Rigidity and stiffness of the muscles associated with death. Onset varies with temperature, usually setting in 3-4 hours following death, becoming fully complete at about 12 hours and lasting up to 72 hours postmortem.
Following death, the cellular mechanisms that exclude calcium from the intracellular space cease. Influx of calcium causes actin and myosin to associate. Normal contraction of muscles requires cyclical breaking and reforming of the actin-myosin bridges, an ATP-dependent process. In the absence of ATP, the actin-myosin interactions become fixed, manifesting as stiffness until further biochemical decomposition intervenes. The process is accelerated in warm environments and retarded by cold.
If ambient temperature is known, it can be used to estimate time of death.
An approximate on the spot guide is:
- If body is warm and flaccid death was less than 3 hours ago
- if body is warm and stiff death was 3-8 hours ago
- If body is cold and stiff death was 8-36 hours ago
- If body is cold and flaccid death was more than 36 hours ago
From a more practical point of view, patients who are recently deceased should have their eyes closed, heads straightened and the rest of their body laid reasonably flat. Some departments will even put a bandage over the head and round the bottom of the jaw to ensure the mouth remains closed. This will ensure that body looks more peaceful.