Rigor mortis typically begins in which muscle group and follows which sequence?

Explanation

## Onset and Spread of Rigor Mortis **Key Point:** Rigor mortis follows a characteristic anatomical sequence that is crucial for forensic assessment. ### Sequence of Appearance Rigor mortis typically appears in the following order: 1. **Eyelids** — first affected (within 2–4 hours post-mortem) 2. **Muscles of mastication** (jaw muscles) 3. **Neck muscles** 4. **Trunk and upper limbs** 5. **Lower limbs and feet** — last affected This progression is sometimes remembered as **descending order** — from head to toe. ### Mechanism The sequence reflects: - **Muscle mass and metabolic rate**: Smaller muscles with higher metabolic activity (facial muscles, jaw) stiffen first - **ATP depletion**: Muscles with greater oxidative capacity deplete ATP faster, leading to earlier actin–myosin cross-bridge fixation - **Temperature gradient**: Core muscles may cool slower, affecting the timing of rigor onset ### Timeline | Time Post-Mortem | Location | Notes | | --- | --- | --- | | 2–4 hours | Eyelids, jaw, neck | Earliest signs | | 4–8 hours | Trunk, upper limbs | Progressive spread | | 8–12 hours | Lower limbs, feet | Complete rigor | | 24–48 hours | All muscles | Maximum rigidity | | 48–72 hours onwards | Variable | Begins to disappear (rigor mortis lysis) | **High-Yield:** The **descending (cephalocaudal) pattern** is the classic teaching point and is frequently tested in forensic exams. **Clinical Pearl:** Deviation from this expected sequence may suggest ante-mortem muscle activity, heat exposure, or pathological conditions (e.g., tetanus, strychnine poisoning). [cite:Park 26e Ch 3]