The pelvic floor is made up of a series of muscles, tendons, and ligaments which form a kind of “hammock” from ligaments in the pelvis and support and hold in place the pelvic organs. The largest muscle in the pelvic floor is the "levator ani". All of the muscles in the pelvic floor attach near the rectum. This series of muscles is in constant use. It relaxes only when you urinate or defacate.
Just like any other muscle in the body, the pelvic floor weakens or “atrophies” with age, and with not enough protein in the diet. In women, many of the tendons and ligaments become stretched or torn with childbirth and never heal completely, and weaker scar tissue sometimes replaces muscular tissue in these cases. Obesity, lack of use, heavy lifting, chronic cough, and chronic constipation place additional pressures on the pelvic floor. Genetically weak collagen, and smoking (which weakens collagen), both play a role in weakness of the pelvic floor. Pudendal nerve injury (usually from childbirth or surgery, but sometimes from diabetes and other conditions that affect nerves) also plays a role in muscle atrophy, since muscles need a tonic level of nerve input to maintain their tone and strength.
The strength of the pelvic floor is not always easy to assess through physical exam, but a test called anorectal manometry can easily quantify it. Another test, called EMG, can determine the nerve input and how long it is lasting for a particular contraction. It is used to assess whether or not the muscle is spastic, since weak muscles are more prone to injury and spasm. Bulbocavernosus reflex latency is a simple test done to assess the pudendal nerve in particular. If your provider determines that you have pelvic floor weakness, s/he will likely order one or more of these tests to further quantify what is happening and how best to treat this condition.
Kegal exercises are the best way to maintain the tone of the pelvic floor and the muscles that support the bladder neck and vagina and give these organs tone and strength. A strong healthy set of pelvic floor muscles goes a long way to preventing urinary stress incontinence and pelvic floor prolapse.
There is a fun Australian website that explains in detail how to do Kegals, how to remember to do them, and how to actually make them fun. Check it out! (Women's Health Connection has no financial ties to this company.)