The arrival of material in the rectum induce a reflex distension (a relaxation) of the internal sphincter and a contraction of all the intestinal musculature. The voluunter relaxation of the external sphincter permit in this case the expulsion of the material.
Unfortunately it is obvious than some people (essentially elderly people but some junger people to) are suffering from incontinence problem. Various studies [1] allow to deduce that this anomaly is due in major part to the abnormal functioning of the internal sphincter who don't play is warning signal role.
The experiment of ano-rectal manometry exposed above permit studie the comportment of this msucular complex, both on normal and pathological subjects.