What causes Abdominal rigidity?

Abdominal rigidity, a hard board-like belly you cannot relax, is most often caused by peritonitis: inflammation of the lining of the abdomen from a perforated ulcer, a burst appendix, diverticulitis, severe pancreatitis, bowel injury, or a ruptured ectopic pregnancy.

True rigidity is involuntary, so the muscles stay tight even when you try to relax them and even when no one is touching you. That is different from guarding, where you tense up only when someone presses on a tender spot, which happens with many milder causes of stomach pain. In people with liver disease and fluid in the abdomen, infection of that fluid can produce the same picture. Blunt trauma to the belly, such as from a crash or a fall, can cause bleeding that stiffens the abdominal wall over minutes to hours. Rigidity with fever, vomiting, a fast heartbeat, or pain that gets worse when you move or cough needs emergency evaluation the same day, not watchful waiting at home.