What Causes Rigor Mortis? Muscle Changes After Death

Dead fly

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A few hours after a person or animal dies, the joints of the body stiffen and become locked in place. This stiffening is called rigor mortis. The phrase is Latin, with rigor meaning stiffness and mortis meaning death. Rigor mortis is a a temporary condition. Depending on body temperature and other conditions, rigor mortis lasts approximately 72 hours. The phenomenon is caused by the skeletal muscles partially contracting. The muscles are unable to relax, so the joints become fixed in place.

Key Takeaways: Rigor Mortis

  • Rigor mortis is a recognizable indication of death characterized by muscles stiffening and locking into place.
  • At normal temperatures, rigor mortis starts around four hours following death.
  • Rigor mortis is a temporary condition. After a total of about eight hours after death, muscles relax again.
  • The main cause of rigor mortis is depletion of the cell's energy molecule, ATP. ATP separates actin-myosin bridges during muscle relaxation. Without ATP, cross-bridging locks muscles in place. Eventually, decomposition breaks the bridges and muscles relax.

The Role of Calcium Ions and ATP

After death, the membranes of muscle cells become more permeable to calcium ions. Living muscle cells expend energy to transport calcium ions to the outside of the cells. The calcium ions that flow into the muscle cells promote the cross-bridge attachment between actin and myosin, two types of fibers that work together in muscle contraction. The muscle fibers ratchet shorter and shorter until they are fully contracted or as long as the neurotransmitter acetylcholine and the energy molecule adenosine triphosphate (ATP) are present. However, muscles need ATP in order to release from a contracted state (it is used to pump the calcium out of the cells so the fibers can unlatch from each other).

When an organism dies, the reactions that recycle ATP eventually come to a halt. Breathing and circulation no longer provide oxygen, but respiration continues anaerobically for a short time. ATP reserves are quickly exhausted from the muscle contraction and other cellular processes. When the ATP is depleted, calcium pumping stops. This means that the actin and myosin fibers will remain linked until the muscles themselves start to decompose.

Factors That Affect Rigor Mortis

Temperature is the primary factor influencing when rigor mortis begins and ends, but there are other considerations:

  • Temperature: Warmer temperature speed the pace of rigor mortis.
  • Physical Exertion: If a body engages in strenuous exercise prior to death, rigor mortis may set in immediately. This is because exertion uses oxygen and ATP.
  • Age: Rigor mortis occurs more rapidly in the very young and very old because they have lower muscle mass.
  • Illness: Illness is another physiological stress that leads to a rapid onset of rigor mortis.
  • Body fat: Fat insulates the body, slowing the rate of rigor mortis.

How Long Does Rigor Mortis Last?

Rigor mortis can be used to help estimate the time of death. Muscles function normally immediately after death. The onset of rigor mortis may range from 10 minutes to several hours, depending on factors including temperature (rapid cooling of a body can inhibit rigor mortis, but it occurs upon thawing). Under normal conditions, the process sets in within four hours. Facial muscles and other small muscles are affected before larger muscles. Maximum stiffness is reached around 12-24 hours post mortem. Facial muscles are affected first, with the rigor then spreading to other parts of the body.

Rigor mortis affects joints, too. The joints are stiff for 1-3 days, but after this time general tissue decay and leaking of lysosomal intracellular digestive enzymes will cause the muscles to relax. It is interesting to note that meat is generally considered to be more tender if it is eaten after rigor mortis has passed.

Sources

  • Bear, Mark F; Connors, Barry W.; Paradiso, Michael A. (2006). Neuroscience, Exploring the Brain (3rd ed.). Philadelphia: Lippincott Williams & Wilkins. ISBN 0-7817-6003-8.
  • Hall, John E., and Arthur C. Guyton. Guyton and Hall Textbook of Medical Physiology. Philadelphia, PA: Saunders/Elsevier, 2011. MD Consult. Web. 26 Jan. 2015.
  • Hammer, R., Moynihan, B., Pagliaro, E. (2006). "Chapter 15, Death Investigation". Forensic Nursing: A Handbook for Practice. Jones and Bartlett Publishers. pp. 417-421.
  • Moenssens, Andre A.; et al. (1995). "Chapter 12, Forensic Pathology". Scientific Evidence in Civil and Criminal Cases (4th ed.). Foundation Press. pp. 730-736.
  • Peress, Robin. Rigor mortis at the crime scene. Discovery Fit & Health, 2011. Web. 4 December 2011.
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Helmenstine, Anne Marie, Ph.D. "What Causes Rigor Mortis? Muscle Changes After Death." ThoughtCo, Aug. 2, 2021, thoughtco.com/what-causes-rigor-mortis-601995. Helmenstine, Anne Marie, Ph.D. (2021, August 2). What Causes Rigor Mortis? Muscle Changes After Death. Retrieved from https://www.thoughtco.com/what-causes-rigor-mortis-601995 Helmenstine, Anne Marie, Ph.D. "What Causes Rigor Mortis? Muscle Changes After Death." ThoughtCo. https://www.thoughtco.com/what-causes-rigor-mortis-601995 (accessed September 18, 2021).