Rigor mortis

Rigor mortis is a recognizable sign of human 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 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.