You hear about cases every so often: a mother lifting a car to rescue a pinned child, performing a feat of strength not usually available to humans. Evidence is almost always anecdotal -- like when a man in Arizona lifted a car to free a cyclist who'd been hit and dragged in 2006, witnessed only by those involved in the accident -- but these things happen so rarely (if they do indeed happen at all) that it's difficult to observe them under repeatable, scientific circumstances. But while proof of the events themselves remains hearsay, there is a theory, at least, among researchers that such things are at least possible.
It's all about adrenaline. Some believe that on a day-to-day basis we only use a small percentage of our muscles' capability. But adrenaline and noradrenaline, hormones that creates a state of fight-or-flight readiness in stressed humans, have the capability to raise the heart rate, dilate the pupils, increase respiration, slow digestion and, yes, allow muscles to contract more than they would normally. Here's a more technical blow-by-blow from Discovery Health:

When adrenaline is released by the adrenal medulla -- an interior region of the adrenal glands, which are located just above your kidneys -- it allows blood to flow more easily to your muscles. This means that more oxygen is carried to your muscles by the extra blood, which allows your muscles to function at elevated levels. Skeletal muscles -- those attached to bones by tendons -- are activated by electrical impulses from the nervous system. When they're stimulated, muscles contract, meaning they shorten and tighten. This is what happens when you lift an object, run or throw a punch. Adrenaline also facilitates the conversion of the body's fuel source (glycogen) into its fuel (glucose). This carbohydrate gives energy to muscles, and a sudden burst of glucose also allows muscles to strengthen further.

So does that mean it's possible to perform superhuman feats of strength? The evidence is frustratingly thin here -- but there's one example of our muscles' amazing capabilities that may point to the possibility of hysterical strength, and that's what happens when our bodies are shocked with electricity. You've heard of people being thrown across rooms by shocks, or their hands clamping down on live wires so hard they can't be loosed? It's not the current that causes it, but our muscles' reaction to the current. If nothing else, it demonstrates the potential for muscle use that's not normally available to us.

So the verdict is: maybe! But until scientists begin staging lab experiments where moms have to pull their children out from underneath cars, we may never know for certain.