Not all lower back pain comes from the spine. The sacroiliac joints, where the base of the spine meets the pelvis on each side, are a frequent source of pain low in the back, to one side, sometimes spreading into the buttock or the top of the thigh. A sacroiliac belt is a support designed specifically for this area.
What the sacroiliac joints do
The two SI joints transfer load between the upper body and the legs. They move only a few millimetres, but when they are irritated or feel unstable, everyday movements such as rolling over in bed, standing on one leg to dress, or getting out of a car can be surprisingly painful. SI pain is common in pregnancy and after childbirth, after a fall onto the bottom, and in people whose work involves repeated bending and twisting.
What an SI belt does
An SI belt is a relatively narrow, firm belt worn low around the pelvis, across the top of the hip bones rather than around the waist. It applies compression across the pelvis to reduce movement at the SI joints and give a feeling of stability. Many people describe the relief as similar to pressing their hands firmly on the hips.
SI belt or lumbar back belt?
- Lumbar back belt: wider, worn around the waist and lower back, designed to support the lower spine and remind you to keep a good lifting posture.
- SI belt: narrower, worn lower across the pelvis, designed to compress the pelvis and limit movement at the SI joints.
If your pain is central and in the small of the back, a lumbar belt is usually the better fit. If it is low, to one side and worse with single-leg activities, an SI belt is more likely to help. Some supports combine both.
How to wear an SI belt
- Stand or lie down and place the belt so it sits across the top of the pelvis, over the bony points at the front of the hips, not around the waist.
- Fasten it firmly enough to feel supportive, then adjust the secondary straps if the belt has them.
- Walk around. It should feel stable but not restrict breathing or dig into the hips.
- Wear it for the activities that provoke pain, such as walking, standing and household tasks, and take it off for rest.
Fit and comfort tips
- Measure around the hips at the level of the hip bones, not the waist.
- Choose a belt with a silicone or grippy inner surface if it tends to ride up.
- Wear it over a thin layer of clothing if the edges rub.
- Loosen it when sitting for long periods.
During and after pregnancy
Pelvic girdle pain is common in pregnancy and an SI or pelvic support belt is often suggested by midwives and physiotherapists. Choose a design that sits below the bump and can be adjusted as your shape changes. Always check with your midwife or physiotherapist before using any support during pregnancy.
When to get advice
See a healthcare professional if pain spreads down the leg below the knee, if you have numbness or weakness in the legs, any change in bladder or bowel control, pain after a fall or injury, or if symptoms have not improved after a few weeks. A physiotherapist can confirm whether the SI joint is the source and suggest strengthening exercises for the hips and core.
Frequently asked questions
Can I sleep in an SI belt?
Some people find it helps with turning over in bed, but most belts are designed for daytime use. Ask your physiotherapist if night use is appropriate for you.
Will it weaken my core?
Used for the activities that hurt and combined with exercise, a belt is unlikely to weaken you. Avoid wearing it constantly for months without also building strength.
How tight should it be?
Firm enough that you notice support and relief, but you should still be able to breathe deeply and slide a finger under the edge.
Explore back supports
Browse the Venollo back supports and braces and read our guide to back support belts.
This article is general information and is not a substitute for medical advice. If you have an injury, a health condition or ongoing symptoms, please speak to a qualified healthcare professional.
