What are the stages of rigor mortis and their typical post‑mortem time intervals?

Stages of Rigor Mortis

Rigor mortis progresses through three distinct phases—onset, maximum stiffness, and resolution—with timing heavily dependent on ambient temperature, typically completing within 6-28 hours at room temperature but persisting much longer in cold environments.

Three Primary Phases

Phase 1: Onset and Development

• Rigor mortis begins approximately 2-6 hours post-mortem at room temperature (24°C), starting in smaller muscle groups and progressing to larger muscles 1, 2
• At 37°C (body temperature), onset occurs as early as 3 hours post-mortem, while at 6°C it may not begin until 48-60 hours post-mortem 2
• The intensity of rigor increases progressively during this phase, with measurements showing a characteristic rising curve when monitored serially 1
• If increasing intensity is observed on repeated examination, death occurred no more than 5 hours prior to initial assessment 1

Phase 2: Maximum Rigidity (Plateau)

• Full development occurs at 5 hours post-mortem at 24°C, 3 hours at 37°C, or 48-60 hours at 6°C 2
• During this plateau phase, the body exhibits maximum stiffness with no further increase in rigidity 1
• This phase represents complete ATP depletion in muscle fibers, preventing actin-myosin dissociation 1

Phase 3: Resolution

• Resolution begins at 16 hours post-mortem at 24°C, 6 hours at 37°C, or 168 hours (7 days) at 6°C 2
• If only decreasing intensity is observed, death occurred at least 7 hours prior to initial assessment 1
• At 24 hours post-mortem at room temperature, resolution is typically complete and no further changes occur 1
• If a body is discovered with no rigor mortis present, death either occurred very recently (<2 hours) or more than 24 hours ago 1

Critical Temperature Effects

• Higher temperatures accelerate all phases: at 37°C, the entire cycle completes in approximately 6 hours versus 16 hours at 24°C 2
• Cold environments dramatically prolong rigor: at constant refrigeration (4°C), complete rigor can persist for 10-16 days, with partial rigor lasting up to 28 days 3
• Cold temperatures also produce "cold rigidity," a separate phenomenon independent of true rigor mortis 2

Re-establishment Phenomenon

• Mechanical loosening of rigor followed by re-establishment can occur up to 20 hours post-mortem, contradicting older teaching that limited this to 8-12 hours 4
• In 21% of cases, rigidity at re-establishment equals or exceeds the original degree, contrary to traditional doctrine 4
• This phenomenon appears independent of body or ambient temperature 4

Practical Forensic Algorithm

For estimating time since death using rigor mortis:

1. If rigor is increasing on serial examination → death occurred ≤5 hours ago 1
2. If rigor is at maximum (plateau) → death occurred 5-8 hours ago at room temperature 1, 2
3. If rigor is decreasing only → death occurred ≥7 hours ago 1
4. If rigor is completely absent → death occurred either <2 hours or >24 hours ago 1
5. If ambient temperature is cold (near 4°C) → extend all timeframes dramatically; complete rigor may persist 10+ days 3

Common Pitfalls

• Never rely on rigor mortis alone for time-of-death estimation, as individual variation, physical activity before death, and environmental factors create substantial variability 1, 4
• Do not assume rigor always weakens upon re-establishment—it may equal or exceed original intensity 4
• Account for temperature meticulously—a body found outdoors in winter may show complete rigor 2+ weeks post-mortem 3
• Serial measurements are far superior to single assessments for accurate time-since-death estimation 1