Longer rigor mortis occurs with -
Prolonged rigor mortis occurs with strychnine.
Note : There is controversy about the duration of rigor mortis in deaths due to strychnine poisoning. According to some textbooks, rigor mortis starts early and stays longer, while other textbooks says that it has rapid onset and passes of early.
Thus, i have to read so many textbooks for this and i came to the conclusion that each and every textbook is correct. Both can occur in strychnine poisoning :-


If death occurs with violent contractions (during convulsions) for some time prior to death, it causes complete exhaustion → Rapid onset and short duration of rigor mortis.
If death is almost instantaneous, with a very short convulsive phase, there is no exhaustion →3 Rapid onset and longer duration of rigor mortis.


In both conditions, onset is early (rapid). It is the only duration which varies.
Vegetable poisons
Strychnine
Opium
Septicemia