Why does rigor mortis occur soon after death?

As a forensic expert with extensive knowledge in the field of post-mortem changes, I can explain the process of rigor mortis in detail. Rigor mortis is the stiffening of the body after death due to the depletion of adenosine triphosphate (ATP) in the muscles. ATP is the primary energy source for muscle contraction. When a person dies, the body stops producing ATP, which leads to the inability of the muscles to relax. The myosin, a protein filament in muscle fibers, remains attached to actin, another protein filament, causing the muscles to stay contracted. This is the fundamental biological mechanism behind rigor mortis. The onset of rigor mortis typically begins within a few hours after death and is first noticeable in the smaller muscles of the face and jaw. It then progresses to the larger muscle groups throughout the body. The process can take anywhere from 6 to 12 hours to affect the entire body, and the rigor can last for 24 to 72 hours before the muscles begin to relax again. It's important to note that the timing of rigor mortis can be influenced by various factors, including the individual's physical condition, the temperature of the environment, and the pH level in the muscles at the time of death.

The muscle contracts when the myosin shifts, but the lack of ATP prevents it from detaching, and the muscle remains contracted. Such a process occurs in all muscles as the body becomes rigid. Rigor mortis usually sets in within four hours, first in the face and generally smaller muscles.