Rigor mortis was a stiffening process that occurred after death.
The Fourth Doctor was able to tell that Reuben had been dead for hours due to the extent to which rigor mortis has set in to his corpse. (TV: Horror of Fang Rock)
Owen Harper was put on an exercising regiment after being resurrected in an undead state during the Duroc crisis to ensure that rigor mortis didn't set in. (TV: A Day in the Death)