The postmortem stiffening of the muscles by natural processes.
Rigor results in the muscles inability to re-synthesize adenosine triphosphate (ATP).
Once rigor has passed (or is broken up) it does not reoccur.
“Rigor when it develops involves all the muscles at the same time and at the same rate. However becomes most evident in the smaller muscles.”
Rigor may seem to develop in smaller muscles first; moving to larger muscles last. The eyes, jaw, face, neck…Then the upper extremities, trunk, and lower extremities.
This directional occurrence of rigor from face to feet is
historically known as Nysten’s Law. Named after Pierre Nysten (1744-1817) a French pediatrician.