The phase of primary relaxation of the muscles continues for an hour or more after death. After lapse of this period, the muscles of the body gradually become stiff or rigid. This rigidity of the muscles after death indicates molecular death of the concerned muscles.

Definition — Rigor mortis is that state of the muscles of dead body when they become stiff or rigid with some degree shortening.

Mechanism of formation of rigor mortis

RIGOR MORTIS OR CADAVERIC RIGIDITYMuscle fibres contain bundles of myofibrils which consist of two types of protein filaments, actin and myosin. These two types of protein filaments are arranged in the myofibrils as inter-digitating arrays, one type of the filaments extending in between the filaments of the other type. In the arrangement the myosin filaments have lateral projections.

At rest, during life, the interdigitation of the filaments is of small extent. The muscle fibres also appear soft and supple. Maintenance of softness; suppleness and extensibility of the muscles is due to the presence of A.T.P. (adenosine-triphosphate) above a certain level. When the level of the A.T.P. falls below this critical value, there is contraction of the muscle fibres. With nervous stimulation, breakdown of A.T.P. occurs to A.D.P. (adenosine-di- phosphate) and phosphate. This process of breakdown of A.T.P. causes liberation of energy which causes contraction of the muscle fibres with extension of the actin filaments more inside the myosin filaments. But during life, very soon there is re-synthesis of A.T.P., from the A.D.P. and phosphate by the process of phosphorylation, with the help of energy liberated due to the break down of glycogen to lactic acid.

After death there is continuous break down of the A.T.P. For some time so long glycogen is available in the muscle, there is re-synthesis of A.T.P. Due to break down, there is constant decrease in the level of glycogen in the muscle. In this process, once the muscle glycogen is exhausted, no further re-synthesis of A.T.P. is possible and the muscle loses softness, elasticity and extensibility due to formation of viscid acto-myosin complex in which both actin and myosin filaments take part, giving rise to the formation of the state of rigor mortis in the muscle.

In rigor mortis, there is stiffening of the muscles with loss of elasticity and extensibility. There is only mild degree shortening of the muscle fibres.

The extent of rigidity achieved is related with the extent of break down of the A.T.P. With 30% break down of the A.T.P., there is 15% loss of extensibility of the muscles. When break down of A.T.P. is optimum, then loss of extensibility of the muscles is 66%.

The muscles again become soft, during the stage of secondary relaxation, due to decomposition.

Rigor mortis occurs both in the voluntary and in the involuntary muscles. It occurs earlier in the involuntary or smooth muscles, than in the voluntary or striated muscles. In the voluntary muscles, though breakdown of the A.T.P. starts everywhere at a time, rigidity is generally noticed earlier in the smaller group of muscles than in the larger ones. The small muscles of the hands and feet are exceptions, where the rigidity comes last.

Onset of rigor mortis

In the summer season in our country, rigor mortis first appears in the heart muscle, towards the end of the first hour after death. During this season again, among the voluntary muscles, rigor mortis first appears in the muscles of the eye lids (orbicularis oculi), by the end of the first hour after death. Rigidity spreads gradually, within about the next three hours, chronologically, in the muscles of face, neck, jaw, thorax, upper limb (from shoulder to the hand), the abdomen, and lower limb (from the hip to the foot), It comes last in the small muscles of fingers and toes. In summer, after about 12 – 18 hours’ stay, rigor mortis passes off in the order of its appearance. Among the whole body, its stay is maximum in the muscles of the lower limbs.

Period of stay of rigor mortis

In summer, it takes 1 hour to appear and another 3-4 hours for all round distribution. During this season the average period of stay of rigor mortis is between 12 to 18 hours.

In winter, it takes about 2-3 hours to appear and another 4-6 hours for all round distribution, with an average period of stay for about 24 – 48. hours.

In cold countries the above timings are prolonged according to the temperature of the countries.

Some features observed during the period of stay of rigor mortis —

1. Due to rigidity of the heart muscles during rigor mortis, there is emptying of the chambers of the heart.

2. Pupils are constricted due to rigor mortis of the muscles of the iris.

3. There is goose skin appearance of the body due to rigor mortis of the erector pilae muscles.

4. Due to rigor mortis in the muscles of the seminal vesicles, there is post mortem ejaculation of seminal fluid in males.

