shoulders, hips and knees line up, squeezing the buttocks at the top. Hold 2 seconds, lower with control. Ten to fifteen repetitions, two to three sets. Progress to single-leg bridges only once the two-leg version is comfortable and symmetrical. Technique detail in our bridge exercise guide.
Clamshell
Lie on your side with hips and knees bent to roughly 45 degrees and feet together. Keeping the feet touching and the pelvis still, lift the top knee. Hold 2 seconds, lower. Twelve to fifteen each side, two to three sets. Targets the gluteus medius and the hip external rotators. Add a resistance band above the knees when it becomes easy.
Bird dog
On hands and knees, wrists under shoulders and knees under hips. Extend the opposite arm and leg together while keeping the pelvis level and the spine neutral. Hold 5 seconds, return, alternate. Eight to ten each side. Trains the multifidus and trunk control, which is precisely the demand the joint faces during walking.
Side-lying hip abduction
Lie on your side with legs stacked and straight. Lift the top leg about 30 to 45 centimetres, keeping it in line with the body rather than drifting forward. Hold 2 seconds, lower slowly. Twelve to fifteen each side, two to three sets. Strengthens the gluteus medius directly.
Our phased at-home back programme covers how to build these into a wider routine.
Manual therapy
Manipulation and joint mobilisation of the sacroiliac region can produce short-term pain relief and improved movement for some people. The effects are modest and short-lived, so it works best as something that makes the strengthening work easier rather than as a treatment in itself. The audible click is not necessary for benefit, and gentler graded mobilisation appears to produce comparable outcomes.
Sacroiliac belt
A pelvic belt worn low, around the level of the greater trochanters rather than at the waist, adds external compression and substitutes for force closure while you build it. It is genuinely useful during acute flares and for pregnancy-related pelvic girdle pain. Treat it as a temporary aid rather than a permanent fixture, since relying on it indefinitely works against the muscular strengthening that produces lasting change.
What Tends to Aggravate It
- Standing with your weight on one hip. The single most common habit to change, and it costs nothing.
- Crossing your legs while sitting, and sitting on a wallet or phone in a back pocket.
- Deep lunges, wide-stance squats and single-leg gym work during a flare, since these load the joint asymmetrically at end range.
- Getting in and out of a car, and in and out of bed, one leg at a time. Moving both legs together as a unit is noticeably easier during a flare.
- Sleeping without support between the knees if you are a side sleeper, which lets the top hip drop and rotate the pelvis all night.
Particular Situations
| If you are | What tends to matter most |
|---|---|
| Pregnant | A pelvic belt worn low, keeping the legs together when turning and getting out of bed, avoiding wide-stance movements, and early referral rather than waiting it out |
| Postpartum | Graded strengthening rather than stretching. Symptoms often improve over months, but persistent pelvic girdle pain should be treated rather than endured |
| A runner | Check for a sudden increase in volume, camber running on the same side of the road, and single-leg control. Gluteus medius endurance is usually the limiting factor |
| Hypermobile | Prioritise strength and stability work over stretching and manipulation, both of which can make things worse by adding movement to a joint that already has too much |
| Post lumbar fusion | Load transfer through the sacroiliac joints increases after fusion. This is a recognised pattern and worth raising with your surgical team |
| Sitting all day | Position changes every 30 to 45 minutes, an even sitting base, and not sitting on a wallet |
Medical Options
- Anti-inflammatory medication can help short term, but suitability depends on your other conditions and medicines. Ask a pharmacist or doctor rather than self-prescribing. If an inflammatory cause is suspected, a strong response to anti-inflammatories is itself diagnostically meaningful and worth reporting.
- Image-guided corticosteroid injection can provide a period of meaningful relief for some people, commonly measured in weeks to a few months, and simultaneously helps confirm the joint as the pain source. Duration varies considerably between individuals.
- Radiofrequency denervation targets the small nerves carrying pain signals from the joint. It is considered for persistent pain that has not responded to conservative treatment and where injections gave temporary relief. Reported benefit varies, and it is not a permanent fix.
- Sacroiliac joint fusion is a surgical option reserved for confirmed, disabling instability after conservative management has failed. It is uncommon and warrants a second opinion.
For the wider picture on lower back pain, see our lower back pain guide. If your pain is worst on standing and walking and eases when you sit or lean forward, read back pain when walking or standing instead.
