Rigor mortis is the most recognisable sign of death where the corpse becomes stiff and is difficult to move or manipulate.
What is Rigor Mortis?
Rigor mortis is the most recognisable sign of death where the corpse becomes stiff and is difficult to move or manipulate. It is derived from the Latin words: rigor which means stiffness and mortis which means death. In humans, it typically commences after about three to four hours and reaches maximum stiffness after 12 hours. It totaly dissipates after 48 to 60 hours after death.
After death, cellular respiration ceases, depleting the corpse of oxygen and the body stiffens due to chemical reactions. Rigor Mortis is extremely important in forensic sciences. A body goes stiff in the position it was in when the person dies. If the position in which a body is found does not match where it is found. For example, if it is flat on its back with one leg up, that could mean someone moved it.
It also helps investigators determine the time of death. However, rigor mortis slows down in cold temperatures. When conditions are warm, the onset and pace of rigor mortis are sped up by providing a conducive environment for the metabolic processes that cause decay. Therefore, a person who dies outside in freezing temperatures may experience rigor mortis over several days more than normal. So investigators may have to abandon it as a tool for determining time of death.
What causes Rigor Mortis?
Rigor mortis is the reason why the word "stiff" is a slang term for a dead body. Two or three hours after a person or animal dies, the muscles start to stiffen. This phenomenon progresses in a downward, head-to-toe direction. In 12 to 18 hours the body is, as the saying goes, stiff as a board. At this stage, you can move the joints only by force, breaking them in the process.
It takes about two days for rigor mortis to fade, and once it does, decay sets in. If the body isn't embalmed or cooled to 38 degrees Fahrenheit (3.3 degrees Celsius) or below, it will quickly decompose.
To people who work in mortuaries, rigor is an unimportant, temporary condition. It may require them to massage the deceased's extremities to reduce stiffness and allow for easier, more effective embalming. But to police, medical examiners and lawyers in the criminal justice system, rigor mortis has much more significance. It's a clue to understanding the circumstances of someone's unexpected -- and possibly violent -- death. Rigor mortis is a piece of the forensic jigsaw puzzle, and combined with other details, it can help detectives and medical examiners figure out what happened.
But what turns flexible joints into immovable structures, and why does the process reverse itself later? Next, we'll look at why muscle tissue goes through this transformation after death.
The Chemicals of Life and Rigor Mortis
Why does a dead body go board-stiff for two or more days? The easiest answer boils down to this: A biochemical chain reaction that causes a living person's muscles to move stops working when someone dies. When the reaction stops, the muscles become locked in place.
You have to look deep inside muscle cells to find a more detailed explanation. At the microscopic level, skeletal muscle fibers -- the ones that attach to your bones -- have two main parts:
- Thick filaments, made of the protein molecule myosin
- Thin filaments, made of the protein molecule actin
The fibers include other proteins as well, but actin and myosin are at the heart of rigor mortis.
When you lift a weight or scratch your head, a nerve impulse sets off a biochemical reaction that causes myosin to stick to actin. These two molecules lock together, pulling the muscle's thick and thin filaments toward each other. When thousands of filaments pull together all at once, over and over, you have a muscle contraction. You can read more about all the steps of this process in How Muscles Work.
Once the actin and myosin molecules stick together, they stay that way until another molecule, adenosine triphosphate (ATP), attaches to the myosin and forces it to let go. Your body uses the oxygen you breathe to help make ATP. That oxygen supply ends, of course, with death. Without ATP, the thick and thin filaments can't slide away from each other. The result is that the muscles stay contracted -- hence rigor mortis.
During rigor mortis, another process called autolysis takes place. This is the self-digestion of the body's cells. The walls of the cells give way, and their contents flow out. Rigor mortis ends not because the muscles relax, but because autolysis takes over. The muscles break down and become soft on their way to further decomposition.
Although this helps explain why rigor mortis comes and goes, it's the outward appearance -- the relative stiffness of the body -- rather than the process that's of most interest to investigators. Next, we'll explore how the gradual progression of rigor mortis plays a part in solving crimes.