Onset of rigor mortis may start from 15 minutes to 15 hours after death, but as a general rule, it starts ________ hours after death.

• A. 1-2
• B. 3-4
• C. 7-8
• D. 5-6

Answer: (D)

Rigor mortis happens because after death the body no longer has ATP to detach actin and myosin cross-bridges in muscle fibers. With ATP depleted, calcium remains in the cells and the muscles lock into a stiff state. In a typical case at normal room temperature, this stiffness begins a few hours after death and is usually noticeable around five to six hours, spreading from small muscles to larger ones. It often peaks around about 12 hours and then fades as decomposition progresses, with temperature playing a big role: warmer conditions speed onset and progression, cooler conditions slow it down. That’s why five to six hours is the general rule for when rigor mortis starts.