What is rigor mortis?

In rigor mortis, the muscles of the body stiffen in response to biochemical changes in the cell. Once blood supply to a cell stops it loses its ability to produce ATP. ATP is used twice in every cross- bridge cycle – once to rotate the myosin head attached to actin and once to break the connection and return the myosin head to its original position. Rigor mortis develops when insufficient ATP is available to allow the cross bridges to be broken. In other words, the muscle stays in a contracted state. Rigor mortis doesn’t develop immediately because anaerobic metabolism can continue to generate ATP for a time after blood supply to the tissues stops. But eventually, energy stores are depleted and ATP production stops. So rigor mortis is due to cross bridges in muscle fibres, but why do cross bridges form in dying cells? The answer appears to be that calcium leaks into the cells enabling cross bridge formation to occur. Eventually, early decomposition of the muscle ends rigor mortis because the contractile filaments start to break down.