A 52-year-old man is found dead in his apartment after 8 hours. On examination, rigor mortis is observed. Which muscle group shows the earliest onset of rigor mortis?

Explanation

## Rigor Mortis: Onset and Progression **Key Point:** Rigor mortis follows a characteristic cephalocaudal (head-to-toe) progression, beginning in the smaller muscles and advancing to larger muscle groups. ### Sequence of Onset | Stage | Muscle Group | Timeline | |-------|--------------|----------| | **First** | Eyelids, jaw, neck | 2–6 hours | | **Second** | Trunk, upper limbs | 6–12 hours | | **Third** | Lower limbs | 12–24 hours | **High-Yield:** The **muscles of the eyelids and jaw** are the smallest and most metabolically active, depleting ATP fastest. They are invariably the first to stiffen. ### Mechanism 1. After death, ATP production ceases. 2. Myosin-actin cross-bridges remain locked (no ATP to break them). 3. Smaller muscles with higher metabolic rate exhaust ATP first. 4. Rigor then spreads to progressively larger muscle groups. **Clinical Pearl:** The presence of rigor in the eyelids and jaw but absence in the limbs suggests death occurred 2–6 hours prior to examination. This helps in **time-of-death estimation** in forensic investigations. **Mnemonic:** **"Small First, Large Last"** — rigor follows the rule of smallest-to-largest muscles, not anatomical proximity. ### Factors Affecting Onset - **Accelerated rigor:** High ambient temperature, muscular exertion before death, fever, convulsions. - **Delayed rigor:** Cold environment, cachexia, sepsis. - **Absent rigor:** Decomposition, putrefaction (rigor is masked).