Rigor mortis onset timing: which statement is correct?

• A. It begins immediately at death due to calcium influx.
• B. It is caused by the accumulation of ATP.
• C. It is prevented by respiration after death.
• D. It begins several hours after death due to ATP depletion causing myosin heads to remain bound to actin.

Answer: (D)

Rigor mortis occurs because the cell’s energy source stops after death, so the muscles can no longer relax. In life, ATP binds to myosin and causes the myosin head to detach from actin after a power stroke. After death, ATP production ceases, so the myosin–actin cross-bridges can form but cannot be released. Calcium leakage after death further promotes contraction, speeding stiffening, but the crucial point is that ATP is depleted, not present to allow detachment. This leads to stiffening that begins several hours after death, not immediately. Respiration after death cannot prevent this, since energy production has stopped. So the onset timing—beginning hours after death due to ATP depletion making myosin heads stay bound to actin—best explains rigor mortis.