Rigor Mortis is the state of muscles of a dead body that become stiff with some degree of shortening that follows the period of primary flaccidity. In general, rigor mortis is the stiffening of the muscles after death. Along with stiffening, shortening of the muscles is also observed. The term is derived from ‘rigor‘ which means ‘rigidity‘and ‘Mortis‘ meaning of ‘death’. It is also known as Cadaveric rigidity. 

Rigor mortis is the third stage of death and is a sure sign of death. When it is completely developed, the body and the joints become extremely stiff with the flexion attitude of upper limb muscles. It indicates the death of individual cells that is molecular death. In children, rigor mortis may fix a dilated anal orifice and this finding may persist after it has faded. The position of the body in which the rigor mortis is established indicates the position of the body at the time of death unless the position is disturbed by external forces or putrefaction.

The first person to demonstrate postmortem rigidity in the investigation was Nysten in 1811. Rigor mortis is usually followed up by four different stages namely autolysis, bloat, active decay, and skeletonization. It can be confused with cadaveric spasm, heat stiffening, cold stiffening, or gas stiffening.

Rigor mortis is not related to the nervous system functionally and so it will develop in the case of paralyzed limbs also. It might affect the pupil making them of unequal sizes. It can cause the ventricles of the heart to contract and so it can be mistaken for ventricular hypertrophy. It can also affect the dartos muscles of the scrotum that will compress the testes and epididymis causing postmortem emission of semen.

Mechanism of Rigor Mortis

When we are alive the muscles contract and relax with the help of ATP. ATPs are found in higher concentrations in the resting muscles. Generally, ATP is synthesized by glycogen. When we die no more ATP is resynthesized, and thus the level decreases within the muscles.

This reduction in the concentration of ATP within the muscles will lead to the hardness and rigidity of the muscles. The extensibility is affected as and when the level drops to 95% and the muscles are least extensible when the ATP falls to the level of 15% of normal.

At the time of somatic death rigidity does not happen because there is enough glycogen in the body for the resynthesis of ATP with the depletion of glycogen amidst the anaerobic respiration and constant accumulation of lactate and phosphate in the muscles, ATP synthesis stops.

Thus there is the formation of viscid gel, like actomyosin complex leading to the shortening and stiffening of the muscles. This rigidity is lost when autolysis happens due to the release of enzyme lysosomes which will destroy the muscle proteins and cause decomposition.

Onset and Disappearence of Rigor Mortis

Rigor mortis occurs in all types of muscles but it is seen first in the involuntary and then in the voluntary muscles.

In the voluntary muscles, it first appears in the eyelids. At the end of the first hour after death, rigor mortis starts to set in the heart muscles. There is a chronological order for the spread of this rigidity. It appears in the muscles of the face, neck, jaw, trunk, upper limbs( from shoulder to limbs), lower limbs (hips to foot), small muscles of hand and feet. The disappearance of rigor mortis also follows the same order.

Conditions Affecting the Onset of Rigor Mortis

  • Early-Onset and Passes Off Early:- Causes may be electrocution, cancer, convulsions, hyperpyrexia, metabolic acidosis, uremia, and hot environmental conditions.
  • Rapid Onset and Stays Longer: – This can be due to poisoning using hydrocyanic acid, strychnine.
  • Delayed Onset:- The causes include asphyxia, apoplexy, hypothermia, cold environmental conditions.

RULE OF 12: It is considered that the time taken for rigor mortis to develop is 12 hours after death and it remains for another 12 hours and takes about 12 hours to pass off. This is the rule of 12 and is also known as the March of Rigor. This can vary as there are a lot of factors affecting rigor mortis. 

Factors Affecting Rigor Mortis

  • Age: It is said that rigor mortis will not develop in a fetus less than 7 months of intrauterine life.
  • Physique: Rigor mortis appear and disappear early in thin built and people with weak musculature when compared to those with a well-built body.
  • Season: When the temperature is high like in summers, it will appear early and pass off.
  • Cause of Death: If the death of a person is preceded by high muscular activity which will cause rapid depletion in the glycogen, then rigor mortis will set in quickly. The onset and the duration depends upon the presence of some medical condition or poison and so on.
  • Condition of Muscle: If the muscles are relaxed, then rigor mortis sets late and if the muscles are completely exhausted it sets in and pass off early. 


The presence of rigor mortis is an obvious sign of death but many a time it is confused with cadaveric spasm. It indicates the position of the body. Time since death can also be estimated. Rigor mortis can be broken during the improper handling of the body or when more force is used, then the rigidity is lost and estimation of the time since death becomes difficult.  


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