## Post-mortem Rigidity (Rigor Mortis) **Key Point:** Rigor mortis is the stiffening of muscles after death due to depletion of ATP and accumulation of lactic acid, causing actin–myosin cross-bridges to lock. ### Timeline of Rigor Mortis Development | Phase | Interval | Features | |-------|----------|----------| | Onset | 2–6 hours | Begins in small muscles (eyelids, jaw) | | Full establishment | 12–24 hours | Involves entire body; maximum rigidity | | Resolution | 24–48 hours | Begins as putrefaction progresses | | Complete loss | 48–72 hours | Muscles soften due to decomposition | **High-Yield:** The progression of rigor mortis follows a predictable sequence: **eyelids → jaw → neck → trunk → limbs → lower extremities**. This sequence is useful in estimating time since death in the early post-mortem period. **Clinical Pearl:** Rigor mortis is NOT instantaneous; its absence in the first 2–6 hours does not rule out recent death. Conversely, its presence does not indicate a specific time window beyond 2–6 hours to 48–72 hours. **Mnemonic:** **RIGOR** = **R**igidity **I**s **G**radual **O**nset **R**eaching peak at 12–24 hours. ### Factors Affecting Rigor Mortis Timing - **Accelerate onset:** High ambient temperature, muscular exertion before death, fever, sepsis - **Delay onset:** Low ambient temperature, starvation, cachexia, electrolyte depletion - **Cadaveric spasm:** Instantaneous rigidity of isolated muscle groups (rare; associated with violent death or extreme emotion) **Warning:** Do not confuse cadaveric spasm (instantaneous, localized) with true rigor mortis (gradual, generalized).
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