Dead bodies stiffen after death in a process called rigor mortis.
Within a few hours of person's death, the body begins to stiffen. This is due to the loss of a protein in the muscle tissue known as ATP, which allows muscles to contract. This stiffening process is called "rigor mortis."
Features
According to deathonline.net, under normal conditions, rigor mortis sets in approximately three hours after death. The body to reach its maximum stiffness within 12 hours, and then continues to dissipate for another three days.
Function
ATP helps pump calcium from the muscles. Upon death, ATP can no longer be produced, so calcium can no longer be pumped out, which causes the calcium concentration to rise, in turn causing the muscles to remain contracted.
Misconceptions
Rigor mortis is not a permanent condition. According to the website, Ask A Scientist, when muscle proteins begin to degrade, they relinquish their grip and the body will lose its stiffness.
Considerations
According to the book Forensic Pathology, in warmer climates, decomposition begins quickly, meaning that rigor mortis can, in certain cases, set in only for a short time or not at all.
Fun Fact
Violent muscular exertion will often lead to a decrease in ATP levels in certain parts of the body, meaning that, in certain cases, rigor mortis can have a rapid onset. This phenomenon, known as cadaveric spasm, can leave the body posed in bizarre contortions.
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