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Rigor mortis (Latin: rigor "stiffness", mortis "of death") is one of
the recognizable signs of death, caused by chemical changes in
the muscles after death, causing the body to stiffen and locked in
place, making it difficult to move or manipulate.
It commences after about 3 to 4 hours, reaches maximum
stiffness after 12 hours, and gradually dissipates from approx 24
hours after death, thus lasting for about 72 hours.
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, the joints can be moved 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
In 1812, a French pediatrician named Pierre Nysten
recorded his observation that rigor mortis follows a
downward path that begins in the upper region of the
body, around the face and head, and travels in a set
pattern down to the rest of body and the extremities.
Known as Nysten's law, this principle likely reflects
the fact that rigor mortis - while affecting all muscles in
the same way at the same time - becomes noticeable
first in small muscle groups, such as those around the
eyes, mouth and jaws, and becomes pronounced
somewhat later in the larger muscles of the lower
At the time of death, a condition called "primary flaccidity"
occurs. Following this, the muscles stiffen in rigor mortis. All
muscles in the body are affected.
Starting between two to six hours following death, rigor mortis
begins with the eyelids, neck, and jaw. The sequence may be due
to different lactic acid levels among different muscles, which is
directly related to the difference in glycogen levels and different
types of muscle fibers.
Rigor mortis then spreads to the other muscles within the next
four to six hours, including the internal organs.
The onset of rigor mortis is affected by the individual's age, sex,
physical condition, and muscular build. Rigor mortis may not be
perceivable in many infant and child corpses due to their smaller
The phenomenon is caused by the skeletal muscles
partially contracting. The muscles are unable to relax,
so the joints become fixed in place.
What happens is that 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.
After death, cellular respiration in organisms ceases to occur,
depleting the corpse of oxygen used in the making of
adenosine triphosphate (ATP) allowing the corpse to harden
and become stiff.
ATP is no longer provided to operate the SERCA pumps in the
membrane of the sarcoplasmic reticulum, which
pump calcium ions into the terminal cisternae.
This causes calcium ions to diffuse from the area of higher
concentration (in the terminal cisternae and extracellular
fluid) to an area of lower concentration (in the sarcomere),
binding with troponin and allowing for crossbridging to occur
between myosin and actin proteins, two types of fibers that
work together in muscle contraction.
Unlike normal muscular contraction, after death, the body is
unable to complete the cycle and release the coupling
between the myosin and actin, creating a state of muscular
The muscle fibers ratchet shorter and shorter until they are
fully contracted or as long as the neurotransmitter
acetylcholine and the energy molecule 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).
ATP reserves are quickly exhausted from the muscle
contraction and other cellular processes. This means that the
actin and myosin fibers will remain linked until the muscles
themselves start to decompose.
As part of the process of decomposition, the myosin heads are
degraded by the enzymes, allowing the muscle contraction to
release and the body to relax.
The degree of rigor mortis may be used in forensic pathology to
determine the approximate time of death. A dead body holds its
position as rigor mortis sets in.
If the body is moved after death, but before rigor mortis begins,
forensic techniques such as Livor mortis can be applied.
If the position in which a body is found does not match the location
where it is found (for example, if it is flat on its back with one arm
sticking straight up), that could mean someone moved it.
Several factors also affect the progression of rigor mortis, and
investigators take these into account when estimating the time of
One such factor is the ambient temperature. When conditions are
warm, the onset and pace of rigor mortis are sped up by providing a
conducive environment for the metabolic processes that cause
decay. Low temperatures, however, slow them down. Therefore, for
a person who dies outside in frozen conditions rigor mortis may last
several days more than normal, so investigators may have to
abandon it as a tool for determining time of death.
Contrary to common perception the process of rigor
mortis actually does reverse and the body returns to
a flaccid state; the muscles losing their tightness in
the reverse of how they gained it: i.e. those larger
muscles that contracted last will lose their stiffness
first and return to their pre-rigor condition.
Rigor mortis is a good means of indicating time of
death as is normally visible within the first thirty-six
to forty-eight hours after death; after which it leaves
Body temp Body stiffness Time since death
warm not stiff dead not more than three
warm stiff dead 3 to 8 hours
cold stiff dead 8 to 36 hours
cold not stiff dead more than 36 hours