What causes your muscles to become stiff after death, or rigor mortis?

• A. Oxygen depletion after death
• B. Increased body temperature after death
• C. The loss of ATP which lasts for about 3 hours after death
• D. Formation of new ATP after death

Answer: (C)

Rigor mortis happens because ATP is no longer available to detach the myosin heads from actin after death. In living muscle, contracting events are followed by relaxation when ATP binds to myosin and releases it from actin. After death, cells stop producing ATP, calcium remains elevated, and the cross-bridges between actin and myosin remain locked. Without ATP to disengage them, the muscles stay in a contracted state, causing stiffness that appears a few hours later and lasts until the muscle proteins are broken down. Oxygen depletion and temperature can influence how quickly this process occurs, but they don’t cause the stiffness itself, and there is no new ATP being formed after death.