Rigor mortis

Rigor mortis is one of the recognizable signs of death (Latin mors, mortis) that is caused by a chemical change in the muscles after death, causing the limbs of the corpse to become stiff (Latin rigor) and difficult to move or manipulate. Eerily, during rigor mortis, muscles may contract in seemingly random and bizarre ways even though the body is dead.

Biochemistry
After a muscle contracts, ATP expenditure is required to release the myosin head of a thick filament from its binding site on the thin filament. Since all metabolic processes have come to a halt in a dead body, no ATP is being produced. Therefore, because of a lack of ATP, the myosin head cannot be released from the actin filament, and the sarcomere cannot relax. Because this happens in muscles all over the body, they become "stiff" and "locked" into place.

To understand the contractions associated with rigor mortis, it's important to know that ATP is required to reuptake calcium into the sarcomere's sarcoplasmic reticulum. Additionally, when a muscle is relaxed, the myosin heads are returned to their "high energy" position, ready and waiting for a binding site on the actin filament to become available. Because there is no ATP available, previously released calcium ions cannot be reuptaken into the SR. These leftover calcium ions move around inside the sarcomere and may eventually find their way to a binding site on the thin filament's regulatory protein. Since the myosin head is already ready to bind, no addition ATP expenditure is required and the sarcomere contracts. When this process occurs on a larger scale, the disturbing twitches and gruesome postures associated with rigor mortis can occur.

After about 72 hours, the body relaxes again, this time as a result of decomposition. This is known as resolution of rigor.

Rigor mortis and the meat industry
Rigor mortis is very important in meat technology. The onset of rigor mortis and its resolution partially determines the tenderness of meat. If the post-slaughter meat is immediately chilled to 15 °C, a phenomenon known as cold shortening occurs, where the muscle shrinks to a third of its original size. This will lead to the loss of water from the meat along with many of the vitamins, minerals, and water soluble proteins. The loss of water makes the meat hard and interferes with the manufacturing of several meat products like cutlet and sausage.

Cold shortening is caused by the release of stored calcium ions from the sarcoplasmic reticulum of muscle fibers in response to the cold stimulus. The calcium ions trigger powerful muscle contraction aided by ATP molecules. To prevent cold shortening, a process known as electrical stimulation is carried out, especially in beef carcass, immediately after slaughter and skinning. In this process, the carcass is stimulated with alternating current, causing it to contract and relax, which depletes the ATP reserve from the carcass and prevents cold shortening.