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What causes rigor mortis after death?

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surgeon; American Civil War — embalming An embalming surgeon at work on a soldier's body during the American Civil War.

Rigor mortis is a physiological process caused by the cessation of breathing and oxygen intake following death, which terminates aerobic respiration and the cellular production of adenosine triphosphate (ATP).12

In living muscle, ATP is required for the myosin filaments to release from actin filaments, allowing muscles to relax. After death, the depletion of ATP prevents this dissociation, causing the filaments to remain locked in a contracted, stiffened state.12

Additional factors contributing to the onset and duration of this stiffness include:
* Lactic acid accumulation: As muscle cells switch to anaerobic metabolism, lactic acid builds up, decreasing pH levels.2
* Calcium leakage: Postmortem leakage of calcium ions into the sarcomere binds to muscle filaments, reinforcing their continued contraction.2

Rigor mortis typically begins in smaller muscles, such as those in the face and hands, within two to six hours after death, eventually affecting the entire body as ATP is steadily depleted.12