The obturator internus, coccygeus, and piriformis all work together but have slightly different roles. The obturator internus is also a hip rotator and is the conduit for a very important nerve, the pudendal nerve, that helps control a great deal of pelvic floor function. A dysfunction in the obturator internus can therefore result in a slew of problems. The coccygeus muscle assists the levator ani muscles in moving the coccyx bone or tail-bone when the muscles are contracted. The piriformis, like the obturator internus, is also a hip rotator.
Experts in evolution theorize that the pelvic floor musculature once controlled the tails of our apelike ancestors, before hands proved to be more effective. Eventually, the use of hands naturally selected a new branch of evolutionary development. In that new branch, which eventually became us, the tails went away, but the muscles remained. Now, however, instead of controlling wagging and hanging from trees, the muscles took on the function of helping to support the body's core. So today, the evolved pelvic floor serves three vital purposes:
It upholds and cushions the organs within the pelvis and lower abdomen: urinary organs, digestive organs, and reproductive organs.
It controls continence by signaling elimination urges to the bladder and bowel and by opening and closing the urethra and anal canals to allow voiding.
It is the mechanism of sexual function, contracting the muscles around the female and male genitalia to respond to arousal and to enhance appreciation.
These are big and important jobs, which may be why so many thick, closely connected muscles are involved. We can characterize these pelvic floor muscles in several ways. First, they're voluntary muscles. That is, we control them consciously. This is different from the smooth muscles of the bladder, intestines, lungs, and blood vessels, which are involuntary; that is, they're controlled by our nervous systems in such a way that they operate automatically.
Second, the muscles of the pelvic floor are skeletal muscles. That means they are attached to the skeletal frame. When we contract the pelvic floor muscles, the energy of the contraction applies force to the tailbone.
The pelvic floor muscles also come in two "speeds." About 70 percent of the pelvic floor muscle fibers are slow-twitch or slow-contraction fibers; the rest are fast-twitch fibers. Put simply, the slow-twitch muscle fibers fuel endurance. They're the marathoners of muscles, providing support and resisting fatigue. Think of the muscles in your lower back: they are mostly slow-twitch fibers that can work for a long time without tiring. That's essential, because these are the muscles responsible for helping keep you upright. The slow-twitch muscles that make up the bulk of your pelvic floor are that kind of slow-to-tire, persistently supportive muscles.
Fast-twitch fibers, by contrast, provide the quick forcefulness of sprinters. The muscles that move your eyes, for example, are fast-twitch muscles. In the pelvic floor, the fast-twitch muscles assist in controlling the contraction and relaxation that open and close the bladder and bowel and that serve the sexual function so essentially.