Well, at least in part the relative strengths of primates vs humans has been a little exaggerated.For a long time it was kind of assumed that primates were 5-8times stronger. This is now thought to be an exaggerated estimate.That said, chimps for instance are, pound for pound, probably 2-3 times stronger than an equivalent human which stems from a number of causes.
First is body architecture - limb length, leverage, subtle anatomical differences all of which added together mean that primates can generate more power in legs and most especially arms.
Second is most likely the reflex governing fine muscle control. Humans have greater neurological control over muscle response, leading to a kind of damping down reflex, whereas with primates the same reflex becomes much more an "all or nothing" response.
As to the other point of your question - incredible feats of strength by humans is often referred to as an example of "hysterical strength". There have been many instances documented in the media, but few have been scientifically validated, mostly because such events are not reproducible. It would also very probably be unethical :)
Adrenaline does a lot of synergistic things to the body - it improves blood flow to the muscles, and over-rides the normal muscle contraction cap, which in turn grants more strength - but that increased strength is short- lived, as is the adrenaline release triggered by the fight of flight response. Were it not short lived, there would be a very real danger of overstressing muscles, tearing tendons, or even displacing joints.
So feats of hysterical strength are certainly feasible, and have been documented. The level of feat or strength exhibited will be affected by muscle tone - so someone who exercises would likely be able to generate a larger response than those untrained.