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.
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.