<P> Normal relaxation would occur by replacing ADP with ATP, which would destabilize the myosin - actin bond and break the cross-bridge . However, as ATP is absent, there must be a breakdown of muscle tissue by enzymes (endogenous or bacterial) during decomposition . As part of the process of decomposition, the myosin heads are degraded by the enzymes, allowing the muscle contraction to release and the body to relax . Decomposition of the myofilaments occurs 48 to 60 hours after the peak of rigor mortis, which occurs approximately 13 hours after death . </P> <P> At the time of death, a condition called "primary flaccidity" occurs . Following this, the muscles stiffen in rigor mortis . All muscles in the body are affected . Starting between two and six hours following death, rigor mortis begins with the eyelids, neck, and jaw . The sequence may be due to different lactic acid levels among different muscles, which is directly related to the difference in glycogen levels and different types of muscle fibers . </P> <P> Rigor mortis then spreads to the other muscles, including the internal organs, within the next four to six hours . The onset of rigor mortis is affected by the individual's age, sex, physical condition, and muscular build . Rigor mortis may not be perceivable in many infant and child corpses due to their smaller muscle mass . </P> <P> Rigor mortis is very important in meat technology . The onset of rigor mortis and its resolution partially determine the tenderness of meat . If the post-slaughter meat is immediately chilled to 15 ° C (59 ° F), a phenomenon known as cold shortening occurs, whereby the muscle sarcomeres shrink to a third of their original length . </P>

Why is rigor mortis not a permanent condition in cadavers