Lack of oxygen after clinical death results in temperature-dependent physiochemical changes in muscles. Energy cannot be produced using oxidative phosphorylation, resulting in cessation of Adenosine Tri-Phosphate (ATP) production. Anaerobic processes then take over producing lactic acid as a by-product, leading to increasing cellular acidity. This decreased ATP and high acidity causes actin and myosin filaments to bind together and form a gel, giving rise to stiffness in muscles. Shortening does not occur unless the muscle is under tension. Hence, in conditions where muscles are acidic (heavy exercise, electrocution, epileptic seizures) rigor mortis develops faster. In extremes of ages or in emaciation, rigor mortis is hard to detect due to low muscle mass.
Rigor mortis develops uniformly throughout the body but is first detectable in smaller group of muscles like those around eyelids, jaws and finger. It advances down the body from head to legs as muscles get larger. Using this information, time since death is estimated or rather guessed. No accurate postmortem interval can be derived as rigor is influenced by a multitude of factors.
An important factor affecting rigor mortis is temperature. Cooler the temperature, the longer the process of rigor. It is therefore imperative to investigate the micro-environment around the body at the scene of death before estimating time since death.
Rigor mortis is seen after the stage of primary flaccidity, which may last for 1-2 hours after death. It is estimated that it takes 12 hours for rigor to complete, it stays in the body for the next 12 hours and then starts disappearing in the same head to toe pattern once decomposition sets in. These numbers are very unreliable since factors like diurnal variation in temperature, body mass, age, clothing, etc. plays a vital role in the development of rigor mortis.
It is best to test for rigor across a joint using very gentle pressure from one or two fingers only – the aim is to detect the presence and extent of stiffness, and not to ‘break’ it. If rigor is broken by applying too much force, those muscle groups cannot reliably be tested again.