Rigor mortis or postmortem rigidity, is the fourth stage of death.
It is one of the recognizable signs of death.
RM is characterized by stiffening of the limbs of a corpse caused by chemical changes, mainly calcium, in the muscles postmortem.
Rigor mortis can occur as soon as four hours after death.
Rigor mortis is not permanent and begins to pass within hours of onset.
Typically, RM lasts no longer than eight hours at room temperature.
With death, aerobic respiration in an organism ceases, depleting the source of oxygen used to make adenosine triphosphate (ATP).
ATP is required to cause separation of the actin-myosin cross-bridges during relaxation of muscle.
When oxygen is no longer present, the body continues to produce ATP via anaerobic glycolysis, but when the body’s glycogen is depleted, the ATP concentration diminishes, and the body enters rigor mortis because it is unable to break the actin-myosin bridges.
Calcium enters the cytosol after death due to the deterioration of the sarcoplasmic reticulum.
The calcium activates the formation of actin-myosin cross-bridging, binding to the troponin of thin filaments, which causes the troponin-tropomyosin complex to change shape and allow the myosin heads to bind to the active sites of actin proteins.
In rigor mortis myosin heads continue binding with the active sites of actin proteins via adenosine diphosphate (ADP), and the muscle is unable to relax until further enzyme activity degrades the complex.
Normally relaxation occurs by replacing ADP with ATP, which would destabilize the myosin-actin bond and break the cross-bridge.
As ATP decreases with death, there is a breakdown of muscle tissue by enzymes occur as the the myosin heads are degraded and decomposed, allowing the muscle contraction to release and the body to relax.
The peak of rigor mortis occurs approximately 13 hours after death, and the decomposition of the myofilaments occurs between 48 and 60 hours after
The degree of rigor mortis may be used in forensic pathology to determine the approximate time of death.
Rigor mortis is transient evidence of the degree to which it is affects a body that degrades over time.