What muscle is used when going on the toilet?

You know how when you’re pooping and straining a little while waiting for latecomers to the departure lounge there’s like abdominal strain happening to bear down, as it were. What muscle(s) is this? Is it the diaphragm pushing down? Obliques pulling in?

http://en.wikipedia.org/wiki/Sphincter

http://en.wikipedia.org/wiki/Sphincter_ani_internus_muscle

from the wikithing article on defecation:
Defecation is normally assisted by taking a deep breath and trying to expel this air against a closed glottis (Valsalva maneuver). This contraction of expiratory chest muscles, diaphragm, abdominal wall muscles, and pelvic diaphragm exerts pressure on the digestive tract. Ventilation at this point temporarily ceases as the lungs push the chest diaphragm down to exert the pressure. Thoracic blood pressure rises and as a reflex response the amount of blood pumped by the heart decreases. Death has been known to occur in cases where defecation causes the blood pressure to rise enough to cause the rupture of an aneurysm or to dislodge blood clots (see thrombosis).

I guess if you take the phrase “muscle you use” to mean (or include) “muscle you relax”, true.

eschereal already discussed in another post the rather large set of abdominal muscles (including the diaphragm) that are tensed to pressurize the bowels and help empty them.

The other muscle (or group of muscle tissue) involved is the actual Muscular Layer of the digestive tract, the external layer of which is responsible for peristalsis: the squeeze-and-release wave that pushes the contents of the digestive tract from entry to exit.

Isn’t this how Elvis died?

The Valsalva maneuver is also used by some with spinal cord injuries who have flaccid bladders to void their bladders. The Crede method is another technique used to manually push on the bladder to contract it and get it to empty. These techniques are used in place of, or sometimes in addition to, catheters. They are usually seen, by most experts at least, as poor long term alternatives for catheterization.

However, catheterization is the number one cause of urinary tract infection for those with spinal cord injury, as well as a number of other chronic health issues. I have used the Crede and Valsalva techniques excusively for the 14+ years that I have been spinal cord injured and I have suffered no ill effects. Not a single UTI either. Plus I have full sensation, so the idea of a tube getting stuffed down my urethra every time I have to piss is kinda unimaginable to me. /hijack