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Cadaveric spasm, also known as postmortem spasm, instantaneous rigor, cataleptic rigidity, or instantaneous rigidity, is a rare form of muscular stiffening that occurs at the moment of death, persists into the period of rigor mortis and can be mistaken for rigor mortis. The cause is unknown, but is usually associated with violent deaths happening under extremely physical circumstances with intense emotion.
Cadaveric spasm may affect all muscles in the body, but typically only groups, such as the forearms, or hands. Cadaveric spasm is seen in cases of drowning victims when grass, weeds, roots or other materials are clutched, and provides proof of life at the time of entry into the water. Cadaveric spasm often crystallizes the last activity one did prior to death and is therefore significant in forensic investigations, e.g. holding onto a knife tightly.
ATP is required to reuptake calcium into the sarcomere's sarcoplasmic reticulum (SR). When a muscle is relaxed, the myosin heads are returned to their "high energy" position, ready and waiting for a binding site on the actin filament to become available. Because there is no ATP available, previously released calcium ions cannot return to the SR. These leftover calcium ions move around inside the sarcomere and may eventually find their way to a binding site on the thin filament's regulatory protein. Since the myosin head is already ready to bind, no additional ATP expenditure is required and the sarcomere contracts.
When this process occurs on a larger scale, the stiffening associated with rigor mortis can occur. It mainly occurs during high ATP use. Sometimes, cadaveric spasms can be associated with erotic asphyxiation resulting in death.