For a long time, I had major problems with my SI joints.
In may cases, SI joint pain is a result of a neuromuscular pattern that Postural Restoration calls the left AIC pattern.
While pain on the right side of the pelvis is often due the joint being pulled apart, pain in the left SI joint is often related to either compression or weakness in the left hip. Or both.
More specifically, the left leg (femur) doesn’t sit snug inside the hip socket (acetabulum).
This occurs in response to a forwardly rotated left pelvis. When a left hemi-pelvis is rotated forward, the left leg will often compensate into external rotation. It does this in order to stay straight. This is a smart thing to do so that it makes it easier for you to walk and run and do everything you need to do.
But all compensations come with consequences. In this case, certain muscles, particularly the left IC adductor and left hamstrings will weaken because they are now positioned in an elongated state.
As the muscles weaken, the pelvis becomes unstable, and the left SI joint is often a place that hurts as a result.
The following video explains more in depth.