rigor mortis (rĭˈgər môrˈtĭs) [key], rigidity of the body that occurs after death. The onset may vary from about 10 min to several hours or more after death, depending on the condition of the body at death and on factors in the atmosphere, particularly temperature. Rigor mortis affects the facial musculature first and then spreads to other parts of the body. It is caused by chemical changes in the muscle tissue. The state of rigor usually lasts about 24 hours or until muscle decomposition takes place by acid formation.