What is rigor mortis, and why does it happen?
After death, a series of natural processes begin in the human body. The corpse cools, turns pale and stiffens before beginning to decompose. Stiffening of the corpse is called rigor mortis. It is considered macabre, frightening, and unsettling, and it has been used as a plot device in crime shows, as a scare in horror films, and as a clue in cinema mysteries.
But what exactly is this process, and why does it happen to almost all bodies?
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After death, the body stops producing ATP and its store of the energy molecule is used up within a few hours on average, Jorden said.
The body needs ATP not only to move but also to relax. As the body’s ATP stores become depleted, muscle filaments – proteins linked together in muscle cells – begin to stick together.
A misconception is that the body instantly goes into rigor. Although rigor mortis begins immediately in all muscles after death, in most cases it can take several hours before it becomes visible to the naked eye because the filaments stick together.
Between two and six hours after death, stiffness first appears in the smallest muscles of the face. After six to 12 hours, it progresses to the larger muscles of the hands, arms, neck and chest. Finally, the largest muscles in the lower body stiffen – full-body rigor mortis occurs between 12 and 24 hours after death, completing this sequence known as “rigorous walking“.
Another misconception about rigor mortis is that it is permanent, meaning that once stiffened, the body will remain that way forever. “It’s a transitional phase, not a permanent state,” said Shawn’te Harvelllicensed funeral director in New York and New Jersey and president of the National Funeral Directors & Morticians Association.
Rigor mortis lasts about 24 to 48 hours and then goes away once decomposition beginswhen “muscles become flabby again as breakdown destroys protein structures,” he said. The body becomes limp in the same order in which it stiffened – starting with the face and hands, moving through the neck and torso, then the legs.
This timeline is a guideline, Harvell said, and the process is unique for each agency. Factors such as temperature, age and activity level at the time of death; general health and body composition; and medications taken over the course of life affect how quickly or slowly the body goes through rigor mortis before decomposition sets in. “No two cases follow the exact same timeline or intense pattern,” he said.
Jorden, a practicing forensic pathologist and neuropathologist, said determining the timing of rigor mortis can be “a very valuable tool” in assessing the scene of a death.
“[It] “can actually provide clues as to whether the body was handled or moved,” she said. Additionally, if the timing of rigor mortis doesn’t quite match what they think the circumstances of death were, that may be a clue that “causes us to pause and ask more questions.”
