Rigor mortis occurs because

1) no atp is available to release attached actin and myosin molecules
2) the cells are dead
3) sodium ions leak into the muscle causing continued contractions
4) proteins are beginning to break down, thus preventing a flow of calcium ions

No ATP is available to release attached actin and myosin molecules.

Explanation:

Two to six hours later after death, all skeletal muscles of the body become rigid and stiff. This situation is defined as Rigor mortis. It occurs from the lack of ATP along with the production of lactic acid. When breathing stopped permanently in the body, oxygen level also reduced; that’s why body muscles cannot go for any aerobic respiration to produce any ATP. 

As we know already, during every muscle construction, ATP needs to break the cross-bridge between Myosin and Actin. So if there is no ATP produced in the body, separation of Myosin molecules from Actin molecules cannot happen anymore. This situation leads for rigor mortis.

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