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Rigor mortis is the stiffening of all muscles in the body following death. In humans, this begins approximately three to four hours after death, with maximum stiffness setting in around 12 hours after death.
After death, cellular respiration in organisms ceases to occur, depleting the corpse of oxygen used in the making of adenosine triphosphate (ATP). ATP is no longer provided to operate the SERCA pumps in the membrane of the sarcoplasmic reticulum; these pumps move calcium ions into the terminal cisternae. Physiologically, rigor mortis is caused a release of calcium facilitating crossbridges in the sarcomeres; the coupling between myosin and actin cannot be broken, creating a constant state of muscle contraction until enzymatic decomposition eventually removes the crossbridges. This release of calcium is caused by the loss of ATP-mediated function of calcium pumps of the sarcoplasmic reticulum, due to ATP depletion in the absence of cellular respiration. Diffusion of the calcium through the pumps occurs, facilitation binding of myosin and actin filaments. This cycle of the sliding filament mechanism cannot be completed, so muscles maintain ridigity until enzymatic degradation of muscle fibers.
Unlike muscular contractions during life, the body after death is unable to complete the cycle and release the coupling between the myosin and actin, creating a state of muscular contraction until the breakdown of muscle tissue by enzymes (endogenous or bacterial) during decomposition . As part of the process of decomposition, the myosin heads are degraded by the enzymes, allowing the muscle contraction to release and the body to relax.