What causes rigor mortis?

Rigor mortis can play an important role in crime scene investigation. See more death pictures.
Rigor mortis can play an important role in crime scene investigation. See more death pictures.
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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

Myosin, actin and ATP are central to muscle contractions.
Myosin, actin and ATP are central to muscle contractions.
2008 HowStuffWorks

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.

Rigor Mortis at the Crime Scene

A body goes stiff in the exact position it was in when the person died. If the body's position doesn't match up with the location where someone found it -- for example, if it's flat on its back in bed with one arm sticking straight up -- that could mean someone moved it.

Although it's an imperfect marker of the time of death, rigor mortis is useful because it's like an alarm clock set to go off and stop ringing within a known time span. Several variables affect the progression of rigor mortis, and investigators must take these into account when estimating the time of death. These include:


  • Ambient temperature: Warm conditions speed up the onset and pace of rigor mortis by providing a hospitable environment for the bacteria and processes that cause decay. Cold temperatures, on the other hand, slow it down. If someone dies outside in freezing temperatures, rigor mortis can last for days. Investigators might abandon it entirely as a tool for estimating the time of death.
  • Physical exertion just prior to death: If someone dies while engaged in strenuous activity like exercising or struggling against drowning, rigor mortis can set in immediately. This instant onset, sometimes called cadaveric spasm, happens because the person's muscles, at the moment of death, were depleted of oxygen energy and ATP. This is why the victim of a violent attack may still be clutching the attacker's hair or a piece of clothing.
  • Fat distribution: Fat acts as insulation, causing rigor mortis to develop more slowly.
  • Age or illness: In people with low muscle mass, such as children and the elderly, or in those with a fever or a debilitating disease, rigor will progress quickly.

Because rigor mortis leaves a lot of room for doubt, forensic pathologists rely on other indicators that provide greater certainty as to time of death. These include:

  • Body temperature: The body cools at the rate of 1.5 to 2 degrees per hour. A body that registers approximately 92 degrees Fahrenheit (33.33 degrees Celsius) has been dead about four hours.
  • Stomach contents: By determining the degree of digestion of the last meal, examiners can gauge how long the person lived after eating.
  • Insect activity: Flies gather around the eyes, mouth and other openings to feed on the body's fluids. Forensic entomologists can determine approximately how long someone's been dead by observing the life cycle of the flies, as well as their eggs and larvae.

But without an eyewitness, investigators can only estimate the time of death -- not pinpoint it for certain. To learn more about crime scenes, forensics and related topics, see the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links

  • Australian Museum. "Decomposition: What happens to the body after death?" (5/3/2008)http://www.DeathOnline.net/decomposition/body_changes/index.htm.
  • Blassino, Edwin A. Personal Interview. New York City Police Department (retired). 5/2/2008
  • Forensics Medicine. "Signs of Death." (5/6/2008) http://www.forensicmedicine.ca/Forensics/Signs-Of-Death.html
  • Hammer, R., Moynihan, B., Pagliaro, E. "Forensic Nursing: A Handbook for Practice." Jones and Bartlett Publishers, 2006. Chapter 15, Death Investigation, pp. 417-421.
  • HBO. "Autopsy: Postmortem with Dr. Michael Baden." (5/2/2008) http://www.hbo.com/autopsy/baden/.
  • King, Michael W., Ph.D., "Muscle Biochemistry." 4/5/2006 (5/4/2008) http://www.TheMedicalBiochemistryPage.org/muscle
  • Massey, Peter. Personal Interview. Training Coordinator, Henry C. Lee Institute of Forensic Science, New Haven, Conn. May 9, 2008.
  • Mayer, Robert G., Taylor, Jacquelyn. "Embalming: History, Theory, and Practice." McGraw-Hill Professional, 2005. Chapter 5, Death--Agonal and Pre-embalming Changes, pp. 112-14.
  • Moenssens, Andre A., et al., "Scientific Evidence in Civil and Criminal Cases," Fourth Edition. Foundation Press, 1995. Chapter 12, Forensic Pathology, pp. 730-736.
  • Tsokos, Michael, M.D., ed. "Forensic Pathology Reviews." Humana Press, 2005. Postmortem Changes, pp. 200-204.