Rigormortis

Muscle rigidity can manifest without neural stimulation, a condition known as rigor or physiological contracture, differing from typical muscle contraction. In this state, actin and myosin filaments are locked together due to an insufficient supply of ATP necessary for muscle relaxation. Postmortem, this lack of ATP leads to rigor mortis, where muscles remain rigid until lysosomal enzymes initiate decomposition after 12-24 hours. Muscles that were most active prior to death exhibit rigor mortis first due to their greater ATP depletion. Once death occurs, ATP synthesis ceases, preventing any further muscle relaxation.