In what order do the muscles appear during rigor mortis?

During rigor mortis, the order in which muscles stiffen reflects their metabolic and structural characteristics, which tend to influence the onset of stiffness. After death, the body experiences a depletion of ATP (adenosine triphosphate), which is necessary for muscle relaxation. As ATP levels fall, the binding of actin and myosin in muscle fibers occurs, leading to the characteristic rigidity associated with rigor mortis.

The orbicularis oculi, a muscle surrounding the eye, is typically the first to show signs of rigor mortis. This is because the smaller muscle groups often contract more quickly and are more influenced by the lack of ATP. Following this, the sternocleidomastoid, a larger neck muscle, will stiffen next.

Next in the sequence is the diaphragm, which plays a crucial role in respiration. It is larger and more complex than the previous muscles but is still affected relatively quickly after they become rigid. The gastrocnemius, a major calf muscle, is usually the last to undergo rigor mortis due to its larger mass and slower metabolic processes.

Consequently, the correct order of muscle stiffness—starting from the muscles that are least massive and most metabolically active to those that are larger and slower—align