What is the primary cause of rigor mortis?

Rigor mortis occurs primarily due to a lack of ATP in muscle cells after death. When an organism dies, cellular functions cease, including the production of ATP, which is essential for muscle relaxation. In living muscles, ATP binds to myosin, allowing it to release actin and detach from muscle fibers. Without ATP, myosin heads remain attached to actin, causing the muscle fibers to stay contracted and leading to the stiffening of the body associated with rigor mortis.

As the body undergoes decomposition, the ATP levels drop to zero, maintaining the contraction in a locked state until proteolytic enzymes eventually break down the muscle proteins. Hence, the absence of ATP is the critical factor that initiates this process, making it the primary cause of rigor mortis.