What is rigor mortis?

• A. Rigor mortis is the postmortem stiffening of muscles due to calcium influx and lack of ATP preventing cross‑bridge detachment.
• B. Muscle fatigue after intense exercise.
• C. A type of muscle spasm during sleep.
• D. Loss of muscle tone with aging.

Answer: (A)

Rigor mortis is the postmortem stiffening of muscles that occurs after death. After death, calcium leaks into the muscle cytoplasm and cross-bridges form between actin and myosin, but there’s no ATP left to detach them. In living muscle, ATP is needed to release these cross-bridges after contraction; without ATP, the links stay in place, locking the muscles in a rigid state. This stiffness typically begins a few hours after death, peaks as energy stores are depleted, and then passes as proteins degrade. It isn’t muscle fatigue from exercise, a sleep spasm, or a gradual loss of tone with aging.