When Lauren Kornacki discovered her father crushed beneath his BMW 525i, which had slipped off the carjack as he was working on it, the 22-year-old wedged herself under the mid-sized vehicle -- and pulled it off her father.

We hear tales from time to time of people exhibiting superhuman strength in life-and-death emergencies. After experiencing amazement over such a feat, we all wonder: How can a regular person lift something that weighs more than a ton?

Actually, most people "can lift six to seven times their body weight," says Michael Regnier, professor and vice chair of bioengineering at the University of Washington. But most people don’t push themselves so hard, though athletes often push themselves more than most. Fear, fatigue and pain prevent people from attempting feats of amazing strength in daily life, says Dr. Javier Provencio, director of the neurological ICU at Cleveland Clinic.

Regnier, a former world-class weightlifter, has experienced bouts of incredible strength both as an athlete and as someone who helped after an accident. About 20 years ago, Regnier was driving on a Los Angeles freeway when he spotted a wrecked car on the side of the road. The driver sat slumped over his steering wheel so Regnier pulled over to help. It was instinct; he couldn’t fathom leaving the man without doing something. The driver’s door had caved in and Regnier couldn’t get him out any other way—he ripped the door off to pull the man out.

Regnier remembers his hands hurting from cuts he sustained while tearing off the car door, but he doesn’t know what happened with the driver because he left when the EMTs arrived.

Ripping doors off cars or lifting vehicles from people could be considered hysterical strength. Little medical evidence exists about such cases; most of it remains anecdotal.

Physicians once believed that the adrenaline that flooded the system caused an extra boost to the muscles, allowing people to be stronger. But that’s not quite accurate. Adrenaline certainly primes the body for emergency action, it speeds up the heart and lungs, dilates the blood vessels and releases nutrients, both of which ready the muscles for quick responses.

And while the adrenaline fueled fight-or-flight reflex spurs people into action, the body’s entire stress response contributes to superhuman strength. Cascades of enzymes and proteins release, helping people sustain the activity.

“Endorphins are very important,” says Provencio. Our bodies release endorphins when we exercise, providing that “runner’s high.”

These neuropeptides make people feel good and suppress pain as well as providing people with an extra boost to finish their superhuman task.

“[Endorphins] sort of make the brain available to handle these stressful situations. You focus on the task you are doing,” says Regnier. “The endorphins will have a longer lasting affect.”

While the body’s stress response enables humans to turn into less angry Incredible Hulks, our emotions truly motivate people to attempt such actions. In most cases, the rescuers believe the victim will die without help. Take 21-year-old Danous Estenor, a University of South Florida football player, who lifted a car off a 34-year-old tow truck driver pinned under a tire in 2011. He believed Pedro Arzola would have perished without his intervention.

“The people who do these things are really under a lot of stress,” says Provencio. “It really touches them personally.”

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