What Causes Rigor Mortis?Esther Inglis-Arkell1/30/15 3:05pm204EditPromoteGo to permalinkWhen you're dead, you stiffen up. We all know this. But why? What is the process that goes on, inside a dead body, that causes muscle tissue to contract so tightly that the limbs can be impossible to move? We'll take you into the wonderful world of rigor mortis.