Sacroiliac Joint Dysfunction
If the SIJ is irritated and/or inflamed, in other words there is a ligamentous sprain, muscles tighten for sacroiliac stabilization, attempting to prevent the joint from excessive movement that might further irritate/damage it or cause pain to the intrinsic fascial tissue of the joint. If there is a muscular strain of the associated musculature of the SIJ, the muscles might be directly injured causing them to tighten up and armor the region. If instead, there is movement dysfunction, for example the SIJ is hypermobile, musculature might tighten to stabilize the joint to decrease its excessive movement. And even if it is hypomobile dysfunction, a hypomobile SIJ often does not move/glide smoothly and pain can occur during motion. Finally, if there is degenerative joint disease (osteoarthritis) of the SIJ joint surfaces, then musculature might tighten to minimize this motion and guard against the pain.
Source: https://www.instagram.com/p/CjHfAm3rAAF/?utm_source=ig_web_copy_link