|Signs of death|
Rigor mortis, also called postmortem rigidity, happens after a person or animal 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 chemicals in the 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 hour when the body is at room temperature.
Some people think rigor mortis is permanent, but it's not true. The stiffness gradually goes away as time goes on after death.
Body Processes change
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 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, releasing the muscle contraction and allowing the body to relax.
The breakdown of muscle fibers during decomposition happens between 48 and 60 hours after the peak of rigor mortis, which usually happens around 13 hours after death.