Rigor mortis
Rigor mortis is a biochemical phenomenon characterized by the temporary stiffening of skeletal and cardiac muscles that occurs shortly after death, resulting from chemical changes within the muscular tissue. Initially, the body becomes flaccid, but within hours, the muscles gradually lock into a rigid position due to the depletion of adenosine triphosphate (ATP), which is crucial for muscle contraction. The process typically follows a sequence where stiffness begins in the small facial muscles and progressively affects the upper and lower extremities, fully manifesting within six to twelve hours in a temperate climate.
Rigor mortis, alongside algor mortis (cooling of the body) and livor mortis (discoloration), helps forensic investigators estimate the time of death and can indicate whether a body has been moved postmortem. However, it is important to note that the progression of rigor mortis is influenced by various factors, such as ambient temperature and the circumstances of death, making it unreliable as a standalone method for determining the exact time of death. Generally, rigor mortis dissipates within thirty-six hours as the body begins to decompose. Understanding rigor mortis is essential in forensic science, as it provides valuable insights into the postmortem interval.
On this Page
Subject Terms
Rigor mortis
DEFINITION: Temporary stiffening of the cardiac and skeletal muscles that ensues shortly after death as the result of chemical changes within the muscular tissue. The limbs of the deceased may lock into position and become difficult to manipulate.
SIGNIFICANCE: Together with algor mortis (cooling of body temperature) and livor mortis (discoloration from gravitational blood seepage), rigor mortis provides information that is useful in the assessment of time of death. In some cases, rigor mortis may be also useful for helping investigators determine whether bodies have been moved after death.
Rigor mortis (a Latin term meaning “the stiffness of death”) was first studied systematically in the nineteenth century. Immediately after a human being dies, the body’s muscles become limp, or flaccid. The circular sphincter muscles of the anus, for example, may relax to the point that defecation occurs. After a few hours (in some instances after a few minutes), the muscles grow rigid, locking the body into a fixed position. This stiffening occurs when, in the absence of oxygen and nutrients, the body’s adenosine triphosphate (ATP) is depleted. This complex energy-carrying molecule, a neurotransmitter released by nerve cells, is critical for muscular contraction; without it, muscular fibers remain interlocked.
In a temperate climate with an ambient temperature of 70 degrees Fahrenheit, the rigidity of a body ideally proceeds, with some variations, according to chronological parameters. It generally progresses downward following Nysten’s law (named for a nineteenth century researcher): It appears first in the small facial muscles (such as the muscles of the eyelids and jaws), then in the upper extremities, and then in the lower extremities. The facial muscles stiffen within thirty minutes to three hours after death; the entire body stiffens within six to twelve hours. After thirty-six hours (again in a temperate climate), rigor mortis tends to disappear in the same downward progression as the body begins to putrefy.
Although this multistaged biochemical process would seem to allow some precision in determining time of death, in fact no competent investigator would use information about rigor mortis in isolation to estimate time of death because the progression of rigor is subject to too many variables. Contrary to popular misconceptions fostered by films and television, observation of rigor mortis stages is one of the least reliable means of determining the postmortem interval; further, such observation is at all useful only before begins. When investigators base estimations of time of death on the progress of rigor mortis, they do not furnish specific times; rather, they make statements regarding probable intervals.
Cool ambient temperatures retard the progress of rigor mortis, and high temperatures accelerate it. How an individual died can also affect the progress of rigor. For example, if a person kicked at an attacker before dying, the ATP in the legs might be depleted first. Strychnine poisoning induces violent physical convulsions that would quickly deplete the supply of ATP and shorten the time for onset of rigor. Elevated body temperature caused by an infection would also accelerate the process.
Bibliography
Bergland, Christopher. "What Is ATP? Adenosine Triphosphate (ATP) Is Every Living Cell's Source of Energy." Verywell Health, 19 July 2024, www.verywellhealth.com/atp-6374347. Accessed 19 Aug. 2024.
DiMaio, Vincent J., and Dominick DiMaio. Forensic Pathology. 2d ed. Boca Raton, Fla.: CRC Press, 2001.
Innes, Brian. Bodies of Evidence. Pleasantville, N.Y.: Reader’s Digest Association, 2000.
Krompecher, Thomas. "Rigor Mortis: Estimation of the Time Since Death by Evaluation of Cadaveric Rigidity." In Estimation of the Time Since Death, 4th ed., Burkhard Madea, ed., CRC Press, 2023, doi.org/10.1201/9781003244974.
Nickell, Joe, and John F. Fischer. Crime Science: Methods of Forensic Detection. Lexington: University Press of Kentucky, 1999.
Shrestha, Rijen, Tanuj Kanchan, and Kewal Krishan. "Methods of Estimation of Time Since Death." National Library of Medicine, 30 May 2023, www.ncbi.nlm.nih.gov/books/NBK549867/. Accessed 19 Aug. 2024.