Rigor mortis typically begins to appear in which muscle group first?

Explanation

## Onset Pattern of Rigor Mortis **Key Point:** Rigor mortis does NOT begin simultaneously in all muscles. It follows a predictable sequence that is clinically important for time-of-death estimation. ### Typical Sequence of Appearance 1. **Eyelids** — usually the first site (within 2–12 hours post-mortem) 2. **Muscles of mastication (jaw)** — closely follows eyelid involvement 3. **Neck muscles** 4. **Trunk muscles** 5. **Large limb muscles** — last to be affected **High-Yield:** The sequence is **cephalocaudal** (head to toe), progressing from smaller muscles to larger muscle groups. This is sometimes remembered as the "**Nysten's Law**" — rigor spreads in a predictable anatomical order. ### Why This Sequence Occurs - Smaller muscles with higher metabolic rates deplete ATP more rapidly - Muscles with greater innervation density are affected earlier - The process is driven by ATP depletion and calcium-mediated actin–myosin cross-linking **Clinical Pearl:** When examining a body for time-of-death estimation, the **absence of rigor in the eyelids and jaw** in a body that has been dead for >12 hours should raise suspicion for either very early death or post-mortem disturbance (e.g., decomposition, heat exposure). ### Timeline Reference | Time Post-Mortem | Expected Rigor Pattern | |---|---| | 0–2 hours | Absent | | 2–12 hours | Eyelids, jaw, neck | | 12–24 hours | Trunk and proximal limbs | | 24–48 hours | Complete rigidity | | 48+ hours | Begins to disappear (cadaveric spasm may persist) | [cite:Parikh Textbook of Forensic Medicine Ch 4]