What principle accounts for legs stiffening last in rigor mortis?

• A. Nysten's law
• B. Hansen's principle
• C. Newton's law
• D. Boyle's law

Answer: (A)

The progression of rigor mortis from smaller muscles to larger muscles explains why the legs stiffen last. After death, ATP production halts, so muscles can’t relax and cross-bridges form permanently. How fast and where rigor appears depends on muscle size and activity before death, with smaller, more active muscles like those in the face and jaw stiffening first and larger muscle groups, such as those in the legs, stiffening later. This pattern is described by Nysten’s law, which states that rigor mortis develops in a head-to-toe, or smaller-to-larger, sequence. The legs are last because they consist of large, thick muscle groups that take longer for the stiffening process to propagate through. Other choices refer to physical laws that don’t govern the sequence of rigor mortis.