Nysten's Law states that rigor mortis begins with which muscles?

• A. The muscles of mastication
• B. The abdominal muscles
• C. The leg muscles
• D. The pectoral muscles

Answer: (A)

Rigor mortis follows a predictable pattern because muscles with rapid energy use and small size lose ATP quickly after death, causing the cross-bridges between actin and myosin to lock first. The muscles of mastication are small, highly active in life, and rely on fast energy turnover, so they exhaust ATP soon after death and stiffen earliest. The other large muscle groups—abdominal, leg, and pectoral—have greater mass and different activity profiles, so they tend to stiffen later as rigor progresses from head toward the rest of the body. So, the earliest onset of rigor mortis occurs in the muscles of mastication.