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.
Before Rigor Mortis – 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
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.
The delay in onset of rigor mortis is predictable and frequently enables forensic scientists to approximate time of death.
- San Diego State University: Anaerobic Glycolysis
- U. of Washington: Structure and Function of Skeletal Muscle