According to Nysten's law, rigor mortis begins with which muscle group?

• A. The muscles of mastication
• B. The calf muscles
• C. The back muscles
• D. The abdominal muscles

Answer: (A)

Rigor mortis follows a predictable sequence after death, tied to how quickly muscles lose ATP and become unable to detach the actin-momyosin cross-bridges. In this pattern described by Nysten, stiffness starts in the muscles that were most active before death—the muscles of mastication. The jaw muscles are heavily used and have high metabolic demand, so they exhaust their ATP sooner and stiffen first. From there, rigor spreads to other muscle groups in a typical progression (neck, then trunk or upper limbs, and finally the lower limbs). So the muscles of mastication are the earliest to become rigid, which is why that option is correct. The calf, back, and abdominal muscles typically stiffen later in the process.