Immediate stiffness postmoem occurs in -
Cadaveric rigidity
Cadaveric spasm
Rigor mois
Algor mois
Immediate rigor occurs in cadaveric spasm which is contraction of muscles during life which become stiff and rigid immediately after death. Dr. Narayana Reddys Synopsis of Forensic Medicine & Toxicology 27 th edition pg. 81.
Death and postmortem changes