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Rigor mortis is one of the recognizable signs of death (Latin mors, mortis) that is caused by a chemical change in the muscles after death, causing the limbs of the corpse to become stiff (Latin rigor) and difficult to move or manipulate. Assuming mild temperatures, rigor usually sets in about 3-4 hours after clinical death^{[citation needed]}, with full rigor being in effect at about 12 hours, and eventually subsiding to relaxation at about 36 hours. Times for the onset of rigor mortis can vary from a few minutes to several hours depending on the ambient temperature. Factors influencing rigor mortis include the age and condition of the body, as well as the mode of death and the surroundings. For example, rigor mortis will tend to set in faster in those who were active immediately prior to death.

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 to form actomyosin complex, causing muscles to become rigid. After the pH of the muscle has become 5.5, release of autolytic enzymes stored in lysosomes will take place. The major proteolytic enzymes are Cathepsins and Calpains. These enzymes act at the myofibrillar proteins and hydrolyse them. As a result, the actomyosin complex is broken down and muscles become "soft" again. This is known as resolution of rigor.

Rigor Mortis is important in forensics as it allows the pathologist to estimate time of death and to determine position of the body at time of death. Conditions similar to rigor mortis include freezing of the body, heat coagulation, putrefaction and cadaveric spasm.

Rigor mortis is very important in meat technology. The onset of rigor mortis and its resolution determines the tenderness of meat. If the post-slaughter meat is immediately chilled to 15°C, a phenomenon known as cold shortening occurs, where the muscle is shrunken to a third of its original size. This will lead to the loss of water from the meat along with many of the vitamins, minerals, and water soluble proteins. The loss of water makes the meat hard and interferes with the manufacturing of several meat products like cutlet and sausage.

Cold shortening is caused by the release of stored calcium ions from the sarcoplasmic reticulum of muscle fibers in response to the cold stimulus. The calcium ions trigger powerful muscle contraction aided by ATP molecules. To prevent cold shortening, a process known as electrical stimulation is carried out, especially in beef carcass, immediately after slaughter and flaying. In this process, the carcass is stimulated with alternating current, causing it to contract and relax, which depletes the ATP reserve from the carcass and prevents cold shortening.

• Cadaveric spasm