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

Explanation

## Onset and Spread of Rigor Mortis **Key Point:** Rigor mortis characteristically begins in the smaller muscles of the eyelids, jaw, and neck, then progresses downward to involve the trunk, upper limbs, and finally the lower limbs. **High-Yield:** The sequence is **eyelids → jaw → neck → trunk → upper limbs → lower limbs**. This cephalocaudal (head-to-toe) progression is a cardinal feature of rigor mortis and helps distinguish it from cadaveric spasm, which is instantaneous and localized. ### Mechanism Rigor mortis results from depletion of ATP in muscle cells after death. Without ATP, the myosin heads remain bound to actin filaments, creating sustained muscle contraction. The smaller muscles of the face and jaw have less ATP reserve and deplete first, explaining why rigor begins there. ### Timeline - **Onset:** 2–6 hours post-mortem (variable with temperature, physical exertion, and body composition) - **Peak:** 12 hours - **Resolution:** 24–48 hours (due to muscle autolysis and bacterial decomposition) **Clinical Pearl:** In infants and very young children, rigor mortis may be absent or minimal due to lower muscle mass and different muscle fiber composition.