It's hard to believe, for example, that a four-year-old girl could keep her stricken grandfather afloat at sea for several hours but it happened earlier this summer in a part of the Chesapeake Bay in the United States, when the 60-year-old man began sinking while swimming and the girl supported his body until rescuers arrived. Although he drowned, 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.

With a broken pelvis, fashion model Petra Nemcova spent hours clinging to a palm tree after she and her boyfriend were swept away by the Boxing Day tsunami. He was killed but she survived, even after feeling her bones break several times. She was one of many tsunami survivors 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 devotes all of the body's available resources to an emergency. It's known as going into alarm stage, says Sonja Batten, coordinator of the trauma recovery program at the Baltimore Veterans Affairs Medical Center. The reaction is also known as the fight or flight response.

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 says. 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.

The system can release the body's 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.

"People can be shot in the head or the leg and not realize until after the dust settles and they're able to take stock of what's going on,'' she says.

In alarm stage, the brain also releases a cascade of neuro-chemicals including adrenalin, noradrenalin, and cortisol, Batten adds.

Immediately, the nervous system sends signals to individual organs. Vision narrows to focus on a threatening object. Perception of time can change, causing some people to feel as though seconds are stretched into minutes or compressed into an instant.

"It does not require conscious decision,'' says Robert Kass, chairman of the pharmacology department at Columbia University. "It just takes over. It's a primal response.''

As the body prepares for combat or escape, its resources are pushed to the extreme. "All the systems are opt-imized, and you're performing at the optimum level your body is capable of,'' Kass says.

That's when people perform feats of strength or survival, like Aron Ralston, a hiker who amputated his own hand when it was pinned under a boulder in a remote Utah canyon, or the Georgia woman who lifted a Chevrolet Impala off her son in 1982, saving him from serious injury.

There can be consequences to such extraordinary feats of strength, according to Thomas Scalea, physician-in-chief at the University of Maryland's R Adams Cowley Shock Trauma Center.

"If you lift a car up, it doesn't mean you haven't strained your back,'' he says. "It just means you ignore that because you're focused on lifting the car off your kid or keeping your grandfather alive.''

Not everyone might be capable of such feats, and Columbia University's Kass says genetics might 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 says.

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 - takes charge.

Proving how complete the switch can be, he says, is the fact that people might become incapable of solving the simplest problems, such as recalling 911 in an emergency.

"People end up dialing 411 instead,'' he says. "That's how effectively the limbic system `turns off' the neocortex. That's how stupid you get.''

Humans are capable of incredible strength but are taught to suppress it, Gonzales says, because thinking has usually been more beneficial than brute strength for the species' long-term survival. The neocortex might 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 says.

Young children are very strong for their size, he says, because they haven't yet learned to moderate their strength. It is often the acquired belief that humans are incapable of great feats of strength that makes people unable to perform them, he says.

"People are surprisingly strong. They're constantly inhibiting their strength through their neocortex.''

He points out that muscles in chimpanzees and humans are similar, so we should be capable of the same actions. Yet an average 1.5-meter, 45kg chimp is much stronger than a typically larger human. While working in a lab, Gonzales says, he saw chimps who could crush truck tires with their hands.

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.

"They realize the full potential of their muscles,'' Gonzales says. "What you think is possible makes a big difference.''