rigor mortis

rigor mortis, physiological process that occurs within hours after death, wherein muscle tissue stiffens. Rigor mortis can be assessed manually by attempting to flex or extend each joint during an autopsy.

Rigor mortis is initially apparent in small muscles, such as the muscles of the hands and face, which become noticeably stiff within three to four hours after death. In some instances, immediate muscle relaxation and rigor mortis-induced fixation upon death causes an open-eyed grimacing facial expression (which usually is not an accurate reflection of the individual’s facial expression in their final moments). Larger muscles are affected later, usually around 12 hours, leading to stiffening of the entire body.

Cause and mechanism

Rigor mortis results from the cessation of breathing and oxygen intake following death, which leads to the termination of aerobic respiration, the main process by which cells generate energy in the form of adenosine triphosphate (ATP). ATP serves a critical role in muscle contraction by binding to a type of filament in muscle tissue known as myosin. Myosin and a second type of filament known as actin slide past one another to generate a muscle contraction. In order for this process to occur, globular headlike regions at the end of myosin filaments attach to binding sites on actin. When ATP attaches to myosin, the ATP molecule is hydrolyzed to form ADP, with the reaction simultaneously inducing a conformational change in myosin that allows the globular head region to release from actin. Myosin is then free to bind to a new site on actin for the generation of another muscle contraction.

In rigor mortis and the absence of ATP, however, myosin is unable to change its conformation to dissociate from actin, and muscles remain in a contracted, stiffened state. Other chemical characteristics of rigor mortis include the accumulation of lactic acid in muscle tissue and postmortem leakage of calcium ions into the sarcomere, where myosin and actin filaments are located. Lactic acid is produced postmortem as still-living muscle cells switch from aerobic to anaerobic metabolism; as lactic acid builds up inside cells during rigor, it causes a decrease in pH levels. Calcium that leaks into the sarcomere binds to the muscle filaments, reinforcing their continued contraction, contributing to the onset and duration of rigor.

Stages

There are six stages of rigor mortis, the names of which reflect the overall process: absent, minimal, moderate, advanced, complete, and passed. In the absent stage, the body is still relaxed, and its parts can be moved into different positions. As rigor mortis progresses from minimal to complete, the body becomes increasingly stiff, with parts remaining in their positions until the passed phase, when the muscles begin to lose their stiffness and liver mortis and decay set in. In general, rigor mortis reaches the passed stage around 36 hours after death.

The point at which a body reaches the passed stage of rigor mortis varies, depending on a range of factors, including the individual’s muscle mass and fitness level, the ambient temperature, the individual’s body temperature, the cause of death, and whether the person engaged in drug use or was experiencing an infection. Low levels of ATP prior to death also can cause variations in the timing of muscle stiffening in rigor; ATP levels may be reduced by factors such as strenuous activity or convulsions.

Use in forensics

Rigor mortis is used to aid in forensic investigations. It is especially useful in estimating time of death, since it is indicative of whether or not a body has been moved. Usually, the position of the body in rigor mortis is the same as that at the time of death, unless it has been disturbed (or moved) by someone or something else.