The "pelvic floor" is a catch-all term for the group of muscles that covers the bony opening at the base of the pelvis. As a group, the pelvic floor muscles (or PFM) provide support for the pelvic organs, control the bladder, aid in positioning and fine-tuning the spine, and support sexual function.
There are two layers of muscle in the pelvic floor – the superficial perineal muscles and the deep pelvic diaphragm.
Superficial Muscles of the Pelvic Floor
The most superficial layer of muscles in the female pelvic floor looks like a figure 8. The top loop of the 8, attached to the pubic bones near the clitoris, is made of the bulbospongiosus muscles. The bottom loop of the 8, attached near the tailbone to the anococcygeal ligament, is made of the anal sphincter. The 8 crosses at the central tendon of the perineum.
The superficial transverse perineal muscles each run straight across from the central tendon to the ischial tuberosity, or sitz-bone. The ischiocavernosus muscles run from the pubic bones to the ischial tuberosities, along the ischiopubic ramus on each side of the front of the pelvic opening.
In addition, the external sphincter of the urethra is wrapped around the urethra and sandwiched between the two layers of the perineal membrane, and the deep transverse muscles of the perineum are stretched horizontally between the ischiopubic rami, providing a triangular base for the anterior half of the pelvic floor.
Deep Muscles of the Pelvic Floor
The deep pelvic floor is also called the pelvic diaphragm. This portion of the pelvic floor looks like a bowl, or like the hull of a boat. It is made up of a muscle group called levator ani, and a muscle called coccygeus.
The levator ani group consists of the puborectalis, which attaches above the pubis and travels around the rectum, the pubococcygeus (popularly called the PC or Kegel muscle), which makes a wider loop around the rectum and attaches to the ischium via a tendinous arc, and the iliococcygeus, slung between the tendinous arc and the coccyx.
Coccygeus is attached to the ischial spine and to the sacrum and coccyx.
As a whole, the deep pelvic floor muscles form a bowl-shaped structure covering most of the pelvic opening. There is an area free of muscle, between the symphysis pubis and the rectum, called the urogenital hiatus or the levator buttonhole, which allows for childbirth.
Overall Structure of the Pelvic Floor Muscles
Because of the different orientations of their fibers, the different attachment sites at bones and ligaments, and their different layers, the pelvic floor muscles are able to form a highly adjustable, flexible and strong muscle group. When properly functioning, the female pelvic floor muscles adapt to even the stresses of childbirth and are able to heal and return to previous levels of function.
Source and Further Reading: Blandine Calais-Germain, The Female Pelvis (Seattle: Eastland Press, 2003)
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