Hi nice question.
Rigor Mortis is an process that occurs in muscles. After death of an living being.
During rigor mortis the Calcium channels are opened and contraction happens in muscles.
But to undergo relaxation ATP is not available in enough quantitiies due to no breathing (respiration).
These days rigor mortis is applied in the Meat industry to prevent rigor mortis and keep the muscles straighten and uncontracted by freezing the meat at 15-5oC