People in life-threatening predicaments can sometimes perform feats that seem impossible, exhibiting superhuman strength or surviving for days in the wilderness to save themselves or others.
It's hard to believe a 4-year-old girl could keep her stricken grandfather afloat for several hours. But it happened this summer in Herring Bay, a part of the Chesapeake, when the 60-year-old man began sinking while swimming and the girl supported his body until rescuers arrived. Although he did not survive, she escaped with only a case of hypothermia.
In similar extreme situations, women have been known to lift a car to save a life, and hikers have moved huge boulders.
Despite a broken pelvis, model Petra Nemcova clung for to a palm tree after she and her boyfriend were swept away by the Asia tsunami in December. He was killed, but she survived, even after feeling her bones break several times. She was one of many tsunami victims who struggled between life and death.
How do ordinary people perform such feats of strength and endurance?
Scientists credit a part of the nervous system that kicks in and devotes all of the body's available resources to the emergency. It's known as going into alarm stage, said Sonja Batten, coordinator of the trauma recovery program at the Baltimore Veterans Affairs Medical Center.
The autonomic nervous system, stimulated by stress, can cause the heart to beat faster and respiration to increase, thereby getting more blood, oxygen and energy into the muscles, Batten said. It also halts functions such as digestion, which are unimportant during an emergency, and diverts blood flow from the skin to prevent potential blood loss.
It can also release natural opiates to relieve pain. That explains why soldiers or police officers who have been wounded sometimes can carry on with their duties -- often until someone else notices they've been hit.
In alarm stage, the brain also releases a cascade of neurochemicals such as adrenaline, noradrenaline, and cortisol, Batten said.
Immediately, the nervous system sends signals to individual organs. Vision narrows to focus on a threatening object. Memory can become more selective, and perception of time can change.
The reaction is also known as the fight or flight response.
"It does not require conscious decision," said Robert Kass, chairman of the pharmacology department at Columbia University. "It just takes over, in a sense. It's a primal response."
As the body prepares for combat or escape, its resources are pushed to the extreme. "All the systems are optimized, and you're performing at the optimum level that your body is capable of," Kass said.
That's when people perform feats of strength or survival, like Aron Ralston, a hiker who cut off his arm when it was pinned under a boulder in a remote Utah canyon.
Not everyone may be capable of such feats, and Columbia University's Kass said genetics may affect the reaction to signals sent by the brain. "We're just at the beginning of this next phase of [investigating] how gene variations work," he said.
Laurence Gonzales, author of "Deep Survival: Who Lives, Who Dies and Why," has another theory. He suggests that people are socialized to control their natural, animal strength. But under stress, they can tap into it.
He says the rational part of the brain, the neocortex, is turned off in a dangerous situation, and the limbic -- the emotional and more animal part of brain -- takes charge.
Proving how complete the switch can be, he said, is the fact that people may become incapable of solving the simplest problems, such as recalling 911 in an emergency.
Humans are capable of incredible strength but are taught to suppress it, Gonzales said, because thinking has usually been more beneficial than brute strength for the species' long-term survival. The neocortex may have evolved when early humans found it better to stop and think where to aim before they threw a rock at an animal during a hunt, he said.
Young children are very strong for their size, he said, because they haven't yet learned to moderate their strength and use their thinking abilities first. .
"People are surprisingly strong," he said. "They're constantly inhibiting their strength through their neocortex."
When the neocortex is turned off during an emergency, people don't stop to think that they can't lift a boulder or car. Their bodies, in survival mode, just do it.