## Sequence of Rigor Mortis Onset **Key Point:** Rigor mortis follows a characteristic **descending pattern** — it begins in the small muscles and progresses to larger muscle groups. ### Classic Progression Order 1. **Eyelids** (first to show rigor) 2. **Muscles of mastication** (jaw clenches) 3. **Neck muscles** 4. **Trunk and upper limbs** 5. **Lower limbs and feet** (last to become rigid) **High-Yield:** This descending pattern is sometimes remembered as **"eyelids → jaw → neck → trunk → limbs"** and is consistent across most bodies, though individual variation exists. ### Mechanism Rigor mortis develops due to: - Depletion of ATP in muscle cells post-mortem - Actin-myosin cross-bridge formation becomes irreversible - Lactic acid accumulation lowers muscle pH, promoting rigor **Clinical Pearl:** The small muscles of the face and eyelids have lower ATP reserves relative to their mass, making them the first to become rigid. Large muscles of the limbs have greater ATP stores and therefore show rigor later. ### Timeline - **Appearance:** 2–6 hours post-mortem (variable with temperature, physical exertion before death, age) - **Maximum rigidity:** 12 hours - **Disappearance:** 24–48 hours (due to muscle autolysis and decomposition) **Warning:** Do not confuse rigor mortis with **cadaveric spasm** (instantaneous muscle contraction at the moment of death due to extreme heat or electrical injury) — cadaveric spasm is localized and does not follow the descending pattern.
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.