The muscles of the body initially become flaccid after death. Within 1-3 hours following death, the muscles begin to become noticeably increasingly rigid and the joints immobile (freeze) due to a process known as rigor mortis (postmortem rigidity, rigor). The chemical processes causing the muscles to stiffen are not clearly understood but are similar, although not identical, to physiologic muscle contraction in that both of these processes involve calcium and ATP/ADP. Rigor mortis involves the formation of locking chemical bridges between the muscle proteins actin and myosin and does not involve muscular shortening, while physiologic muscular contraction involves shortening of muscle as the actin molecules reversibly slide over the myosin molecules.
When a body stiffens, it remains in that position until rigor passes or it is physically overcome (broken), such as when a joint is forcibly moved. Since the chemical process of rigor is irreversible, fully developed rigor will not recur in an area in which it has been broken or in an area in which it has already passed. However, if rigor is overcome prior to its full development the process will continue to completion, resulting in the apparent recurrence of rigor in that area, albeit to less than expected "full" stiffness.
All muscles of the body begin to stiffen at the same time after death. However, the stiffening becomes noticeable in the smaller muscle groups before the larger groups, giving the appearance that rigor mortis proceeds at different rates in the various muscle groups. Stiffness is usually apparent first in the jaw, then the elbows and finally the knees. A body is said to be in complete or full rigor when the jaw, elbow and knee joints are immovable. The ability to passively move a joint is dependent on the amount of muscle controlling the joint. Rigor involving a joint with a small amount of muscle such as the finger is easily overcome, while it may be difficult to move a joint such as the elbow, which is connected to relatively large muscles. As a rule, men will have stronger rigor than women since men typically have a larger muscle mass than women. Large muscles, especially in muscular individuals, may become so resistant to stretching that it may require the efforts of more than one person to move a large joint. Occasionally, the bone may break before the rigor mortis is overcome. Conversely, rigor may be poorly formed or not apparent in individuals with little muscle mass, such as infants or emaciated adults.
Complete rigor takes approximately 10-12 hours to fully develop in an average size adult when the environmental temperature is 70-75°F. The body will remain stiff for 24-36 hours at this same temperature before decomposition causes the muscles to begin to appreciably loosen, apparently in the same order they stiffened.
The problem with relying on this time sequence for rigor mortis in estimating the postmortem interval is that a number of factors influence the course of rigor mortis. Rigor mortis is affected by the environmental temperature, the internal body temperature and the decedent's activity prior to death.
Rigor mortis is most affected by the environmental temperature. Elevated temperatures will accelerate the appearance and disappearance of rigor. Rigor involving a body lying in a field will come and pass quicker on a hot summer day than on a cold winter one. The rate of development and disappearance of rigor will be affected by temperature changes experienced by the body, such as occur during the heat of day and the coolness of night.
Rigor is also affected by the decedent's internal body temperature and activity prior to death. Higher body temperatures at the time of death and conditions causing more lactic acid production cause rigor to develop more quickly. For example, a person who dies having a fever from an infection such as pneumonia may develop rigor sooner than a person with a normal body temperature. Accelerated rigor may also be seen in persons dying with hyperthermia even though the environmental temperature may be normal, such as may occur in deaths related to cocaine, PCP or methamphetamine.
The onset of rigor may also occur more quickly if strenuous physical activity takes place immediately prior to death. For example, a person who runs away from an assailant before being shot or stabbed may develop rigor sooner than if there were no intense physical activity. The accelerated rigor is due to a combination of increased body temperature and increased production of lactic acid.
Rarely, rigor mortis may become apparent within minutes of death. This "cadaveric spasm" is usually associated with extreme physical activity just prior to death. It has also been associated with some other circumstances such as electrocution.
In contrast to elevated environmental temperature, cold conditions may retard or prevent rigor mortis. The process will begin or accelerate when the body is allowed to warm. If a body is not in complete rigor and is placed in refrigeration the process will slow down and may stop. Rigor may proceed to completion when the body is warmed. The stiffness of rigor must be differentiated from muscle hardening or freezing due to very cold weather. Under such environmental conditions, rigor may be difficult to evaluate.
Rigor mortis will also aid the investigator in determining if the body has been moved. If an investigator arrives on the scene and finds an unsupported arm or leg sticking up in the air, the investigator knows the decedent has been moved after rigor has set in. A person may die with the arms or legs in the air, but gravity will prevent the unsupported extremities from remaining there after death.
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