Involvement of skeletal muscle leads to joint stiffening. The following sequence occurs:
• Initial flaccidity (exception: instantaneous rigor):
° There is sufficient ATP in the immediate postmortem period to allow muscles to remain relaxed and joints limp. This phase ranges from 0.5 to 7 h (mean = 3 ±2 h ).
• Onset and progression:
° There is simultaneous development of rigor in all muscles, but it is evident sooner in smaller muscle groups (20). The evolution of rigor mortis is not necessarily constant or symmetrical (1,8). Rigor onsets in the jaws, progressing to the upper extremities and then to the lower extremities (Nysten's law; see also refs. 7 and 21). Rigor is
assessed as absent, partial, or complete by manipulation of joints during the external examination (i.e., opening of the mouth to assess the temporomandibular joints and flexion/extension of the elbow and knee joints). The time that rigor is fully established in all joints varies from 2 to 20 h (mean = 8 ±1 h ). An individual who dies in the supine position shows slight flexion of the elbows and knees. Rigor persists for 24 to 96 h (mean = 57 ±14 h ).
• Resolution (secondary flaccidity): ° Rigor lessens and eventually disappears as denaturation of actin-myosin linkages occurs with early decomposition. The disappearance will follow the same pattern as the onset. The time range is 24 to 192 h (mean = 76 ±32 h ).
Various intrinsic and extrinsic factors influence the development of rigor (22,23):
1. Temperature: increased body or environmental temperature decreases the time of onset and resolution (24). The former is likely the result of increased metabolism of ATP, and the latter from rapid denaturation of actin-myosin linkages. Hyperthermia, under various circumstances (e.g., sepsis, cocaine use), enhances the onset. Rigor persists for days under cold conditions (25).
2. Muscle volume and body habitus: increased muscle bulk delays the onset of rigor, but the rigor will be better developed (3,26). Time for resolution may or may not be increased (26). The elderly, cachectic individuals, and infants have a rapid progression of rigor, but it is less developed and disappears more quickly. Rigor can be absent in emaciated and obese individuals (1).
Muscle volume is considered a factor that explains why rigor becomes established in small joints before larger ones (21). Rigor mortis occurs more rapidly in small muscles (e.g., masseter muscle), assuming rigor is a physiochemical process occurring at the same time in all muscles (20,21). Animal experimentation shows a greater decline of ATP in the masseter muscle than larger muscles (20,27). The sequence of rigor could also be explained by the proportion of red and white fibers around the joint being manipulated (21,27). Red muscle undergoes rigor more quickly than does white muscle (21). The muscles of mastication in humans have a high proportional area of red muscle fibers compared with the leg muscles (e.g., gastrocnemius; see also ref. 21).
3. Antemortem muscle contraction: increased muscle activity (e.g., exercise, seizure) prior to death hastens rigor (1). Instantaneous rigor (cadaveric spasm, cataleptic rigidity) is characterized by the sudden development of rigor usually localized to muscles that have
forcefully contracted perimortem (Fig. 4; see also ref. 1). Generalized instantaneous rigidity has been described in situations of considerable excitement and tension.
° Prior to the onset of rigor mortis, a body can be manipulated to any position.
° Once rigor is established, it will not conform to a new position (Fig. 5).
° Because many factors influence the progression of rigor, it is variable as a measure used to determine the time of death. The inaccuracy is enhanced if only one assessment is done (28).
° Rigor mortis is broken by manipulation of the body. If rigor is maximum, then it does not return. If rigidity is not fully established, then it returns to a lesser degree in a particular joint (range 2-8 h ). Forceful manipulation of a stiff joint can tear muscles and fracture a long bone weakened by disease (e.g., osteoporosis, metastasis; see Fig. 6). Manipulation of the body occurs at different times prior to the autopsy—i.e., at the scene, during body removal and transportation, and during removal of clothing (e.g., removal of pants from a shirtless individual selectively diminishes rigor in the legs). Rigid upper limbs are deliberately manipulated to allow appropriate autopsy incisions to be made on the torso. Failure to manipulate rigor and expose certain areas of the body results in injuries being missed (e.g., venipuncture in antecubital fossa of an intravenous drug user).
° Resolution of rigor in the jaws allows the mouth to be examined for injuries, if the mouth could not be opened during the initial examination.
° Flaccidity following resolution of rigor can relax muscles of the pelvic floor, leading to widening of the vaginal and anal orifices. Suspicion of sexual assault arises (Fig. 7).
° The rapid onset of rigor can be linked to certain causes of death associated with increased or abnormal muscle contraction (e.g., electrocution, strychnine poisoning, myotonic dystrophy; see Fig. 8). ° "Instantaneous" rigor may be consistent with the circumstances of the death.
Was this article helpful?