5. Due to rigor mortis in the muscles of the spermatic cord, the testicles are drawn up near the external rings.

6. Contact flattenning continues during the stage of rigor mortis, over those areas which were in contact with the ground, even though the position of the body might have been changed after the formation of the rigor mortis.

7. Rigor mortis in the uterine muscle cannot expel out the foetus from a gravid uterus.

Factors which influence the formation and stay of rigor mortis —

1. Age – Rigor mortis does not develop in a foetus of less than 7 month intra-uterine age.

2. Physique of the subject – Rigor mortis comes early and passes off early in thinly built subjects with weak musculature. In well-built subjects with strong musculature, it comes late and stays longer.

3. Atmospheric temperature – At high atmospheric temperature rigor mortis comes early and passes off early. In cold atmospheric temperature it comes late and stays longer.

4. In death due to exhaustive disease, or when convulsion precedes death, rigor mortis appears and passes off early.

5. In deaths due to wasting diseases, rigor mortis comes early and passes off early.

6. In death due to strychnine and HCN poisoning, rigor mortis comes early and goes late.

Medicolegal importances of rigor mortis

1. It is a sign of death.

2. During the early phase after death, rigor mortis gives good idea about the time of death. During summer, if the whole body is in the state of primary relaxation then, death must have occurred within the last 1 hour. If rigor mortis is present in the face alone, then death has occurred about an hour back. If it is present in the upper limb and chest and not in the abdomen and lower limb then, death might have occurred about 2-3 hours back. If it is present all around the body then, death might have occurred between 4-18 hours back.

In winter season, the above timings are roughly doubled.

3. From rigid contact flattenning, the position in which the dead body was lying for some hours after death, can be known.

4. Rigor mortis indicates molecular death of the muscle involved.

5. Some conditions occurring in dead bodies may be confused with rigor mortis. These are:
(a) Cadaveric spasm or instantaneous rigor
(b) Heat stiffening of the dead body
(c) Cold stiffening of the dead body
(d) Gas stiffening of the dead body.

(a) Cadaveric spasm or instantaneous rigor — It is a condition in a dead body in which the muscles of the body which were in a state of strong contraction immediately before death, continues to be so contracted at the moment of death and after death, without passing through the stage of primary relaxation. Thus, it is not just stiffening of muscles like rigor mortis. The muscles are in a state of contraction in cadaveric spasm. The spasm involves one or a group of muscles of the body usually, but occasionally the involvement is more extensive and though very very rare, in peculiar circumstances muscles of the whole body may be involved. The spasmodic contraction of the muscles reflect antemortem state of excitement of mind, fatigue, nervous exhaustion etc.

Medicolegal importances — Cadaveric spasm being an antemortem phenomenon in origin, which continues after death, reflects the last act of the subject, performed before and at the time of his death. Thus from the cadaveric spasm, sometimes the cause of death and sometimes the nature of death can be guessed. In case of a dead body, if the hand is found flexed in a state of cadaveric spasm and the grip contains sand, mud, gravel or weed, which are present at the bottom of the pond or lake from where the body was recovered, then it indicates that at the time of death, the subject was at the bottom of the pond or the tank where he might have tightly gripped the bottom at the time of his death. Hence, with such findings in such a dead body, it can be safely deduced that, the person must have died when he was at the bottom of the pond or tank or, in other words, his death was due to drowning in that pond or tank. In a case of death due to stab injury over an approachable vital part of the body, if the weapon used (say a dagger) is found in the tight grip (cadaveric spasm) of the hand and if the position of the weapon in the hand is matching with commission of suicide by using that weapon, then it can be taken that, the person might have committed suicide by using the weapon present in his hand. Similarly, in the hand of the deceased which is flexed in a state of cadaveric spasm, if we find foreign scalp hair, some fibres of a cloth not used by the deceased or some foreign article like button of a shirt, we can reasonably presume that those articles belong to the assailant or one of the many assailants with whom he had a struggle at the time of his death. Thus, from such a case, not only the homicidal nature of the death is concluded, but also some clue about the assailant is available.

