(HRI) Postmortem Changes: Rigor Mortis and Algor Mortis Lesson

Postmortem Changes: Rigor Mortis and Algor Mortis

Livor Mortis, Rigor Mortis and Algor Mortis are all post-mortem changes in the body which yield important information to the investigator. The term Mortis originates from Latin and means death. Livor, as discussed in the previous lesson refers to the color of the corpse after death when the blood settles in the lowest point of the body. Rigor and Algor are terms that describe other characteristics of a dead body, namely "stiff or rigid" and "cold", respectively. They are used in Forensic Science as a shorthand way of documenting the post-mortem changes that occur in nearly every body.

Rigor Mortis

Rigor Mortis is the stiffening of a dead body accompanying depletion of adenosine triphosphate (ATP) in the muscle fibers. The body stiffens after death due to proteins known as Actin and Myosin linking in the muscles.  In simplified terms, these proteins are responsible for muscle contraction in a living body; this contraction of muscles is ultimately released by ATP.  Without ATP, the muscles remain contracted.  ATP is a nucleotide that can be thought of as a bank of energy or as energy currency. The body uses ATP to fuel metabolic processes including cellular respiration, cell division and movement such as muscle contraction. When a body dies, ATP is no longer produced and thus the muscle contractions can't immediately be released resulting in the post-mortem systemic muscular stiffness known as Rigor Mortis. Rigor Mortis begins at the head and progresses toward the feet in the next 12-18 hours. By that time, the entire body is generally stiff and the joints can only be moved by force which often breaks them. In about 24-48 hours, Rigor Mortis begins to fade as the process of decomposition picks up. The body must be cooled or embalmed at this point or decomposition will advance quickly. Investigators can use these Rigor Mortis timelines to ascertain an estimated time since death.  It is not always totally accurate because Rigor Mortis is affected by the temperature where the body is located. Rigor Mortis progresses more quickly in warmer climates and is suspended in freezing temperatures.

Did you know?

• Rigor Mortis can settle in the tiny muscles of the hair follicles known as the Arrector Pili giving the corpse the appearance of "goose bumps".
• Very rarely muscles can spasm and "freeze" in the shape they were in at the moment of death.  This is known as "Cadaveric Spasm" or "Post-Mortem Spasm".  Usually muscles completely relax at the time of death until Rigor Mortis sets in.
• In hot climates, Rigor Mortis can begin as soon as 1 hour after death and begin to fade in as little as 12 hours.
• If a body is kept from Rigor Mortis, such as when a body is frozen, it will continue the Rigor Mortis process where it left off once it is thawed until the Rigor Mortis process is complete.

Algor Mortis

Algor Mortis is the third major post-mortem change monitored in investigations. Algor Mortis is the reduction in body temperature and accompanying loss of skin elasticity that occur after death. A body generally loses heat in a predictable way after death, but it can be greatly impacted by the temperature of the environment in which the body is located.  A body usually loses about 1.5° Celsius per hour since death until the ambient temperature of the environment is reached.  Temperature readings are obtained either rectally or through a temperature probe in the liver.  Medical examiners use the Glaister equation (below) to estimate time (in hours) since death:

$\frac{98^\circ - \text{rectal temperature in} ^\circ \text{Fahrenheit}}{1.5}$

Factors affecting the rate of Algor Mortis:

• Ambient temperature
• Surface the body is lying on- is it conductive or heat trapping?
• Diseases or drugs that may have raised the body temperature before death

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