What causes rigor mortis?

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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.