The SIJ is a fantastic structure. It has a simple design for its simple job - to transfer load from the trunk to the legs and from the legs to the trunk.
It receives force from the femur via the hip joint into the innominate (a combination of the Ilium, Ischium and Pubis bones), transfers the load through the SIJ into the sacrum. From the sacrum, the load goes up through (in "normals") the L5/S1 disc and then upwards through the vertebral bodies.
Here is the beauty of the design of the SIJ...the amount of load passing through the joint determines how much the joint "locks up"
You see, the SIJ moves a little (we will deal with this in a later post) - not very much but it moves enough to give you some twisting in the pelvis...but it also moves to help provide nutrition to the joint.
It might be easier to think about the SIJ as being able to "hold" on lightly or strongly depending on the situation.
In everyday life, you hold your knife lightly when spreading margarine but hold it quite firmly when cutting through a through a dense vegetable like a Swede...alternatively, you hold a newborn baby quite gently but if you were hanging off the edge of a cliff, how hard would you hold on?
It all depends on the situation and the load.
In the same way, the SIJ "winds up" as little or as much as it needs to. This makes sense - you don't want to be squeezing as hard as possible for every action. So when you lie down, the SIJ is pretty relaxed. When you hop or run, it winds up quite a bit because all your weight is on one leg and dropping from a height.
I love the SIJ - and the rest of the body - because of amazing facts like these...our bodies are so adaptable and the systems are so simple in design yet so complex that we can't replicate it with the same size restraints.
If you are interested in learning more about the SIJ, please go to www.mypteducation.com and make contact with me about where you would like me to host a course for you.