The unstable SIJ

The sacro-iliac joint (SIJ) is a very stable structure connecting the spine to the pelvis. The stability is achieved by a system of active (force closure) and passive (form closure) stabilisers. Its main role is to helps absorb vertical forces through the body. An unstable SIJ can compromise the effectiveness of transmitting forces, causing symptoms like low back pain or sometimes radiating pain down the legs. Core stability exercises and a sacro-iliac belt can help with an unstable SIJ.


The sacrum sits in between the two iliac bones as they connect to form the SIJ. The sacrum is the tail end of your spine and the iliac bone forms part of the pelvis.

The main role of the SIJ is to allow forces to be transmitted vertically through the body. The SIJ acts like a very tough suspension that absorbs the impact from the legs to the spine during activities like walking, running and jumping.

The SIJ is an inherently strong and stable joint that permits very little movement due to its natural construction. This structural strength provides passive stability or form closure. The rough, groove-like connecting surfaces of the iliac bones and sacrum interlock and help stabilise the joint; like fixing two Lego pieces together. In addition, strong and tight ligaments hold the SIJ firmly together.

Another feature that stabilises the SIJ is described as the active stabilisers or force closure. Imagine wrapping your fingers around the two Lego pieces. This is similar to your muscles surrounding the SIJ, actively contracting to create a compressive force over the SIJ, gripping the joint firmly together. These muscles are also known as the core muscles. They act as a natural corset by providing that compression around the lower back and pelvic region.

This system of form and force closure ensures stability in the SIJ to perform its role optimally. However, an unstable SIJ can happen when the integrity of this system is being compromised.


Passive instability can be compromised when the ligaments joining the SIJ becomes loose or lax. This happens when you fall and land on to the buttocks, or miss a step. The impact going through the SIJ overstretches the ligaments causing it to become loose or lax.

Another cause of SIJ instability is pregnancy. Hormones released during pregnancy relax the ligaments of the body to allow the pelvis to enlarge. At the same time, muscles around the pelvis get overstretched and weakened. This contributes to the loss of form and force closure.


The only way to improve on the stability of the SIJ would be to increase the system of force closure. Thus, core stability exercises are recommended for an unstable SIJ to increase the compressive force. Sometimes, a sacro-iliac belt is prescribed to complement the core stability exercises.

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