Rigor mortis Shortly after death all the muscles in the body become soft and flaccid. At a variable time later, they become firm and rigid. This is known as rigor mortis. Rigor commences in the smallest muscles such as those in the face and the hands, and then extends to the limb muscles. Rigor can be ‘broken’ by stretching the muscle, for example by moving the jaw or the elbow, and does not then return.

Rigor is brought about by a chemical change in the muscle. The normal reaction between adenosine triphosphate and adenosine diphosphate (ATP and ADP) within the muscle fibres, which supplies energy for their contraction during life, ceases and the ATP level in the muscle progressively diminishes. This is accompanied by accumulation of lactic acid and a fall of pH (increase in acidity), which leads to stiffening and firmness. Whether or not the muscle fibres actually shorten has not been established.

Temperature is an important factor in determining the time of onset of rigor. In normal circumstances and at room temperature rigor is complete in about three to six hours. If the temperature is higher the onset is more rapid — perhaps no more than an hour in tropical temperatures. Conversely, the onset of rigor is delayed at low temperatures. In cases of drowning in cold water, for example, rigor may not appear until the body has been removed from the water, even after several days of immersion. The onset of rigor is hastened if there has been intense physical activity shortly before death. Thus, in forensic medical practice, the presence of rigor is a poor determinant of the time of death. Once established, the duration of rigor ranges from 18 to 36 hours.

 http://www.encyclopedia.com/medicine/diseases-and-conditions/pathology/rigor-mortis

I agree with  Dr Mushtaq

Dear Dr. Haifa 

The Rigor mortis is affected by environmental temperature, internal body temperature and decedent activity prior to death. Elevated temperature will accelerate the appearance of rigor mortis.

Ph also affect the rigor mortis 

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t the time of death, a condition called "primary flaccidity" occurs. Following this, the muscles stiffen in rigor mortis. All muscles in the body are affected. Starting between two and six hours following death, rigor mortis begins with the eyelids, neck, and jaw. The sequence may be due to different lactic acid levels among different muscles, which is directly related to the difference in glycogen levels and different types of muscle fibers.

Rigor mortis then spreads to the other muscles, including the internal organs, within the next four to six hours. The onset of rigor mortis is affected by the individual's age, sex, physical condition, and muscular build. Rigor mortis may not be perceivable in many infant and child corpses due to their smaller muscle mass.

llo 

The two factors concerned have been varied experimentally. Activity

preceding death has been controlled by injection of myanesin which causes

complete muscular relaxation and abolishes the death struggle (Berger &

Bradley, 1947; Bate-Smith & Bendall, 1947b). The magnitude of the glycogen

reserve has been varied: (a) by injection of insulin in convulsan-t doses, which

leads in the extreme limit to complete exhaustion of glycogen; (b) by reducing

or increasing the intake of food. These various treatments appear to affect the

post-mortem behaviour of the muscle only in so far as they affect the factors

mentioned, and it is therefore unnecessary in analysing the results to separate

the animals treated in a particular way from the animals not so treated. The

results, at the given temperatures, form homogeneous groups.

For the most part, observations have been made at room temperature.(17° C.

approximately).

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