Medical Definition of Rigor mortis
Rigor mortis: Literally, the stiffness of death. The rigidity of a body after death. Rigor mortis is a good example of a Latin term (one in this case that was coined in the 19th century) remaining intact in contemporary medical usage (and crime writing).
Rigor mortis is due to a biochemical change in the muscles that occurs several hours after death, though the time of its onset after death depends on the ambient temperature. The biochemical basis of rigor mortis is hydrolysis in muscle of ATP, the energy source required for movement. Without ATP, myosin molecules adhere to actin filaments and the muscles become rigid.
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