Rigor mortis, or stiffening of the body after death, is due to the disappearance of adenosine triphosphate (ATP) from muscle. ATP is the basic source of energy for muscle contraction. Muscle needs a continuous supply of ATP to contract because the amount present is sufficient to sustain muscle contraction for only a few seconds. The three metabolic systems responsible for maintaining a continuous supply of ATP in the muscle are the phosphagen system, the glycogen-lactic acid system, and the aerobic system. Under optimal conditions, the phosphagen system can provide maximal muscle power for 10-15 sec, the glycogen-lactic acid system for 30-40 sec, and the aerobic system for an unlimited period of time.1 After exercise, these three systems need time to be replenished. After death, generation of ATP stops, though consumption continues. In the absence of ATP, actin and myosin filaments become permanently complexed and rigor mortis sets in. This complex remains until decomposition occurs.
Any violent muscular exertion prior to death will produce a decrease in ATP and speed up the onset of rigor mortis, since no ATP is produced after death. Some factors that cause a marked decrease in ATP prior to death are violent or heavy exercise, severe convulsions, and high body temperatures. All of these factors may cause rapid onset of rigor mortis, with onset appearing within minutes in some cases, and, in rare instances, instantaneously. The instantaneous appearance of rigor mortis is known as cadaveric spasm. In one instance, a man was chasing his wife with a straight razor when she turned and fired one shot, striking and killing him instantly. The deceased collapsed to his knees, holding the razor in his right hand in an upward position. At the scene, he was found dead, kneeling, with his right arm extended upward with the razor clasped in the hand. In cadaveric spasm, the object will be firmly clenched in the hand (Figure 2.4).
Rigor mortis disappears with decomposition. Cold or freezing will delay the onset of rigor mortis as well as prolong its presence. Rigor mortis can be "broken" by passive stretching of muscles. After rigor mortis is broken, it will not return. If only partial rigor mortis has set in prior to stretching, then the residual unbroken rigor mortis can still set in.
Rigor mortis usually appears 2-4 h after death, and fully develops in 6-12 h. This can vary greatly. In one case seen by the author, a young woman died following an overdose of aspirin. An EMS unit was summoned while she was in an agonal state. On arrival, she was still breathing and had a heart rate. Almost immediately, she suffered a cardiopulmonary arrest. Attempts at resuscitation were made and continued for approximately 15-20 min. Following this, she was pronounced dead. The body was then made ready to be transported to the medical examiner's office. At this time, it was realized that she was in full rigor mortis, only minutes
after being pronounced dead. Two hours later, at the morgue, she had a rectal temperature of 106oF.
Rigor mortis, when it develops, involves all the muscles at the same time and at the same rate. However, it becomes most evident in the smaller muscles. Thus, rigor mortis is said to appear first in the smaller muscles, such as the jaw, and then to gradually spread to large muscle groups. The classical presentation of rigor mortis in its order of appearance is jaw, upper extremities, and lower extremities. It passes off in the order in which it appeared. Rigor mortis is lost due to decomposition. In temperate climates, rigor mortis disappears in 36 h, but may be present up to 6 days. In hot climates, such as in Texas, a body can be in a moderately advanced to advanced state of decomposition in 24 h, in which case, there will be no rigor mortis present.
In a case seen by one of the authors (VJMD), the decomposing body of a 14-year-old boy was pulled from a cold lake. He had drowned 17 days prior. The body was in an early to moderate state of decomposition externally: a swollen face, discolored skin with slippage and marbling. The internal organs were in an early state of decomposition — not as decomposed as one might expect with the external changes. The most unusual aspect of the case was that the body was still in full rigor mortis. One can only speculate that the immersion in the cold water was the reason the rigor mortis persisted.
Rigor mortis may be delayed or be very weak in emaciated individuals. Its onset may also be very rapid in infants. Poisons, such as strychnine, that produce convulsions can accelerate the development of rigor mortis. Any disease or environmental factor that raises body temperature accelerates the development of rigor mortis. Thus, hyperthermia, loss of body regulatory
temperature due to cerebral hemorrhage, and infection may accelerate the development of rigor mortis.
In drowning deaths, rigor mortis may develop fully within only 2 to 3 h. This is apparently due to exhaustion of ATP through violent struggling while drowning. Individuals who are being chased prior to their deaths may show more rapid development of rigor mortis in their legs than in the rest of their musculature. This, again, is due to exhaustion of ATP by the muscles of the legs caused by running. Like livor mortis, rigor mortis can indicate whether a body has been moved (Figure 2.5).
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