Body in full Rigor Mortis. Rigor mortis is a gradual stiffening of the body’s musculature which occurs after death. It does not involve any significant shortening of the muscle fibers. It is not a permanent change; under usual conditions, rigor mortis disappears by 48 hours after death. It is believed to result from biochemical changes in the muscle cells. During life and shortly after death, the cytoplasm of the muscle cell is in a liquid state. With increasing time after death, the cytoplasm is converted to a gel (solid) state — hence the rigidity associated with rigor mortis.