What is the reason for rigor mortis?

ATP depletion and. Calcium leakage. The skeletal muscles continue using sugar to make atp even after death, till the tissue runs out of sugar and atp. One of the consequences of atp depletion is the formation of abnormal links between two components of muscle tissue, actin and myosin which causes muscle stiffening. Calcium leakage into muscle adds to that stiffening.