The primary functions of the rectum and pelvic floor muscles are to prevent incontinence (loss of control) and to allow defecation to occur. This requires intact sensation as well as normal anatomy and coordination to work as it should. In certain situations, an inability to relax the pelvic floor muscles or certain anatomic changes can lead to difficulty getting stool out, which is called pelvic floor dysfunction (also known as pelvic floor dyssynergia, obstructive defecation, or anismus).
On the other hand, damage to the pelvic floor can lead to undesired leakage of stool, called fecal incontinence. This most commonly happens from damage that occurs from childbirth in women, or from radiation for prostate cancer in men, although there are many other causes.
For more information on either of these problems, click here.