It’s a common urban legend: a mother lifts up a car to save her screaming child pinned underneath. And every now and then, this incredible feat of "hysterical strength" seems to really happen.   

In 2012, Lauren Kornacki, a 22-year-old woman in Glen Allen, Virginia, raised a BMW 525i off her father when the car toppled from a jack. Seven years earlier, a man named Tom Boyle hoisted a Chevy Camaro, freeing a trapped cyclist in Tucson, Arizona. The events don't always involve vehicles, like when Lydia Angyiou went toe-to-toe with a polar bear in northern Quebec to protect her son and his friends while they played hockey. 

Scientists have only a tentative understanding of what exactly might be behind hysterical strength

Riveting as these accounts are, scientists have only a tentative understanding of what exactly might be behind hysterical strength. After all, the spontaneous, life-and-death situations that apparently unleash it do not lend themselves to rigorous study.

"You can't really design an experiment to do this in a lab and make people think they're going to die," says E Paul Zehr, a professor of neuroscience and kinesiology at the University of Victoria in British Columbia, Canada. "Something has to happen by fluke."

Nevertheless, numerous lines of research, particularly on athletes, have given us compelling insights into the physiological and psychological elements of hysterical strength. "Clearly, we have it in us," says Robert Girandola, an associate professor of kinesiology at the University of Southern California. "It's not some supernatural force that's causing that strength."

Super human, not superhuman

Before delving further, let's establish a key point about hysterical strength: the amounts of mass often claimed to be involved in the events are less than reported.

Take the archetypal car-lifting example. A person exhibiting hysterical strength is reckoned to have lifted at least 3000lbs (or about a tonne and a half) – the ballpark weight of a mass-market, non-truck, passenger vehicle. This kind of lift is essentially a "dead lift," where someone crouches down and then lifts an object, like a barbell with a bunch of weights on it, completely off the floor.

Could everyday folk really heft three times the world record?

The world record for deadlifting, however, stands at a mere 1,155lbs (524kg), held by Zydrunas Savickas, four-time winner of the World's Strongest Man competition. Could everyday folk really heft three times the world record?

Probably not. Most reported hysterical strength examples describe a person lifting a portion of a vehicle several inches off the ground, and not an entire automobile. There's the catch: three of the vehicle's wheels – or maybe even all four, depending on the suspension – remain on the ground, distributing the total weight of the vehicle. Furthermore, a vehicle's mass is not apportioned evenly; the heaviest part is the engine block, at the front-centre, not at the periphery where the lifting is taking place.

Put that all together – and not to take away from the courage of those who have put themselves at risk to save others – but someone in the standard hysterical strength scenario is probably lifting more on the order of several hundred pounds, not an Incredible Hulk-esque few thousand.

"You're not lifting the whole car, of course," says Girandola.

More from the muscles

Still, for the vast majority of us who aren't natural beefcakes, such a huge deadlift is nothing to sneeze at. So how is hysterical strength, even in this more down-to-Earth conception, possible?

One major clue is that we humans are, quite simply, stronger than we realise. Our movements are controlled by the contraction of muscles through signals relayed by nerves. When going about our daily lives, our bodies tend to use whatever the least amount is of muscle-and-nerve "motor units" to perform an action. 

"Your muscles are normally activated in a very certain way that's really efficient," says Zehr. "Why use your whole muscle mass to lift up a cup of coffee?"

When we need to lug that couch up the stairs, we can simply recruit more motor units. Yet even when we feel we are at our limit, we most certainly are not.

Estimates vary, but researchers have pegged the amount of muscle mass recruited during maximal exercise at around 60%; even elite athletes who have trained to get more output from their musculature might only harness around 80% of their theoretical strength.

Why do we keep so much in reserve? Safety, essentially. If we were to exert our muscles to or beyond their absolute maximum, we could tear muscle tissue, ligaments, tendons and break bones, leaving us in dire straits.

"Our brains are always trying to make sure we don't get pushed too far to where we actually damage something," says Zehr. "If you actually used all the possible force or all the possible energy you could to complete exhaustion, you'd wind up getting into a situation where you might die."

Pain and fatigue as semi-illusions

To duly disincentivise us from incapacitating ourselves, we have evolved to feel pain and distress during periods of high exertion. Experiences of those feelings thus dissuade us from trying to move something that we judge to be too heavy, like a car, under ordinary circumstances. 

Though our muscles seem to be screaming out "stop!", in many cases, we could do more without incurring injury. Until around 15 years ago, exercise science had long chalked up muscle fatigue solely to physiological factors within the muscles themselves. The 1922 Nobel Prize in Physiology winner, AV Hill, established the dogma that the limiting factor in strenuous exercise was simply the body's ability to take in and disseminate energy-unlocking oxygen to muscles.

