Rigor mortis occurs primarily due to ATP depletion in muscle cells following death. ATP is essential for detaching myosin heads from actin filaments during muscle relaxation. After death, ATP synthesis ceases, preventing the myosin-actin cross-bridges from breaking, leading to sustained muscle contraction and stiffness. This process begins within 2-4 hours postmortem and resolves as muscle proteins degrade. Understanding rigor mortis is crucial in forensic medicine for estimating time of death.

Reference: Robbins Basic Pathology, Kumar et al., 10th Edition.