Download Factors Affecting Rigor

Dr. Fisanick
Forensic Chemistry
#5
Name:______________________________ Period:_____________ Date:____________
Rigor Mortis
Your objectives in this activity are to:
 Determine time of death from the extent of rigor
Developing Rigor Mortis
Rigor mortis is caused by a chemical reaction between actin and myosin molecules in
muscle fibers that contracts the fiber structure. The reaction is favored under acidic
conditions in the cell, such as those which develop during anaerobic respiration. Because
this is a chemical reaction, the rate of development of rigor mortis depends on
temperature—the higher the temperature, the more quickly rigor will develop. As a rule
of thumb chemical reaction rates double or triple for every 10°C (or 18°F) increase in
temperature.
This is the opposite of the effect of higher ambient temperatures on algor mortis, or body
cooling. In algor mortis, the higher the ambient temperature the more slowly the body
will cool.
Rigor develops most quickly in the small muscles of the body, such as the eyelids and
face, then in the trunk, and finally in the arms and legs. It relaxes in the same order, from
small to largest muscles.
Factors Affecting Rigor
Many factors can affect such as:
 Ambient temperature. The higher the ambient temperature, the faster the onset of
rigor. This is because the body will cool more slowly, and therefore remain at a
higher temperature for a longer period of time. This in turn means the reaction
will move further towards completion.
 Obesity. Body fat stores oxygen, so the onset of anaerobic respiration can be
delayed, and rigor progresses more slowly, but this is offset to some extent by the
insulating properties of fat, maintaining higher body temperatures.
 Clothing. Clothing will insulate the body and tend to keep it warmer than
otherwise, so it will speed rigor and slow algor mortis.
 Illness. A fever will mean the body is at higher temperature so rigor will develop
faster because the reactions causing rigor will go faster.
 Physical Activity before death. If the person did heavy labor with certain muscles,
say a body builder lifting weights, those muscles will be depleted of stored ATP
and will already be more acidic due to the production of lactic and pyruvic acid
from anaerobic respiration, and so will go into rigor faster. On the other hand, if
the person was sleeping just before the time of death, the muscles will be fully
oxygenated and the rigor will progress more slowly.
 Sun exposure. A body exposed to sunlight will absorb it and be warmer than a
body in the shade, so that rigor will proceed faster because of the higher
temperature.
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Dr. Fisanick
Forensic Chemistry
#5
Name:______________________________ Period:_____________ Date:____________
Relaxing Rigor
Rigor is relaxed when autolysis begins to break down the muscle fiber structure.
Autolysis occurs when the digestive enzymes stored in the lysosomes in the cell begin to
leak out as the membrane breaks down. Rigor breaks down in the small muscles first, and
finally in the largest muscles in the leg.
Rigor Time Table
Time After Death
2-6 hours
Event
Rigor begins
Appearance
Body stiffens.
12 hours
15-36 hours
Rigor complete
Slow loss of rigor
36-48 hours
Rigor totally
disappears
Body is most rigid
Loss of rigor in
small muscles first,
followed by larger
muscles
Autolysis breaks
rigor
Circumstances
Rigor begins earliest
in smallest muscles
and moves to largest
muscles
Entire body is stiff
Rigor is lost first in
the head and neck
and last in the major
leg muscles
Case Scenarios
1. A body is found with rigor mortis present in the face, neck and upper torso. The new
CSI says that the time of death must be more than 15 hours given that the rigor is
beginning to fade since there was no rigor in the legs. Is this time accurate? Explain
why or why not.
2. Estimate the time of death for each of these cases and explain your reasoning:
a. A body is found with no evidence of rigor
b. A body is found with rigor throughout the entire body
c. A body is found with rigor in the chest, arms, face and neck.
d. A body is found with rigor in the legs, but no rigor elsewhere.
e. A body is found with most muscles relaxed except for the face
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Dr. Fisanick
Forensic Chemistry
#5
Name:______________________________ Period:_____________ Date:____________
f. A body is discovered in a weight room at the gym on the arm curl machine.
Rigor is only present in the arms.
g. A frail, elderly woman is found in her apartment on a hot summer night, and
the body exhibits rigor everywhere except in her face and neck.
h. A body is discovered in the woods in full rigor. The man had been missing
two days. The average temperature in the past 48 hours was 50 F.
i. An obese man is discovered in an air-conditioned hotel room in a chair in
front of the TV. The air-conditioner was set for 65 F. When the coroner
arrived the man exhibited rigor in his upper body only.
j. While jogging, a young woman is attacked and killed. The perpetrator hid the
body in the trunk of a car and fled. When the woman was discovered, rigor
was noticed in her thighs only. Approximately how long had she been dead?
k. A victim is found in a snow bank on the side of the road. His body is
completely rigid. How long has he been dead?
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