A few house after a person dies the joints of a dead body stiffen and become looked in place. Rigor mortis is caused by chemical changes in the body after death due to oxygen depletion and the buildup of calcium. When a person is alive the calcium in a body is carried outside the body's cells. This allows muscle fibers to move. But since a dead body is no longer receiving oxygen,calcium builds up causing the muscles to stiffen up and contract.