malmo wrote:
It's called peristaltic rush.
Technically, it's CAUSED by peristaltic rush as the small intestines "force" waste into the large intestine (colon). At this point it still has to travel all the way through the large intestine (via peristalsis AND segmentation). It manifests itself in the sudden gaseous/cramping feeling that precedes the diarrhea-like release most runners have experienced after (or, unfortunately, sometimes during) a run.
Luckily for all of us, even though the colon and most of the rectum is goverened by the autonomic nervous system/involuntary muscle, the terminal end of the rectum and the external anal sphincter are obviously under somatic/voluntary control.
If anyone cares (and I doubt many do), this is similar to the gastroiliac reflex, which is responsible for the sudden urge to defecate after eating a big meal.