Past exam of the medicine course of the University of Oxford / 1864 / First examination / Anatomy and physiology / 10 / Solution

Rigor mortis is a temporary postmortem loss of muscular extensibility caused principally by depletion of ATP and persistent actin–myosin attachment. Its onset, intensity and disappearance depend on the previous state of the muscle and its environment; it is not a fixed clock.

After an initial period of relaxation, surviving muscle metabolism briefly maintains some ATP. When supplies and regeneration fail, cross-bridges cannot detach normally, while disturbed calcium handling facilitates attachment and tension. The resulting stiffness is a local physicochemical change in muscle, not a continuing voluntary contraction and not evidence that its motor nerves remain alive. Later degradation of muscle structures removes the rigidity. This ATP-dependent explanation is modern; an 1864 answer could describe observations and their dependence on muscular condition without possessing the present cross-bridge model.^[ref]

Prior exertion, prolonged convulsive activity, fever or exhausting disease can leave smaller energy reserves and alter the interval before stiffness becomes evident. Starvation, wasting, age and muscle mass can influence how pronounced it appears. A well-nourished resting muscle and an exhausted muscle need not behave alike after death. Cause of death matters through such conditions, through temperature, and through injury or chemicals affecting muscle metabolism; a named cause alone does not impose one universal timetable.

Warm conditions generally hasten biochemical changes, often bringing earlier rigor and faster subsequent loss through decomposition. Cooling generally slows the overall sequence and can prolong its persistence. There are qualifications: muscle shortening caused by low temperature before rigor, freezing of tissue, and heat-induced protein coagulation are not interchangeable with ordinary rigor. Controlled muscle experiments measuring extensibility, pH, temperature and ATP demonstrate that shortening and completed rigor need not occur at the same stage.^[ref]

The stiffness becomes noticeable unevenly because muscles differ in size, condition and the way their resistance is tested. The customary progression from small facial and jaw muscles to larger limb groups is a practical generalization, not a biological wave proving that all other muscles were chemically unchanged until reached. Mechanical movement can break established resistance in a region, and the later appearance depends on how far the underlying process had progressed. Repeated manipulation therefore changes what the observer is trying to use as evidence.

A defensible observation records the environment, body condition, which anatomical joints resist passive movement, whether the resistance is local or widespread, and evidence of decomposition. It is interpreted alongside other observations rather than converted into an exact elapsed time. Rigor alone cannot establish an exact elapsed time.