Why does a dead body go board-stiff for two or more days? The easiest answer boils down to this: A biochemical chain reaction that causes a living person's muscles to move stops working when someone dies. When the reaction stops, the muscles become locked in place.
You have to look deep inside muscle cells to find a more detailed explanation. At the microscopic level, skeletal muscle fibers -- the ones that attach to your bones -- have two main parts:
- Thick filaments, made of the protein molecule myosin
- Thin filaments, made of the protein molecule actin
The fibers include other proteins as well, but actin and myosin are at the heart of rigor mortis.
When you lift a weight or scratch your head, a nerve impulse sets off a biochemical reaction that causes myosin to stick to actin. These two molecules lock together, pulling the muscle's thick and thin filaments toward each other. When thousands of filaments pull together all at once, over and over, you have a muscle contraction. You can read more about all the steps of this process in How Muscles Work.
Once the actin and myosin molecules stick together, they stay that way until another molecule, adenosine triphosphate (ATP), attaches to the myosin and forces it to let go. Your body uses the oxygen you breathe to help make ATP. That oxygen supply ends, of course, with death. Without ATP, the thick and thin filaments can't slide away from each other. The result is that the muscles stay contracted -- hence rigor mortis.
During rigor mortis, another process called autolysis takes place. This is the self-digestion of the body's cells. The walls of the cells give way, and their contents flow out. Rigor mortis ends not because the muscles relax, but because autolysis takes over. The muscles break down and become soft on their way to further decomposition.
Although this helps explain why rigor mortis comes and goes, it's the outward appearance -- the relative stiffness of the body -- rather than the process that's of most interest to investigators. Next, we'll explore how the gradual progression of rigor mortis plays a part in solving crimes.