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Cadaveric spasm, also known as postmortem spasm, instantaneous rigor mortis, cataleptic rigidity, or instantaneous rigidity, is a rare form of muscular stiffening that occurs at the moment of death and persists into the period of rigor mortis. Cadaveric spasm can be distinguished from rigor mortis as the former is a stronger stiffening of the muscles that cannot be easily undone, while rigor mortis can.
Muscles respond to electric stimuli and the muscular reaction is alkaline.
The cause is unknown but is usually associated with violent deaths under extreme physical circumstances with intense emotion, such as the circumstances associated with death via combustion.