Rigor mortis (Latin: Rigor means “stiffness” and Mortis means “of death”) is one of the recognizable signs of death, caused by chemical changes in the muscles after death, causing the limbs of the corpse to stiffen.
After death, respiration in organisms ceases to occur, depleting the oxygen of corpse used in the making of adenosine triphosphate (ATP). As there is a complete loss of ATP, which is required to cause separation of the cross-bridges during relaxation, the myosin heads continue binding with the active sites of actin proteins via adenosine diphosphate (ADP) and the muscle is unable to relax until further enzyme activity degrades the complex.
Normal relaxation would occur by replacing ADP with ATP. However, as it is absent, there must be a breakdown of muscle tissue by enzymes (endogenous or bacterial) during decomposition. As part of the process of decomposition, the myosin heads are degraded by the enzymes, allowing the muscle contraction to release and the body to relax.
বাংলা: রইগর মর্টিস
Reference: wikipedia.org