What is rigor mortis?

• A. Stiffening of muscles due to an excess of ATP
• B. Stiffening of muscles due to lack of ATP
• C. Relaxation of muscles after death
• D. Muscle contraction due to calcium accumulation

Answer: (B)

Rigor mortis refers to the postmortem stiffening of muscles that occurs due to a lack of ATP. After death, cellular respiration ceases, leading to a depletion of ATP in muscle cells. ATP is essential for muscle relaxation because it is required to detach myosin heads from actin filaments in the muscle fibers. When ATP levels drop, the myosin heads remain bound to actin, causing the muscles to become rigid. This stiffness usually develops within a few hours after death and can last for several days as the body continues to break down cellular components.

In contrast, muscle relaxation does not occur postmortem, which negates options that imply relaxation or muscle contraction due to calcium without addressing the underlying energy dynamics. Although calcium may accumulate in the muscles, the key factor in rigor mortis is indeed the absence of ATP, which is central to the mechanism of muscle contraction and relaxation.