Rigor mortis (Latin: rigor "stiffness", mortis "of death") is one of the recognizable signs of death, caused by chemical changes in the muscles after death, causing the limbs of the corpse to become stiff and difficult to move or manipulate. In humans, it commences after about three to four hours, reaches maximum stiffness after 12 hours, and gradually dissipates until approximately 24 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) allowing the corpse to harden and become stiff. ATP is no longer provided to operate the SERCA pumps in the membrane of the sarcoplasmic reticulum, which pump calcium ions into the terminal cisternae. This causes calcium ions to diffuse from the area of higher concentration (in the terminal cisternae and extracellular fluid) to an area of lower concentration (in the sarcomere), binding with troponin and allowing for crossbridging to occur between myosin and actin proteins.