This "brainless model," in the words of Timothy Noakes, an emeritus professor in the Division of Exercise Science and Sports Medicine at the University of Cape Town, has withered in recent years. Noakes' work and that of other researchers has reframed the brain not as a bystander, but as a "central governor", with primary responsibility for performance. Per the new thinking, the pain of muscle fatigue is more of an emotion than a reflection of the physical state of exerted muscles in question.

Zehr offers a telling anecdote to illustrate the central governor hypothesis. He remembers 30-odd years ago, his martial arts teacher asked the class to do a leg conditioning stance that really strained the thigh muscles. When the students finally felt they couldn't take the burn anymore and broke their stance, their teacher asked why. "We'd say, 'our legs just gave out' and we'd 'reached our limits'," says Zehr. The teacher responded that if that were so, instead of standing there offering excuses, shouldn't Zehr and his fellows have collapsed onto the floor?   

The takeaway: our brain, rather than our body, largely says when it's time to quit, based on our psychology instead of the physiology of oxygen-starved muscles.

So what is it psychologically that allows athletes – and for that matter, hysterical strengthers – to overcome their brain's protestations to physical exertion?

There might be an underlying genetic component to the subjective feeling of pain

Training, which can be thought as frequent exposure to the psychological pain of effort, certainly helps in getting one accustomed to and "pushing through" the agony. There might also be an underlying genetic component to the subjective feeling of pain, so some people have a harder time getting as close to their theoretical maximum output, says Zehr. Truly elite athletes might have a built-in advantage of being physiologically predisposed to superior performance. Either way, it's little surprise that, in studies, highly trained triathletes have reported higher pain thresholds than average folk.

A hard-to-calculate, but titanic factor behind hysterical strength is, of course, motivation. "We can force ourselves to ignore some of those pain signals," says Zehr.

As strong as the motivation might be for succeeding in an athletic contest, the motivation can be arguably mightier for someone directly experiencing danger, or empathising with another in harm's way. "If you're in a situation where it's all risk, and the reward is that you live, you're going to risk everything," says Zehr. "There is no next step unless you do that."

"We always act with reserve but that control can be overridden," agrees Cape Town's Noakes. "I am aware of people doing remarkable feats in war when if they were caught, they would be killed. They have run for days without food or drink."

Adrenaline rush

A key actor in pushing the body to extremes is the well-known "adrenaline rush," where hormones such as epinephrine (aka adrenaline) surge out of our adrenal glands, into our blood and throughout the body.

"The release of adrenaline is rapid – seemingly instantaneous – so that we can respond accordingly to fight-or-flight situations," says Gordon Lynch, a physiologist at the University of Melbourne, Australia.

A key actor in pushing the body to extremes is the well-known ‘adrenaline rush’

Physiologically, adrenaline boosts breathing and heart rate, flooding our muscles with extra, oxygenated blood for more forceful contractions. Nerves from the spinal cord running to our bodies' muscles are more easily able to recruit motor units, again harnessing more of a muscle's total strength. "The more motor units that are recruited, the greater the force that can be developed," says Lynch.  

Furthermore, during a stressful, adrenaline-fueled situation, the body's pain sensitivity seems to fall, as suggested by numerous anecdotes of injuries incurred and only later fathomed. For example, according to the writer Jeff Wise, only upon returning home from having lifted a car off of a teenager, Boyle – the Arizonian man – felt pain in his mouth. It turned out he had unknowingly cracked eight of his teeth, apparently from clenching his jaw during the intense lift.

The behaviour of people on certain drugs also sheds light on this pain-and-strength nexus. Users of methamphetamine, cocaine and PCP, just to name a few, have reduced pain sensitivity due to the pharmacokinetics of the substances. That fact likely explains the seemingly exaggerated strength some users display in violent encounters with law enforcement. "Drugs can blunt the pain signals and make you 'superhuman'," says Girandola.

It is difficult to simulate true peril in the lab

The strength increase from an adrenaline rush is unquantified, but for an estimate, Girandola points to a study from 1961. Michio Ikai of the University of Tokyo and Arthur Steinhaus of George Williams College investigated how subjects' grip strengths changed, given certain, ahem, provocations.

"What Ikai did was, he went behind the [study subjects] with a starter pistol," says Girandola, "and he fired it – not at their head – and after the sound, their strength went up dramatically." By 10%, actually, in part based on an adrenaline rush, he says. (Speaking even more convincingly to the reserve vim in our muscles, shouting or grunting by the participants in the 1961 study unlocked 15% more strength, and hypnosis an amazing 30%.)

Clearly, we are capable of more than we realise. That true capacity may well emerge when the stakes are at their highest – or it may not.

"We may think we'd know how we'd react under such stressful situations," says Lynch, "but we could be surprised or disappointed."

--

Join 600,000+ Future fans by liking us on Facebook, or follow us on Twitter, Google+, LinkedIn and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.