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The Mechanics of the Pelvis and Sacroiliac Joint

I am going to do a series of articles covering the pelvis and sacroiliac joint (SIJ), this will be followed by an article on the causes of injury.

The final article will look at the effect that core strength and correct muscular balance has on the stability of the pelvis and the lower back. This first article will look at the pelvis as well as the sacroiliac joint.


The pelvis is made up of two Ilium bones posteriorly and the pubic bones anteriorly, as well as a triangular bone called the sacrum. The sacrum sits at the base of the lumbar spine with the Ilium bones inserting at each side. The pubic bones are joined anteriorly at a cartilaginous joint called the Pubic Symphysis.

Sacroiliac joint functions

The SIJ is designed to transfer large loads and its shape has adapted to this task. The sacrum is essentially the ‘keystone’ between the wings of the pelvis (Gibbons, 2011). There are two ways that the SIJ maintains its integrity, these are form closure and force closure.

Form Closure

This is due to the anatomical alignment of the bones of the Ilium and the sacrum. The anterior surfaces of the joint are relatively smooth to aid the transfer of compression forces and bending movements. This can cause weakness so the SIJ uses the wedge shape of the sacrum to help stabilize along with their irregular shaped surfaces that interlock to aid stabilization.

Force Closure

This is the effect that the ligaments and muscles have on the joint. The main ligaments that stabilize the joint are the sacrotuberous ligament, (this connects the sacrum to the ischium) as well as the long dorsal sacroiliac ligament that secures the sacrum to the posterior superior iliac spine.

The role of the core

The ligaments cannot stabilize the SIJ without the support of the number of groups of muscles. The two most important groups of muscles that contribute to the stability of the lower back and the pelvis are the inner system, called ‘the core’ or the local stabilizers. These are made up of transverse abdominis, multifidus, the diaphram and the pelvic floor. The outer system is called ‘the sling’ or the global stabilizers.

All of this results in something called a force couple. This is “a situation where two forces of equal magnitude, but opposite direction are applied to an object” (Abernethy et al., 2004). Force couples cause the joint to maintain integrity, but still be able to move. It is when this force couple fails and one force is stronger than another that an injury will result.

Next month I will look at the effect this difference in forces has on the pelvis.

Adam Dobson - Revolution Sports Injuries

Adam Dobson started Revolution Health after finishing  a BSc (hons) degree in Sports Therapy at the University of Chichester

Revolution Sports Injuries Clinic Wantage

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