Dr. Sherine Aubert describes the external anal sphincter muscle, part of the superficial (or outermost) layer of the pelvic floor.
The external anal sphincter muscle is, as the name suggests, a sphincteric (constricts and relaxes to allow passage in or out) muscle. It encircles the anus, relaxing to allow stool or gas to pass. Its shape allows a puckering movement around the anus when we contract our pelvic floor muscles and a relaxation when we breath into our pelvic bowls.
How to Self Release Superficial Pelvic Floor Muscles