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A corpse sitting up on the slab due to the effects of rigor mortis
Rigor mortis (Latin: rigor "stiffness", mortis "of death") is one of the recognizable signs of death, caused by chemical changes in the muscles after death, causing the limbs of the corpse to become stiff and difficult to move or manipulate. In humans, it commences after about three to four hours, reaches maximum stiffness after 12 hours, and gradually dissipates until approximately 48 to 60 hours after death.
At the time of death, a condition called "primary flaccidity" occurs. Following this, the muscles stiffen in rigor mortis. All muscles in the body are affected. Starting between two and six hours following death, rigor mortis begins with the eyelids, neck, and jaw. The sequence may be due to different lactic acid levels among different muscles, which is directly related to the difference in glycogen levels and different types of muscle fibers. Rigor mortis then spreads to the other muscles within the next four to six hours, including the internal organs. The onset of rigor mortis is affected by the individual's age, sex, physical condition, and muscular build. Rigor mortis may not be perceivable in many infant and child corpses due to their smaller muscle mass.
Rigor mortis is very important in meat technology. The onset of rigor mortis and its resolution partially determines the tenderness of meat. If the postslaughter meat is immediately chilled to 15°C (59°F), a phenomenon known as cold shortening occurs, where the muscle sarcomeres shrink to a third of their original length.