Which process causes muscles to become rigid after death?

• A. Optical
• B. Cartilage
• C. Remains
• D. Rigor Mortis

Answer: (D)

Rigor mortis is the postmortem stiffening of muscles caused by a chemical change in the muscle fibers after death. In living muscle, contraction happens when calcium is released to expose binding sites on actin, myosin heads attach, and ATP provides the energy to detach the myosin from actin after the power stroke. After death, respiration and circulation stop, so ATP production ceases. Without ATP, the cross-bridges formed between actin and myosin cannot detach, causing muscles to lock in a contracted state. This stiffness typically appears a few hours after death, progresses to involve most or all of the body, and then gradually dissipates as muscle proteins break down during decomposition.

Other options don’t describe this process: optical has no relation to muscle chemistry, cartilage is a different tissue, and remains refers to the body after death rather than a process that causes rigidity.