Rigor mortis is the rigidity of the body that occurs after death. It's caused by changes in muscle protein.

As the flow of blood ceases, energy reserves in the muscles are depleted and anaerobic metabolism takes over. This leads to the breakdown of muscle glycogen and the formation of lactic acid in the muscles. The myofibrillar proteins, actin and myosin, lose their extendability, and the muscles become stiff.

Rigor mortis can start from 10 minutes to several hours or even more after death, depending on the condition of the body at death and on factors in the atmosphere, particularly temperature. Rigor mortis starts in the facial musculature and then spreads to other parts of the body. After 24 hours the muscles usually soften again. Otherwise the state of rigor lasts until muscle decomposition takes place by acid formation.