Rigor Mortis and Other Postmortem Changes

Once the heart stops beating, blood collects in the most dependent parts of the body (livor mortis), the body stiffens (rigor mortis), and the body begins to cool (algor mortis).

The blood begins to settle in the parts of the body that are the closest to the ground, usually the buttocks and back when a corpse is supine. The skin, normally pink-colored because of the oxygen-laden blood in the capillaries, becomes pale as the blood drains into the larger veins. Within minutes to hours after death, the skin is discolored by livor mortis, or what embalmers call "postmortem stain," the purple-red discoloration from blood accumulating in the lowermost (dependent) blood vessels. Immediately after death, the blood is "unfixed" and will move to other body parts if the body's position is changed. After a few hours, the pooled blood becomes "fixed" and will not move. Pressing on an area of discoloration can determine this; if it blanches (turns white) easily, then the blood remains unfixed. Livor mortis is usually most pronounced eight to twelve hours after death. The skin, no longer under muscular control, succumbs to gravity, forming new shapes and accentuating prominent bones still further. The body then begins to cool.

At the moment of death, the muscles relax completely—a condition called "primary flaccidity." The muscles then stiffen, perhaps due to coagulation of muscle proteins or a shift in the muscle's energy containers (ATP-ADP), into a condition known as rigor mortis. All of the body's muscles are affected. Rigor mortis begins within two to six hours of death, starting with the eyelids, neck, and jaw. This sequence may be due to the difference in lactic acid levels among different muscles, which corresponds to the difference in glycogen levels and to the different types of muscle fibers. Over the next four to six hours, rigor mortis spreads to the other muscles, including those in the internal organs such as the heart. The onset of rigor mortis is more rapid if the environment is cold and if the decedent had performed hard physical work just before death. Its onset also varies with the individual's age, sex, physical condition, and muscular build.

After being in this rigid condition for twenty-four to eighty-four hours, the muscles relax and secondary laxity (flaccidity) develops, usually in the same order as it began (see Table 1). The length of time rigor mortis lasts depends on multiple factors, particularly the ambient temperature. The degree of rigor mortis can be determined by checking both the finger joints and the larger joints and ranking their degree of stiffness on a one- to three- or four-point scale. Many infant and child corpses will not exhibit perceptible rigor mortis. This decreased perceptible stiffness may be due to their smaller muscle mass.

During this period, the body gradually cools in a process called algor mortis. The best way to accurately assess a corpse's temperature is with a core (tympanic membrane, liver, or rectal) thermometer. Rectal insertion may be difficult and cause postmortem injury.

A few adult corpses may not undergo perceptible rigor mortis. Folklore in Britain, the Philippines, and elsewhere ascribed fearsome supernatural powers to these "limber corpses."

In the early nineteenth century, the American and British poor often prepared their own dead for burial in a process called "laying-out," "streeking," or rendering the "last offices." Women normally washed the corpse, plugged its orifices, closed its eyes and mouth, straightened its limbs, and dressed or shrouded it. It was ritually important to close the eyes quickly, being that they are the first to rigidify in rigor mortis, and it was thought that a corpse with open eyes posed a threat to its kin. As has long been the case in many cultures, they used coins to keep the corpse's eyes closed. The practice of using coins endures, representing a feeling that money, so important in life, may also be important in death.

Approximate times for algor and rigor mortis in temperate regions Body temperature Body stiffness Time since death warm not stiff dead not more than three hours warm stiff dead 3 to 8 hours cold stiff dead 8 to 36 hours cold not stiff dead more than 36 hours SOURCE: Stærkeby, M. "What Happens after Death?" In the University of Oslo Forensic Entomology [web site]. Available from http://folk.uio.no/mostarke/forens_ent/afterdeath.shtml.

In the embalming process, embalmers first position the body. They then relieve rigor mortis by flexing, bending, and massaging the arms and legs. They then move the limbs to a suitable position, usually with legs extended and arms at the sides or hanging over the sides of the table, so that blood can drain into and expand the vessels for better embalming. They need to position the body before injecting embalming fluid, for no matter what stage of rigor mortis a body is in, once embalming fluid is injected, the muscles begin to firm up, or "set." (Without embalming, they would become flaccid over a period of hours.) After embalming, the muscles harden gradually over the next eight to twelve hours. Once they are set, embalmers cannot alter the body's position.