Adrenaline is best guess for extraordinary feats

LiveScience - How Powerful is Willpower?

There are stories of ordinary people performing incredible feats of strength/endurance that they would ordinarily be unable to do. How is this possible? The experimental evidence is underdeveloped, so this article just lays out the basic structure that scientists believe enable such performance. Under normal circumstances, the "motor units" in muscles needed for a given task are the only ones activated; when adrenaline stimulates the nerve endings & muscle groups, more muscles are able to respond. Furthermore, adrenaline reduces fear and therefore the risk assessment of an activity, enabling someone to perform what they ordinarily wouldn't. Researchers use the "fight or flight" response to refer to the collection of physical changes that take place and allow incredible feats.