Dr. Sherine Aubert describes the superficial transverse perineal (STP) muscle, part of the superficial (or outermost) layer of the pelvic floor.
The STP is a primary stabilizer of the pelvic floor. It is located from sit bone to the perineum on each side of the pelvis. It works to stabilize the ever so important perineal body/central tendon (it's critical for the integrity of your whole pelvic floor).
How to Self Release Superficial Pelvic Floor Muscles