Rigor mortis begins earliest in which muscle group?

Explanation

## Onset Pattern of Rigor Mortis **Key Point:** Rigor mortis follows a predictable anatomical sequence, beginning in smaller muscles and progressing to larger ones. ### Sequence of Appearance 1. **Eyelids and jaw** — first affected (smallest muscles, highest metabolic rate) 2. **Neck and trunk muscles** — follow within 2–6 hours 3. **Large limb muscles** — affected last 4. **Reverse order of disappearance** — rigor resolves in the same sequence it appeared ### Mechanism **High-Yield:** Rigor mortis develops due to depletion of ATP in muscle cells after death. Without ATP, myosin heads remain bound to actin filaments, causing muscle stiffening. Smaller muscles with higher metabolic demand deplete ATP first, hence they stiffen first. ### Clinical Correlation **Clinical Pearl:** The onset and progression of rigor mortis can be used to estimate the **post-mortem interval (PMI)**, though it is influenced by ambient temperature, physical exertion before death, and body composition. | Feature | Timeline | |---------|----------| | Onset in eyelids/jaw | 2–6 hours | | Complete rigor in trunk/limbs | 8–12 hours | | Maximum rigidity | 12–24 hours | | Onset of decomposition (rigor disappears) | 3–5 days | **Mnemonic:** **JETTISON** — **J**aw, **E**yes, **T**runk, **T**highs, **I**nternal organs, **S**maller muscles first, **O**nset predictable, **N**ot simultaneous.