(b) Heat stiffening — If death occurs due to 2nd and 3rd degree burn injuries or if a dead body is subjected to exposure to more than 65°C, then there will be coagulation of the muscle protein, and the body will be stiff with contraction of the muscles, in which the flexor muscles take upper-hand, giving rise to a condition which is known as pugilistic or boxer’s or fencing attitude of the body. It is not necessary that, the burning or exposure to heat should be antemortem in occurrence.

Differences between Rigor Mortis and Cadaveric spasm
Points Rigor Mortis Cadaveric spasm
1. Onset Within 1 or 2 hours after death Instantaneous with death.
2. Muscles involved All muscles of the body are affected gradually Selected voluntary muscles which were in a state of contraction at the time of death.
3. Primary flaccidity Precedes R.M. Does not come in the affected muscles.
4. Intensity of rigidity/ contraction Comparatively moderate Comparatively very strong.
5. Death of the muscles Molecular death of muscles occur No molecular death of the muscles.
6. Duration of stay About 12 -18 hours A few hours, until replaced by rigor mortis.
7. Predisposing factor Nil Excitement, fear, fatigue, exhaustion along with contraction of muscles during death.
8. Body temperature Low Comparatively high.
9. Muscle reaction Acidic Alkaline
10. Reaction to stimulus Does not respond Responds
11. Mechanism of formation Break down of A.T.P. below critical level Not known exactly.
12. Medicolegal importance Mostly helps to know the time of death Speaks sometimes about the cause of death and sometime about the nature of death (whether suicidal, homicidal etc.)
Differences between Rigor Mortis and Heat stiffening
Points Rigor Mortis Heat stiffening
1. Degree of stiffness Moderate High.
2. Time of formation 1 or 2 hours to 4 hours after death May be antemortem or postmortem. Formed due to contact with heat.
3. Role of heat Atmospheric high temperature enhances the process Occurs at a temperature above 65°C.
4. Mechanism of formation Due to breakdown of A.T.P. of muscles Due to heat coagulation of muscle protein.
5. External appearance Nothing specific Signs of exposure to heat will be there e.g. burning, blackening, blister formation etc.
Differences between Primary and Secondary relaxation of muscles
Points Primary relaxation Secondary relaxation
1. Time of occurrence Immediately after death After rigor mortis passes off and decomposition occurs.
2. Death of muscle tissue Has not occurred Molecular death of muscles has occurred.
3. Response to stimuli Responds Does not respond.
4. Other external Nothing particular Signs of decomposition are there.
5. Body temperature Nearer to the normal of 98.4 Low body temperature.

(c) Cold stiffening — This occurs when a body remains in extreme cold atmosphere for a reasonable period. Here stiffening occurs due to, (i) freezing of body fluids, particularly at the tissue level and in the synovial sacs of the joints and, (ii) due to hardening of the subcutaneous fatty tissue. It is not difficult to know if the stiffening is due to exposure to cold. If the stiffening is due to cold then, the body temperature will be below 0°C; there will be cracking sound or crepitation, if the joints , are manipulated ; and the body will be flaccid if kept in ordinary atmospheric temperature for some period after which rigor mortis appears.

(d) Gas stiffening occurs during the stage of  Decomposition when the cause of stiffening is very obvious from the discolouration, swelling and foul smell.

Secondly relaxation of muscles – After some hours of stay, rigor mortis or rigidity of the muscles passes over and the body becomes relaxed or flaccid for the second time. This is secondary relaxation or secondary flaccidity of the muscles. In fact, secondary relaxation occurs only with the onset of decomposition or putrefaction of the dead body. During this phase of the dead body, other signs, of putrefaction will be there. Apart from those signs, the reaction of the muscles will again be alkaline due to breakdown of protein with liberation and accumulation of ammonia. The muscles at this phase stand decomposed and they will not respond to any sort of stimulus.