Cardiac arrest and people going stiff?

Well it has been my experience that no cardiac arrest goes as in the text book. I read page 456 and as I am sure, much study and expertise went into its writing, in the real world events I have witnessed every patient reacts different. I have been an ems provider for about 15 years and can not recall seeing "tonic Posturing". However I probably am not the most observant person to this phenomenon as I am trying to fix the problem rather than to observe it. I do see the book references the fact that the rate of onset and severity influence this, I would also like to ad the age, physical condition and past medical history are relevant.