What Causes Rigor Mortis?

Chemistry of Muscle Fibers in Rigor Mortis

Rigor mortis can be used to provide clues to time of death.
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A few hours after a person or animal dies, the joints of the body stiffen and become locked in place. This stiffening is called rigor mortis. It's only a temporary condition. Depending on temperature and other conditions, rigor mortis lasts approximately 72 hours. The phenomenon is caused by the skeletal muscles partially contracting. The muscles are unable to relax, so the joints become fixed in place.

The Role of Calcium Ions and ATP

More specifically, what happens is that the membranes of muscle cells become more permeable to calcium ions. Living muscle cells expend energy to transport calcium ions to the outside of the cells. The calcium ions that flow into the muscle cells promote the cross-bridge attachment between actin and myosin, two types of fibers that work together in muscle contraction. The muscle fibers ratchet shorter and shorter until they are fully contracted or as long as the neurotransmitter acetylcholine and the energy molecule adenosine triphosphate (ATP) are present. However, muscles need ATP in order to release from a contracted state (it is used to pump the calcium out of the cells so the fibers can unlatch from each other). When an organism dies, the reactions that recycle ATP eventually come to a halt. Breathing and circulation no longer provide oxygen, but respiration continues anaerobically for a short time.

ATP reserves are quickly exhausted from the muscle contraction and other cellular processes. When the ATP is depleted, calcium pumping stops. This means that the actin and myosin fibers will remain linked until the muscles themselves start to decompose.

How Long Does Rigor Mortis Last?

Rigor mortis can be used to help estimate time of death.

Muscles function normally immediately after death. The onset of rigor mortis may range from 10 minutes to several hours, depending on factors including temperature (rapid cooling of a body can inhibit rigor mortis, but it occurs upon thawing). Under normal conditions, the process sets in within four hours. Facial muscles and other small muscles are affected before larger muscles. Maximum stiffness is reached around 12-24 hours post mortem. Facial muscles are affected first, with the rigor then spreading to other parts of the body. The joints are stiff for 1-3 days, but after this time general tissue decay and leaking of lysosomal intracellular digestive enzymes will cause the muscles to relax. It is interesting to note that meat is generally considered to be more tender if it is eaten after rigor mortis has passed.


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