Terms You May Hear
| Term | What it means |
|---|---|
| Sacroiliitis | Inflammation of the sacroiliac joint, as distinct from mechanical irritation. Central to axial spondyloarthritis |
| Pelvic girdle pain | Pain around the pelvis, often including the sacroiliac joints, particularly during and after pregnancy |
| Form closure | The stability the bony shape and ligaments provide. Not trainable |
| Force closure | The additional compression the surrounding muscles provide. This is what rehabilitation builds |
| Provocation test cluster | A group of physical tests that deliberately load the joint to reproduce pain. More reliable together than singly |
| Fortin finger test | Pointing to the painful spot with one fingertip. Suggestive when it lands just below and inside the dimple |
| Diagnostic block | An image-guided anaesthetic injection used to confirm whether the joint is the source of pain |
| Axial spondyloarthritis | A family of inflammatory conditions affecting the spine and sacroiliac joints, including ankylosing spondylitis |
Frequently Asked Questions
Four features point towards the sacroiliac joint: you can usually point to it with one finger just below and inside the dimple on one side, it rarely travels below the knee, it is provoked by single-leg loading and rolling over in bed, and true numbness in a defined band is usually absent. Pain reproduced by coughing or straining points away from the joint and towards the spinal canal. These are indicators, not a diagnosis.
Because its referral pattern into the buttock, groin and back of the thigh overlaps closely with disc problems, hip conditions and piriformis syndrome, and because the physical tests are imperfect. A cluster of provocation tests is reasonably good at ruling the joint out when negative, but considerably weaker at confirming it when positive, so errors happen in both directions.
Not in the way the phrase suggests. The joint moves only a few degrees and a millimetre or two, and it is held by some of the strongest ligaments in the body, so it does not slip out and require putting back. What changes with treatment is how well the surrounding muscles control load through it. Be cautious of anyone offering to realign it permanently.
Exercises that increase force closure, meaning the compression the surrounding muscles apply to the joint. Glute bridges, clamshells, bird dogs and side-lying hip abduction cover the main contributors: gluteus maximus, gluteus medius, multifidus and the deep abdominal wall. Build gradually and judge by how you feel the next morning.
Side-lying with a firm pillow between the knees and ankles is the position most people find easiest, because it stops the top hip dropping and rotating the pelvis through the night. Lying on your back with a pillow under the knees is a reasonable alternative. When getting in and out of bed, move both legs together as a unit rather than one at a time, which is a common flare trigger.
It can help during an acute flare and during pregnancy-related pelvic girdle pain. Wear it low, around the level of the hip bones rather than the waist. Treat it as a temporary aid while you build strength, because relying on it indefinitely works against the muscular support that produces lasting improvement.
It can be. Axial spondyloarthritis, including ankylosing spondylitis, causes true inflammation of the sacroiliac joints. The pattern differs from mechanical pain: onset usually before 45, morning stiffness lasting over 30 minutes, pain waking you in the second half of the night, and improvement with movement rather than rest. That combination warrants rheumatology assessment, as it is commonly missed for years.
Mechanical flares often ease over several weeks with activity modification and strengthening, though it commonly recurs during periods of heavy or asymmetric loading. Pregnancy-related pelvic girdle pain frequently improves in the months after delivery but should be treated rather than endured. If there is no improvement after six to eight weeks of consistent work, get reassessed.
Sources
- Laslett M, Aprill CN, McDonald B, Young SB. Diagnosis of sacroiliac joint pain: validity of individual provocation tests and composites of tests. Manual Therapy, 2005.
- Saueressig T, et al. Diagnostic accuracy of clusters of pain provocation tests for detecting sacroiliac joint pain: systematic review with meta-analysis, 2021.
- Cohen SP. Sacroiliac joint pain: a comprehensive review of anatomy, diagnosis, and treatment. Anesthesia and Analgesia, 2005.
- Vleeming A, Albert HB, Östgaard HC, et al. European guidelines for the diagnosis and treatment of pelvic girdle pain. European Spine Journal.
- National Institute of Arthritis and Musculoskeletal and Skin Diseases — Ankylosing Spondylitis.
- NHS — Ankylosing spondylitis.
- NICE guideline NG59 — Low back pain and sciatica in over 16s.
Last updated: August 2026. Written by Aisha Desai, who covers back and musculoskeletal topics and is not a clinician. This page has been reviewed for accuracy by our editorial team but has not been reviewed by a named clinician. It is general information rather than a diagnosis. Sacroiliac joint pain overlaps closely with several other conditions and cannot be distinguished without examination, so please see a doctor or physiotherapist about your own symptoms.
