What is Rigor mortis?
{{Signs of death}} '''Rigor mortis''', also called postmortem rigidity, happens after a [[person]] or [[animal]] [[death|dies]]. It's a sign that the body is no longer alive. During rigor mortis, the body becomes stiff and hard to move. This happens because some [[chemical|chemicals]] in the [[muscle|muscles]] change after death, especially [[calcium]]. In humans, rigor mortis can start as soon as four hours after death. But it doesn't last forever. After a few hours, the stiffness starts to go away. Usually, rigor mortis lasts no more than eight hours when the body is at [[room temperature]]. Some people think rigor mortis is permanent, but that's not true. The stiffness gradually goes away as [[time]] goes on after death. == Body Processes == When a living [[organism]] dies, it stops using [[oxygen]] to produce [[energy]]. This leads to a decrease in a [[molecule]] called adenosine triphosphate (ATP), which helps muscles relax. Without oxygen, the body tries to make ATP using a different method called anaerobic glycolysis. But when the body runs out of a [[substance]] called [[glycogen]], ATP levels drop, and the muscles become stiff in a condition called rigor mortis. During rigor mortis, the muscles can't relax because certain structures in the muscles can't separate. After death, calcium enters the fluid inside [[cell|cells]], called [[cytosol]]. This happens because certain parts of the cells break down. Calcium causes the formation of structures called actin-myosin cross-bridges, which are important for muscle movement. In rigor mortis, the myosin heads in the muscles keep binding to actin proteins using a molecule called adenosine diphosphate (ADP). The muscles can't relax until enzymes break down this binding. Normally, relaxation happens when ATP replaces ADP, breaking the connection between myosin and actin. But since there is no ATP after death, enzymes (from the body or bacteria) break down the muscle tissue during [[decomposition]]. As the muscle tissue breaks down, the myosin heads degrade, r