If only I had known sooner! I first heard this clear explanation of how the sacroiliac joint should, and should not, move while I was enrolled in Purna Yoga College. Thank you, Aadil Palkhivala, for making things so understandable. (Clarity of safety instruction is a hallmark of the program.)
This is where your SI joint is:
The SI joints have a lot of surface space:
The SI joint is fairly complex; if you like anatomy and physiology, it's a fun one to study. In this post, I am going to keep it simple, giving yoga students and teachers the fundamentals necessary to understand safe movement. A primary principle you need to remember to keep the SI joint (and all other joints) safe is:
Joint Structure Dictates Safe Movement.
Pretty straightforward, isn't it? If a joint is a hinge joint (like the knee), be sure you move it like a hinge to keep it safe. If you go "non-hingey" in your movement, the knee suffers. As for the SI joint, part of it has what I will call a "lock and key" system: the bumps on the sacral side fit into the hollows on the iliac side, and vice versa. This "lock and key" system helps create the stability required of a joint where the entire upper body rests in the lower body. To maintain SI joint health, we have to honour that structure. Don't misalign the bumps and grooves (it HURTS!). Don't encourage movement that wears down the bumps and grooves over time; it can create joint instability, which leads to misalignment, which . . . you guessed it: HURTS! What does that mean in practical terms? The following video explains it all!
You might also be interested in: Keeping your SI Joint Happy in Twists >>