Rigor Mortis – Not a Lucky Stiff

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rigor mortis

When a person or animal dies, it doesn’t take long before the body grows cold and stiff. Cold makes obvious sense, but why stiff? Why does rigor mortis [Latin for stiffness of death] set in?

Chemistry of Rigor Mortis

Rigor mortis results from chemical changes within the muscles – it’s body chemistry. The chemical enabling muscle flexing is ATP (adenosine triphosphate). When breathing ceases, breathing ceases. Lack of oxygen severely diminishes ATP production. If the individual dies, the body begins cooling right away, but muscle stiffening does not set in immediately.

rigor mortis
Adenosine Triphosphate (ATP)

Before R. M. – a Second ATP Production Process

There is a secondary process for producing ATP. It involves anaerobic glycolysis, the breakdown of glucose into lactic acid. Simply written, it is

1 glucose → 2 lactic acid

rigor mortis

but the process includes intermediate steps. These regenerate ATP from ADP (adenosine diphosphate). This source lasts only three or four hours. After that, the lack of ATP, coupled with some calcium cross-bridging, causes the muscles to stiffen.

rigor mortis
Adenosine Diphosphate (ADP)


The delay in onset of rigor mortis is predictable and frequently enables forensic scientists to approximate time of death.

Note: You might also enjoy Components and Functions of Neurons


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