Final answer:
Increased motion that may suggest SIJ instability is associated with the sacrum and ilium at the sacroiliac joint, supported by anterior and posterior sacroiliac ligaments.
Step-by-step explanation:
Signs of Sacroiliac Joint (SIJ) 'instability' have been associated with increased motion at the sacrum and ilium sections of the hip bone, particularly where the anterior sacroiliac ligament and posterior sacroiliac ligament support the joint. These ligaments are crucial for maintaining stability. The ilium forms the superior part of the hip bone with important landmarks such as the anterior superior iliac spine, a bony landmark that can be felt at the anterolateral hip, and the posterior superior iliac spine, which is a point where muscles and ligaments attach that also support the stability of the sacroiliac joint.