Q.Skeletal muscles become stiff and rigid soon after death, a state called rigor mortis. This happens mainly because

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Rigor mortis sets in because, without fresh ATP after death, myosin heads cannot detach from actin. …

Binding of a new ATP molecule to a myosin head is what causes that head to detach from actin at the end of a power stroke, in the normal cross-bridge cycle described in section 18.4. Once cellular respiration stops after death, no fresh ATP is produced anywhere in the body's muscle fibres. Any myosin heads cross-bridged to actin at that moment therefore cannot let go, and stay locked in place. With this happening simultaneously in every sarcomere of every skeletal muscle, the …

Recall that ATP-binding (not ATP hydrolysis) detaches the myosin head from actin; without any fresh ATP after death, that detach …

Assuming rigor mortis is caused by a build-up of calcium -- it is actually caused by the absence of fresh ATP, which is neede …