Rigor mortis refers to the stiffness of the body after the death of a person. Rigor mortis is because of the biochemical change in the muscles which occurs a few hours after the death, although the time of its occurrence after the death totally depends on the ambient temperature. The biochemical basis of the rigor mortis is the hydrolysis in the muscle of ATP, and the energy source that is required for movement. Without the ATP, the myosin molecules tend to adhere to the actin filaments and the muscles thereby become rigid.