These terms represent pretty key concepts that you will inevitably come across as you learn about the SI joint.
In a nutshell, hypomobility refers to a joint that’s moving less than a healthy joint would.
Hypermobility refers to a joint that’s moving more than a healthy joint would.
I have experienced both hypo- and hypermobility in my SI joints. And while it’s important to know exactly why your SI joints are causing pain (are they moving too much or too little?) the two problems can be two sides of the same coin.
When one of my SI joints locks up, it means that the hip bone, or ilium, on one side has rotated so far posteriorly that it becomes wedged against the sacrum. That is technically a case of hypomobility— the ilium is now fixated in that position. In my case, it would remain stuck there, no matter what I did or how I moved, until either my chiropractor or I performed the specific motions necessary to “adjust” the joint.
But, to take a step back for moment conceptually… how did things get there? How did the ilium move so far out of its normal range of motion that it is now wedged abnormally against another bone? Hypermobility is what allowed that to happen in the first place– the ligaments that were supposed to hold the ilium in place weren’t doing their job.
So you can see how in cases of ligament instability like mine, the terms hypo- and hypermobility can simply refer to the same problem, at different moments in time. Both terms simply refer to how much the hip bone (ilium) is moving in relation to the sacrum.
According to my chiropractor, it’s pretty common to find that when one SIJ is hypomobile (stuck), the other is hypermobile. It’s the body’s attempt to compensate– when motion is restricted on one side of the body, the body will find a way to increase motion on the other side.
I don’t really have a very ceremonious way to end this terminology post, except to say that I hope this was helpful and I have more resources to come! Thanks for reading!