Rigor mortis and cadaveric spasm
- Finding
Hours after death the muscles stiffen because the fibres run out of energy, and the stiffness passes off later.
What it is
Rigor mortis is the stiffening of muscles after death. After an initial phase of relaxation, muscle ATP runs out and actin and myosin stay bound, so the muscle cannot be stretched. It is noticed first in the small muscles of the face, eyelids and jaw, then the neck, upper limbs and lower limbs. On average it appears 1.5 to 4 hours after death, becomes generalised between 6 and 12 hours and passes off later, very variably, as decomposition begins. Cadaveric spasm is immediate stiffening without a relaxation phase, described in deaths during intense exertion or emotion; it is rare and its existence is debated.
How to tell it apart
- Rigor vs cadaveric spasm: rigor appears after hours of relaxation; spasm would be immediate and limited to one muscle group.
- Rigor vs heat stiffening: heat coagulates proteins, shortens muscles and produces the pugilistic posture of burned bodies.
- Rigor vs cold stiffening: fat and joint fluid solidify; it disappears when the body is warmed.
- The posture fixed by rigor does not prove the position at death: the body may have been moved during the relaxation phase.
How to recognise it
- Resistance to passive movement of the joints: jaw, elbows, knees, fingers.
- Graded by region: absent, partial or complete, and whether it is the same on both sides.
- Rigor broken by force in the first hours can re-form, though weaker.
- In cadaveric spasm an object (weapon, grass, clothing) is held firmly in the hand.
How it forms
- After death the muscle keeps using ATP but no longer makes it.
- Without ATP, and in an acid environment, the cross-bridges between actin and myosin do not release: the muscle stays rigid.
- Small muscles show it earlier because of their small mass, not because the process starts earlier.
- Rigor passes off when autolysis and putrefaction break down muscle proteins.
Next step: what to do and document
- Examine and record rigor by region (jaw, upper limbs, lower limbs), its degree and whether it is symmetrical, with the time.
- Record the ambient temperature and any exertion, fever or seizures before death.
- If an object is firmly held in the hand, photograph and describe it before removing it, without forcing the fingers.
- Do not break rigor unnecessarily before the inspection, and record if and where it was broken.
- Combine rigor with cooling and lividity and express the time as a range.
Common mistakes
- Using rigor as an exact clock: its pace varies widely with temperature and prior activity.
- Calling ordinary rigor cadaveric spasm and concluding that the victim was holding the weapon.
- Mistaking the heat-induced pugilistic posture for defence or struggle before death.
- Not recording that rigor was broken while handling the body.
Test yourself
Think of the answer, then tap the question to see it.
What is the biochemical mechanism of rigor mortis?
Depletion of muscle ATP: without it, actin and myosin stay bound and the muscle cannot relax.
Why is rigor noticed first in the jaw and eyelids?
Because they are small muscles in which rigor becomes evident earlier, although the process occurs in all muscles.
What distinguishes cadaveric spasm from rigor mortis?
Spasm would be immediate, with no relaxation phase, and limited to one muscle group; rigor appears hours later and becomes generalised.
What factors speed up the onset of rigor?
Heat, fever, heavy exertion, seizures and electrocution before death, because they deplete muscle ATP.
Connections
Educational content on forensic and legal medicine; not legal advice and no substitute for the technical guides of Colombia's National Institute of Legal Medicine and Forensic Sciences. Laws change: each page links the official text and says when it was last checked. For a real case, follow the guide in force and ask the competent authority.
Sources: Accelerated rigor mortis: A case letter (Mesri M, Behzadnia MJ, Dorooshi G) (Journal of Research in Medical Sciences, 2017); Development of a mobile application to estimate time of death based on the compound method (Zirn A et al.) (International Journal of Legal Medicine, 2026); Current Understanding and Future Research Direction for Estimating the Postmortem Interval: A Systematic Review (Strete G et al.) (Diagnostics (MDPI), 2025).
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