A voluntary muscle consists of bundles of long fibres. Each fibre is formed of densely packed myofibrils are the contractile elements, and are made up of protein filaments of two types, actin filaments and myosin filaments which form a loose physico-chemical combination called actomyosin, which is physically shorter than the two substances uncombined. In the relaxed condition, the actin filaments interdigitate with the myosin filaments only to a small extent. Under the influence of the nerve impulse, the arrays of actin filaments are drawn into the arrays of myosin filaments, rather like pistons into cylinders.They are energy dependent(ATP) This causes the muscle to contract.Rigor persists until decomposition(Absence of energy production ATP) of the proteins of the muscle fibres makes them incapable of any further contraction. The muscles then soften and relax.
The order of appearance of rigor:
All muscles of the body, both voluntary and involuntary are affected. It first appears in involuntary muscles; the myocardium becomes rigid in an hour. It begins in the eyelids, neck and lower jaw and passes upwards to the muscles of the face, and downwards to the muscles of the chest, upper limbs, abdomen and lower limbs. Such a sequence is not constant, symmetrical or regular. In individual limbs, it disappears in the same order in which it has appeared. Rigor mortis always sets in, increases and decreases gradually.
Shapiro  suggests that rigor mortis does not follow the anatomical sequence usually described. He suggests that as rigor mortis is a physico-chemical process, it is most likely to develop simultaneously in all the muscles, although the changes are more easily first detected in the smaller masses than in the larger. The proximo-distal progression is more apparent than real, for the sequence is determined by the bulk and kind of muscle involved. This would explain the fixation of elbow or knee joints at an earlier stage than the shoulder or hip joints, but this does not explain why the small muscles of the fingers and toes should be the last to stiffen.
When rigor is fully developed, the entire body is stiff, the muscles shortened, hard and opaque; knee, hips, shoulders and elbows are slightly flexed and fingers and toes often show a marked degree of flexion. Rigor of erector pilae muscles attached to the hair follicles, may cause roughness, pimpling or goose-flesh appearance of the skin with elevation of the cutaneous hairs, known as cutis anserine or goose skin. the testes may be drown up into the groin; semen may be forced out of the seminal vesicles, and the pupils may be partially contracted. Rarely, if the uterus is in labour at the time of death, the rigor mortis may cause the uterus to contract and expel the foetus.
Rigor is tested by trying to lift the eyelids, depressing the jaw, and gently bending the neck and various joints of the body. Note the degree [complete, partial or absent] and distribution. Before rigor mortis develops, the body can be moved to any posture, and te rigor will fix in that posture. When rigor is developing the extremities can be moved and the rigor, temporarily overcome, develops later and fixes the extremities in their new position, although the rigidity will be less than out symmetrical groups, which have not been disturbed. If force is applied when rigor is fully developed, stiffness is broken up permanently and the rigid muscles may show post-mortem ruptures. Frequent handling of the body breaks the rigor in certain places, leaving a patchy distribution. The contraction of the heart muscle due to rigor mortis should not be mistaken for myocardial hypertrophy. Secondary muscular flaccidity may result in distension of the atria or ventricles, which should not be mistaken for ante-mortem dilatation of the chambers, or myocardial degeneration. Because of these post-mortem changes, it is not possible to determine at autopsy whether a heart has stopped in systole or diastole. The development of rigor is concerned with muscles only. It is independent of the integrity of the nervous system, though it is said to develop more slowly in paralysed limbs.
Time of onset:
In Jamaica, it begins one to 2 hours after death and takes further one to 2 hours to develop. In temperate countries, it begins in 3 to 6 hours and takes further 2 to 3 hours to develop.
Duration of rigor mortis:
In Jamaica, usually it lasts 18 to 24 hours in winter and 12 to 18 hours in summer. It lasts for 2 to 3 days in temperate regions. When rigor sets in early, it passes off quickly and vice versa.
Conditions altering the onset and duration:
Conditions simulating rigor mortis:
The nature of cadaveric spasm is obscure, but like rigor mortis, it may be explained on the basis of diminished or exhausted ATP in the affected muscles. The persistence of contraction after death may be due to the failure of the chemical processes required for active muscular relaxation to occur during molecular death. adrenocortical exhaustion which impairs resynthesis of ATP may be a possible cause. It differs only in the speed of onset and the circumstances in which it occurs.
This condition is of great medico-legal importance. Occasionally, in case of suicide the weapon, e.g. pistol or knife is seen firmly grasped in the victim’s hand which is a strong presumptive evidence of suicide. Attempts may be made to simulate this condition in order to conceal murde3r. but, ordinary rigor does not produce the same firm grip of a weapon, and the weapon may be placed in the had in a way which could not have been used by a suicide. If the deceased dies due to assault, some part of clothing , e.g. button of his assailant or some hair may be firmly grasped in the hands. In case of drowning, material such as grass, weeds or leaves may be found firmly grasped in the hands, which indicates that the victim was alive on the entering the water.
Secondary relaxation: Muscles become soft and flaccid due to breaking down of actomyosin due to putrefaction. The reaction of muscle juice again becomes alkaline.