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In 1982, Angela Cavallo, a Georgia mother, lifted a Chevy Impala off of her trapped son. He had been doing repairs to its underbelly when the car jack broke. An average-sized woman, she held the 3,000 pound vehicle up for five minutes while the neighbors pulled his wounded body out from under it.

We've all heard stories like this, but what's the science behind them? Can shear strength of will — "I must lift this heavy object to save my child," for instance — really give you the muscle to lift cars in an emergency? Just how powerful is willpower?

Alas, scientists can't put a discrete number to it, beyond noting that, in some cases, it's clearly enough to amplify one's muscles to the point that a person who might normally max out lifting several dozen pounds can suddenly lift several hundred.

"The adrenaline rush is something that we know about but nobody has ever quantified it," said Bob Girandola, a kinesiologist at the University of Southern California. The main research barrier is that life-or-death situations can't be replicated in the laboratory. And when one arises — when a mother's son gets trapped under a car — no physiologists are around taking notes.

That said, scientists have a pretty solid understanding of how your brain triggers "insta-muscle" in your body.

Revving up

Bursts of seemingly superhuman strength are part of the so-called "fight-or-flight" response. "Situations of 'fight or flight' [were] faced more commonly by our distant ancestors , who had to flee from attacking animals or alternatively fight to the death, just to survive," said Gordon Lynch, a physiologist at the University of Melbourne, Australia.

When faced with a life-or-death-situation, whether you choose to fight or to fly, you need your senses, reflexes and muscles to perform at their peak, or even above what you're normally capable of. And evolution has built in a mechanism to ensure that they will.

According to Girandola, your body's performance ticks up and down like the needle on a tachometer — the gauge in a car's dashboard that displays its engine's revolutions per minute, its raw power output. "On the tachometer, there's a red line above which that model of a car should not normally go, because the engine will get screwed up," Girandola told Life's Little Mysteries. "With performance, there's a similar kind of cutoff above which you normally do not want to go. If you over-exert yourself, bones might break, muscles might tear."

But in moments of extreme stress or danger, adrenaline, also known as epinephrine, gushes from your adrenal glands. "What epinephrine might do is make you exceed that red line on the tachometer," Girandola said.

"The release of adrenaline," said Lynch, who has researched the chemical pathways that influence muscle strength, "promotes blood flow to working muscles, speeds metabolism and enhances the capacity for muscles to contract with more force and power than what we might normally require to perform most of the tasks we perform during daily living. It is possible that during situations of extreme stress and danger, that the adrenaline rush enables us to unlock a muscle's true potential that might otherwise not be achieved voluntarily."

Adrenaline does this by gearing up more "motor units" — nerves and the muscle fibers they control —than are normally used all at once. "When performing most tasks, we're actually only recruiting a small number of motor units, or a sufficient number to complete the task as required. More demanding, intense activities requires that more motor units be recruited. In many cases, we may never actually recruit all of the motor units available, unless we're placed in rare situations of 'fight or flight,'" Lynch explained.

In the brain, adrenaline diminishes fear. "You do things you wouldn't normally do partially because you overlook the fear involved," Girandola said. Kamikazes, Japanese suicide bombers who fought during World War II, took amphetamines, drugs that are chemically similar to epinephrine, in order to fearlessly execute their fatal missions. In short, adrenaline makes us throw caution to the wind, and give everything we've got to the difficult task at hand. [Read: The Psychology of Fear ]

How strong can I be?

Another difficulty in quantifying the fight-or-flight response is that everyone's is different.

"Not everyone will react the same way to the same situations of extreme stress or danger and so not everyone faced in that situation will have this extraordinary capacity to perform at a level never thought possible, including very fit and strong people," said Lynch.

What are you capable of? Someday you may find out.

Follow Natalie Wolchover on Twitter @nattyover.