S. H asked:
What is the "timeframe" for rigor mortis setting in?

On Thursday, July 18, 2002 the Professor responded:

Rigor mortis is the rigidity of the body that occurs after death.

Rigor mortis is Latin for "stiffness of death". It is used medically to describe the stiffness of skeletal muscles that appears soon after death, usually within the first 4 hours. The face tends to stiffen before the hands and feet, and maximal rigor develops in 12 to 48 hours depending on the environmental temperature and other factors (Krompecher et al, 1983). For example, rapid cooling of the body after death can inhibit rigor mortis, but rigor appears rapidly when the body is thawed. For a study of the different rates of onset of rigor mortis in different muscles, see Kobayashi et al (1996).

Rigor mortis sets in as muscle cells run out of the energy substance called ATP (adenosine triphosphate). Even when a person is clinically dead, some cells within their tissues continue to survive for a while. After the circulation of blood ceases, surviving muscle cells resort to anaerobic glycolysis but eventually they become unable to make any more ATP. You will probably recall that in healthy muscle cells ATP is involved in unlocking the cross bridges at the end of the power-stroke and energising them ready for the next contraction. Calcium ions also leak into the compromised muscle cells, moving regulatory proteins away from the molecular cross-bridges between the myofilaments. The myofilaments then become locked in position as a result of these changes, and the skeletal muscles no longer ‘give’ or stretch when parts of the body are moved.

Rigor mortis wears off as the tissues begin to decompose - proteolytic enzymes in the lysosomes of the muscle cells escape and begin to dissolve the myofilaments.

References

1. Kobayashi, M., Takatori, T., Iwadate, K., and Nakajima, M. (1996) Reconsideration of the sequence of rigor mortis through postmortem changes in adenosine nucleotides and lactic acid in different rat muscles. Forensic Science International, 82, 243-253.

2. Krompecher, T., Beregerioux, C., Brandt-Casadevall, C., and Gujer, H.R. (1983) Experimental evaluation of rigor mortis: VI effect of various causes of death on the evolution of rigor mortis. Forensic Science International, 22, 1-9.