why does a chicken go into rigor BEFORE death

[wolfpup]

I've had two chickens in the past that I have found still alive but starting rigor mortise. i.e. they are still alive but stiff as a board.

I am talking two years ago - but am now only asking why sick chickens appear to go into rigor when still alive. I have checked the internet this evening (its late here) to try and avoid this in the future. In future I will end the life rather than let her suffer long enough for a natural death - before I was too much of a coward - now I realize I have to take responsibility for my animals - no matter how I feel - reducing suffering is my goal.

I just want an answer as to WHY a chicken seems to go into rigor before death.