rigor mortis Shortly after death all the muscles in the body become soft and flaccid. At a variable time later, they become firm and rigid. This is known as rigor mortis. Rigor commences in the smallest muscles such as those in the face and the hands, and then extends to the limb muscles. Rigor can be ‘broken’ by stretching the muscle, for example by moving the jaw or the elbow, and does not then return.

Rigor is brought about by a chemical change in the muscle. The normal reaction between adenosine triphosphate and adenosine diphosphate (ATP and ADP) within the muscle fibres, which supplies energy for their contraction during life, ceases and the ATP level in the muscle progressively diminishes. This is accompanied by accumulation of lactic acid and a fall of pH (increase in acidity), which leads to stiffening and firmness. Whether or not the muscle fibres actually shorten has not been established.

Temperature is an important factor in determining the time of onset of rigor. In normal circumstances and at room temperature rigor is complete in about three to six hours. If the temperature is higher the onset is more rapid — perhaps no more than an hour in tropical temperatures. Conversely, the onset of rigor is delayed at low temperatures. In cases of drowning in cold water, for example, rigor may not appear until the body has been removed from the water, even after several days of immersion. The onset of rigor is hastened if there has been intense physical activity shortly before death. Thus, in forensic medical practice, the presence of rigor is a poor determinant of the time of death. Once established, the duration of rigor ranges from 18 to 36 hours.

J. Hume Adams

Rigor Mortis

Rigor mortis, from the Latin for "stiffness of death" is the rigidity that develops in a body after death. This rigidity may begin shortly after death—within 10–15 minutes—or may not begin until several hours later, depending on the condition of the body at the time of death and on environmental factors, such as moisture content of the air and particularly temperature. A colder temperature promotes a slower onset of rigor mortis.

Knowledge of the progression of rigor mortis can be very useful for a forensic investigator in a determination of the time that has lapsed since death.

Typically, rigor mortis affects facial muscles first. Spreading to other parts of the body follows. The body will remain fixed in the rigid position until decomposition of tissue begins, about 24–48 hours after death.

Rigor mortis occurs because metabolism continues in muscles for a short while after death. As part of the metabolic activity, adenosine triphosphate (ATP) is produced from the metabolism of a sugar compound called glycogen. ATP is a principal energy source for muscular activity. As long as ATP is present, muscles continue to maintain their tone. As the store of glycogen is exhausted, ATP can no longer be made and its concentration decreases.

One of the consequences of ATP depletion is the formation of abnormal links between two components of muscle tissue, actin and myosin. The leakage of calcium into the muscle cells also contributes to the formation of abnormal actin-myosin links. The abnormality produces the stiffening of the muscle, which persists until the links are decomposed.

see also Autopsy; Coroner; Fluids; Death, mechanism of.

rigor mortis (rĬ´gər môr´tĬs), 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.

rigor mortis Stiffening of muscle that occurs after death. As the flow of blood ceases, anaerobic metabolism leads to the formation of lactic acid and the soft, pliable muscle becomes stiff and rigid. If meat is hung in a cool place for a few days (‘conditioned’), the meat softens again. Fish similarly undergo rigor mortis but it is usually of shorter duration than in mammals. See also DFD meat; meat conditioning.

rigor mortis The stiffening of the body of an animal after death, due to a temporary rigidity of the muscles. This condition arises because ATP, which is no longer synthesized after death, is required to break down the bridges that form between actin and myosin filaments in muscle tissue during contraction.

rig·or mor·tis / ˌrigər ˈmôrtəs/ • n. Med. stiffening of the joints and muscles of a body a few hours after death, usually lasting from one to four days.

rigor mortis Stiffening of the body after death brought about by chemical changes in muscle tissue. Onset is gradual from minutes to hours, and it disappears within about 24 hours.

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