FI is a complex condition with many impacting factors, so let’s focus solely on the role of pelvic floor. The mechanism of continence is dependent on both the puborectalis (which creates a kink in the rectum), and the external anal sphincter, both of which are under your voluntary control. Weakness in the pelvic floor can result in laxity and atrophy of these and other muscles of the pelvic floor.
A weak pelvic floor typically sit lower providing less support for the organs of the pelvis (bladder, uterus and rectum). If you have read the urinary incontinence and pelvic organ prolapse pages, you will be familiar with the impact of weak pelvic floor muscles. In respect of the puborectalis and levator ani, weakness results in a decrease in the anorectal angle. This can increase the risk of FI as a reduction in this angle allows more, if not all of the fecal content to weigh down on the external urethral sphincter. This reduction in angle is important when you need to poo. Squatting to poo reduces the angle significantly allowing for a faster evacuation.
Weakness in the external anal sphincter is the real killer though. The external anal sphincter needs to have sufficient strength to hold tightly closed. You have voluntary control of the muscle so when it is sufficient in strength, you can maintain the closure. Lack of strength means that the pressure of feces bearing down will be greater than the pressure holding the muscle closed. This will result in a leakage. The muscle can be weak due to atrophy, or in many cases, it can be weak due to injury. If tearing during childbirth is severe, this can cause a sphincter tear. Depending on the quality of repair and on the subsequent muscle rehabilitation, you will have more or less strength in this muscle.
Sometimes fecal incontinence can occur after the bowel has been emptied, if it had failed to empty fully. This can happen due to a rectocele, where weakness in the pelvic floor and the back vaginal wall (posterior) allows the rectum to prolapse into the vagina. You can read more on rectocele and treatments in the pelvic organ prolapse section of the knowledge portal.