Musculature of the Pelvis and Hip Joint. Permission Joseph E. Muscolino. The Muscular System Manual - The Skeletal Muscles of the Human Body, 4th ed. (Elsevier, 2017).

Five Muscles of Sacroiliac Stabilization – Part 2 – Coccygeus and Others

The coccygeus and levator ani are technically muscles of the pelvic floor, however, they are also muscles whose contraction forces cross the sacroiliac joint and therefore often tighten when a sacroiliac joint condition exists. The coccygeus attaches from the sacrum and coccyx to the ischial spine of the pelvic bone.

Musculature around the Sacroiliac Joint. Permission Joseph E. Muscolino. Manual Therapy for the Low Back and Pelvis - A Clinical Orthopedic Approach (2013).

Five Muscles of Sacroiliac Stabilization – Part 1 – Piriformis

Even when the original reason for the sacroiliac stabilization is valid, often the human body overdoes it and tightens the musculature excessively and/or keeps it tight long after it needs to be, so the musculature becomes stuck in a chronic pattern of hypertonicity.