Rupture of the internal anal sphincter (IAS) causes its weakness and it will not withstand increases of abdominal pressure, and fecal incontinence (FI) will occur. Recently, we put forward a novel concept on the physiology of defecation. Defecation is divided into two stages: First stage before training and second stage starts at the age of about 2 years, when the mother starts to teach her child how to hold up himself. This is gained by maintaining high alpha-sympathetic tone at the IAS, thus keeping it closed all the time till there is a need to pass stool or flatus, and the time and place are convenient. On defecation, six neuromuscular actions take place under the control of the CNS: (1) The person will relax the external anal sphincter, (2) he will lower the gained high alpha-sympathetic tone at the IAS, thus opening the anal canal, (3) he will relax the pelvic floor muscles bringing the rectum and the anal canal into one axis, (4) the abdominal and diaphragmatic muscles contract to increase the abdominal pressure, (5) the muscles of the distal colon and rectum contract pushing the stool, (6) sequential contractions of the three parts of the external anal sphincter (EAS) that squeeze any residual contents in the anal canal. Thus, the anal canal is closed and empty under normal circumstances. The IAS is a collageno-muscular tissue cylinder that surrounds the anal canal.

The IAS is intimately related to the posterior vaginal wall, and the vagina is over stretched in labor, childbirth trauma affects both the posterior vaginal wall and the IAS. Rupture of the collagen sheet of the IAS which causes its weakness is better demonstrated by imaging by 3D US.

Normal vagina is a cylinder of collageno-elastic-muscular tissues. Its strong collagen sheet is responsible for keeping it in its normal upward position. Labors cause redundancy and weakness of the vaginal walls with subsequent prolapse; and lacerations of the IAS which is closely related to the posterior vaginal wall leading to FI.