Great info and diagrams.
“The SI joints job is to provide support and some shock absorption as pressure is transitioned from the top and bottom halves of the body. The head, arms, rib cage and spine all load from above and the legs push stress up from the ground into the hips, pelvis and sacrum. The SI joints have strong ligamentous support to help to distribute all of these forces.
One of the most stressful events for the SI joint to deal with is one legged standing, landing or jumping. The body weight load is magnified by gravity and landing on only one side creates a shearing stress not only to the SI joint that takes the weight but also the one not on the ground because some of the weight of that leg is still being pulled towards the ground. These forces happen with every step when walking and are increased greatly when stair climbing, running and certainly jumping onto or off of one leg.
These joints can be injured in several different ways. They can be sprained just as any other joint can when a stress greater than what the joint can handle is placed on in it. This most often happens when a fall occurs onto one side of the pelvis or upper leg or when a forceful one-legged landing occurs. This can lead to low back and buttock pain. The SI joint can also become dysfunctional due to muscle issues. The SI joint does have some ability to move despite its strong ligamentous connections and can be impacted my muscle tightness and weakness especially when the differences are asymmetric. These asymmetric stresses can lead to pain.
(Bold italics mine).