Rigor mortis is the stiffening of muscles after death.
### Why it happens
After death, the body stops producing ATP (energy molecule). Calcium leaks into muscle cells, causing actin and myosin filaments to bind tightly. Without ATP to release the bind, muscles stay locked in a rigid state.
### Typical timeline (at room temperature)
- Starts: 2–6 hours after death (begins in jaw/face).
- Full stiffness: 12–24 hours.
- Lasts: 24–48 hours total.
- Ends: 36–72 hours, as decomposition softens tissues.
### Factors that affect it
- Heat speeds it up; cold slows it down.
- Exercise before death hastens onset.
- Used in forensics to help estimate time of death (along with body cooling and livor mortis).
It’s temporary—muscles eventually become flaccid again.