What is rigor mortis and when does it occur?

• A. Postmortem stiffening due to ATP accumulation causing continuous detachment.
• B. Postmortem stiffening due to ATP depletion causing myosin heads to remain bound to actin; begins several hours after death.
• C. Temporary relaxation of muscles after death due to calcium sequestration.
• D. Delayed stiffening due to protein cross-linking that begins days after death.

Answer: (B)

Rigor mortis is the postmortem stiffening of skeletal muscles caused by loss of ATP after death. In living muscle, ATP binds to myosin to detach the myosin heads from actin and allow relaxation; when death occurs, ATP stores are depleted, so the cross-bridges remain locked between myosin and actin, creating stiffness. Calcium that entered the cytosol after death promotes contraction, but without ATP to reset the cross-bridges, the muscles stay rigid. The timing fits what you’d expect: stiffness typically begins several hours after death (often around 2–6 hours), peaks around about half a day, and then wears off over the next day or two as proteins break down and autolysis occurs. The other ideas don’t fit because rigor mortis hinges on ATP depletion, not accumulation, and the described timing and mechanism don’t match how stiffness develops and resolves after death.