## Onset Timeline of Rigor Mortis **Key Point:** Rigor mortis is a postmortem stiffening of muscles that becomes grossly visible and palpable within **2–6 hours** after death under normal ambient conditions (15–25°C). ### Factors Affecting Onset Speed | Factor | Effect on Rigor Onset | |--------|----------------------| | High ambient temperature | Accelerates (heat increases metabolism) | | Muscular exertion before death | Accelerates (depletes ATP faster) | | Low ambient temperature | Delays (slows biochemical processes) | | Infant or child | May be slower or absent | | Obese individuals | May be delayed | ### Mechanism 1. After death, ATP production ceases 2. Myosin heads remain bound to actin filaments (cross-bridges locked) 3. Muscles become progressively stiff and inextensible 4. Stiffness spreads from smaller to larger muscle groups (jaw → neck → trunk → limbs) **High-Yield:** The **2–6 hour window** is the classic forensic timeline for rigor mortis appearance. This is crucial for estimating the postmortem interval (PMI) in medico-legal cases. **Clinical Pearl:** Rigor mortis is NOT present immediately after death—there is always a lag period (algor mortis and livor mortis may appear first). This distinguishes it from cadaveric spasm, which can occur instantaneously.
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