Basically, what I tell people is that the SI joint is where the pelvis and the spine connect.
The SI joint doesn’t really move in the way you think of when you picture a stereotypical “joint.”
The SIJ only moves a few millimeters, but like anything in the body, when it’s not doing what it’s supposed to do, it causes problems.
The purpose of the SI joint is to just create a little bit of motion and flexibility in that location in your body, in order to assist with larger movements produced by other major joints.
For example, when you walk, your hip bones rotate ever so slightly in order to give you greater freedom to move your leg.
The SI joint is also responsible for a lot of shock absorption. Every time you run, or even walk, an enormous amount of force travels up your leg when your feet connect with the ground.
The SI joint is built to absorb and dissipate that force, allowing your body to move more efficiently over all.
A third function of the SI joint is specific to women: it loosens up dramatically just before and during childbirth, in order to allow more room for a baby to pass through. When a woman’s body is getting ready to go into labor, she will produce higher amounts of the hormone relaxin, which, not surprisingly, tells ligaments to “relax.”
This is why SI joint problems are very commonly related to pregnancy. They are often accompanied by dysfunction at the pubic symphysis as well.
If you liked this post, I have links to more in-depth articles on the SI joint my Resources for Patients section. Definitely check it out!