rigor mortis

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

See also corpse; death.