ATP is required for the contraction of a sarcomere and thus a muscle fiber. At death, the production of ATP ceases and the ATP stores in a cell are quickly used up. The cell's membranes become permeable to calcium and calcium enters the cytosol of the cell and attaches to troponin, removing tropomyosin from the actin active sites. Myosin attaches to the actin active sites and the muscle contracts for several cycles until the ATP stores are depleted. The muscles are then unable to relax. This postmortem contraction of the muscles is called rigor mortis. Rigor mortis is ultimately caused by a lack of ATP that would, if present, (1) allow calcium to be pumped out of the cytosol and into the sarcoplasmic reticulum and (2) .