When we feel fear or are faced with a sudden dangerous situation, the human body undergoes an amazing change. The stressor -- for example, the sight of your son pinned beneath a car -- stimulates the hypothalamus. This region of the brain is responsible for maintaining the balance between stress and relaxation in your body. When it's alerted to danger, it sends out a chemical signal to your adrenal glands, activating the sympathetic system, which sends the body into an excited state. These glands release adrenaline (epinephrine) and noradrenaline (norepinephrine), hormones that create the state of readiness that helps a human confront danger. Together, these hormones raise heart rate, increase respiration, dilate the pupils, slow down digestion and -- perhaps most importantly -- allow muscles to contract.
All of these changes in our normal physical state prepare us to face danger head-on. Combined, they make us more agile, allow us to take in more information and help us use more energy. But adrenaline's effect on muscles accounts for amazing strength. Adrenaline acts on muscles, allowing them to contract more than they can when the body is in a calm or neutral state.
When adrenaline is released by the adrenal medulla -- an interior region of the adrenal glands, which are located just above your kidneys -- it allows blood to flow more easily to your muscles. This means that more oxygen is carried to your muscles by the extra blood, which allows your muscles to function at elevated levels. Skeletal muscles -- those attached to bones by tendons -- are activated by electrical impulses from the nervous system. When they're stimulated, muscles contract, meaning they shorten and tighten. This is what happens when you lift an object, run or throw a punch. Adrenaline also facilitates the conversion of the body's fuel source (glycogen) into its fuel (glucose). This carbohydrate gives energy to muscles, and a sudden burst of glucose also allows muscles to strengthen further.
So does this mean that we have superhuman strength that is unlocked when we're confronted with danger? That's one way to put it.
Some theorize that we normally use only a small percentage of our muscles' capabilities. When we are confronted with danger, we transcend the limitations of our muscles and simply act. The rush of adrenaline, which accounts for a sudden increase in strength, helps to facilitate a person lifting a car. In other words, when confronted with extreme stress, we involuntarily use our muscles beyond the limitations of their normal voluntary use.
This theory is supported by what happens when a person is electrocuted. Someone who is shocked can be thrown a notable distance from where the shock took place. But this is not due to the electric shock. Instead, it's a sudden and violent contraction of the person's muscles as a result of the electrical charge flowing through the body. This demonstrates a potential for muscle contraction that isn't utilized under normal circumstances. In much the same way that people can't throw themselves across the room, they also can't normally lift a car -- the resources aren't available without the threat.
But why don't we possess superhuman strength all the time? Wouldn't it be beneficial? Read the next page to find out why it's better that we only have bursts of strength.