Final answer:
Rigor mortis is a postmortem stiffness of muscles due to myosin heads remaining bound to actin without ATP and generally fades after 24 to 48 hours as tissues break down.
Step-by-step explanation:
Rigor mortis, which is the stiffness of the muscles that sets in after death, is a biological phenomenon that occurs when the myosin heads within muscle cells do not detach from actin-binding sites due to a lack of adenosine triphosphate (ATP), the energy currency of the cell. After death, ATP production ceases, and the myosin heads remain bound to actin, causing muscles to become stiff. Typically, rigor mortis sets in within 2 to 6 hours after death, peaks at about 12 hours, and gradually fades after 24 to 48 hours as the body undergoes further postmortem changes, such as the breakdown of muscle tissue.