Rigor mortis
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Rigor mortis is a recognizable sign of death that is caused by a chemical change in the muscles, causing the limbs of the corpse to become stiff ("rigor") and impossible to move or manipulate. Typically rigor sets in several hours after clinical death and subsides spontaneously in about two days, though the time of its onset and duration depends on ambient temperature. The average, assuming mild temperatures, is about 3-4 hours post-death for onset, with full rigor being in effect at about 12 hours, and eventually subsiding to relaxation at about 36 hours.
The biochemical cause of rigor mortis is hydrolysis of ATP in the muscle tissue, the chemical energy source required for movement. Myosin molecules devoid of ATP become permanently adherent to actin filaments and muscles become rigid.
The signs of death are usually noted as algor mortis, rigor mortis, livor mortis, and decomposition.