A 16-year-old boy lifts a Volkswagen to free an immobilized neighbor. A mother pushes away a polar bear to protect her children. A girl tips over a tractor that had fallen on her father. These feats are made possible by an adrenaline surge and by the unlocking of bodily systems and muscular capacities that are only fully accessible in situations of intense distress. These demonstrations of "superhuman strength" are indeed real, but the phenomenon is difficult to study in the laboratory, as recreating such situations could be dangerous for participants. Instead, neuroscientists rely on what is known about the brain and body's fight-or-flight response and on the feedback mechanisms of stress associated with these manifestations of extreme strength. These same reaction systems served our ancestors when they encountered a saber-toothed tiger and had to choose between confrontation and flight. However, these systems have evolved so that less extreme mechanisms are triggered by situations of contemporary life: receiving a troubling text from a loved one, having to brake suddenly when an animal crosses the road at high speed, or having to speak in public. In each case, "it is the same stress reaction, but today it is more frequently activated in situations that do not pose a life-threatening danger," explains Marc Dingman, associate professor of biocomportmental health at Pennsylvania State University. These mechanisms originate in the autonomic nervous system, which can be viewed as a continuum, as suggested by Andrew Huberman, a renowned researcher and neuroscientist at Stanford University's School of Medicine. "On one side of this continuum, you have absolute panic and the physiological reactions associated with it, he explains. And on the other side, you have coma." Between these two extremes exists a range of biological stress responses, some of which are familiar, such as losing appetite or having difficulty falling asleep. Other reactions are known only to a very limited circle of initiates, the circle of those who have been briefly inhabited by superhuman strength. "Superhuman strength" is a term sometimes used to describe "physical feats occurring in intense stress situations that far exceed the capacities that would normally be attributed to a person and would be impossible to reproduce in calmer circumstances," explains E. Paul Zehr, professor of sensorimotor neuroscience at the University of Victoria, Canada. We are likely to be subject to this phenomenon when we are in extreme danger: walking through the ice of a frozen lake, being attacked by a human or animal, being trapped by an object, or being confronted with a human or natural disaster, for example. "The same reaction can also occur when intervening to protect another person in danger, so it is not only about self-protection," notes Massimo Testa, sports physician at the Intermountain Medical Group in Utah. Research shows that in such circumstances, complex brain structures, neurotransmitters, and specific bodily systems come into play to release a flow of hormones that will better recruit muscular capacity and increase blood flow to the limbs and organs most essential for dealing with the emergency. To further help the body reach this state of overactivation, the energy normally used in other systems of the body, such as that allocated to searching for and digesting food, managing reproductive health, or regulating body temperature, is diverted to be redirected toward immediate survival. "All organisms, human or otherwise, essentially have three elementary reactions to any stress factor: stay in place, move forward, or retreat," states Andrew Huberman. If bodily resources are needed for any of these reactions, it is the fight and flight options that mobilize them the most and concentrate all attention on the same objective. "In this state, the sequential speed of your perception of time increases spectacularly, and you start to divide time into micro-slices and absorb much more information than usual, and this much more rapidly," explains Andrew Huberman. In such intense stress situations, we are also likely to mobilize our muscles more, activate them more than we normally could. "We generally only use a fraction of the maximum strength and power of our muscles, and there is generally a good reserve that remains untapped," reveals Gordon Lynch, director of the Muscle Research Centre at the University of Melbourne, Australia. Studies show that there are several underlying safeguards that specifically prevent muscles from being overexerted. However, in an emergency, he explains, these safeguards can "be overridden to allow the immediate recruitment of the largest and most reactive muscle fibers, those without which there would be neither explosive strength nor realization of true muscular potential." Fight-or-flight reactions such as these originate in the amygdala, a complex brain structure "that integrates your experiences in terms of their emotional content," as explained by Donald Katz, psychologist and behavioral neuroscientist at Brandeis University in Massachusetts. According to him, when this structure is subjected to a stress factor, it sends a distress signal to the hypothalamus, another region of the brain. The hypothalamus serves as a command center for the autonomic nervous system, a system that divides into two parts: the sympathetic nervous system and the parasympathetic nervous system. These systems control several automatic bodily functions such as cardiovascular and respiratory performance and the constriction and dilation of blood vessels and critical small airways in the lungs. When a stress reaction is activated in the hypothalamus, neurotransmitters are released by neurons throughout the body, and a signal is sent to the adrenal glands, which are located on top of the two kidneys. From there, a rapid secretion of two hormones, adrenaline (epinephrine) and noradrenaline (norepinephrine), occurs. "This release of hormones 'increases heart rate and arterial pressure, dilates the respiratory passages to maximize oxygenation, and causes blood vessels to contract, which contributes to the redirection of blood to important muscle groups, such as the heart and lungs," explains Holly Blake, professor of behavioral medicine at the University of Nottingham School of Medicine in England. Sensations related to touch, sight, and hearing are also heightened by the secretion of these hormones (and others) that help us better process sudden changes occurring in our environment and better deal with them, regardless of their nature. Particularly important, adrenaline can also temporarily reduce the sensation of pain. "Adrenaline can influence the perception of pain by inhibiting the signaling pathways of this latter," explains Mihail Zilbermint, physician and director of the hospital endocrinology program at Johns-Hopkins Hospital. It achieves this in part by intercepting and blocking the pain signals traveling in the brain and spinal cord. Endorphin floods are also involved, and science knows that they act as natural painkillers. It is thanks to these hormones that one can overwork or overexert a muscle in a situation of immense stress. "The pathways used by the pain feedback loops in your body usually function in a way that protects you, but when these pathways are inhibited, you no longer have to worry about tearing a biceps or dislocating a shoulder, because you are actually trying to defend yourself or defend a loved one from a catastrophic harm," explains Andrew Huberman. Extreme fight-or-flight reactions in stressful situations can be beneficial, even crucial in emergencies, but the hormones secreted in these contexts can also be present in smaller quantities in more ordinary circumstances. "All that occurs in this reaction system exists on a continuum, so a person a little stressed will experience some of its effects, while a person in full panic will unlock all the effects of this system," details Andrew Huberman. Research has shown that most of us often secrete stress hormones such as cortisol, adrenaline, and noradrenaline. "Adrenaline is produced every time there is stress," explains Melissa Leber, physician and director of the Emergency Sports Medicine Service at the Mount Sinai Health System in New York. "This can happen during a competition or performance, due to a test or an important presentation, during a fight or when your body is facing a disease or infection." Some people who already experience stress more often than others tend to have stress reactions more frequently, and often to a greater degree. A person in a demanding job or a person who regularly lacks sleep is, as Andrew Huberman puts it, more likely to be "tired and entirely designated for stress" than a person not dealing with these things. Conversely, higher up the continuum, "athletes practicing endurance or strength at extreme levels activate this stress system more and for longer periods than the average person," asserts E. Paul Zehr. In any case, it is likely that an individual benefits from short-term secretion of stress hormones, but the long-term consequences of frequent flooding of the system with these hormones can be concerning. "We need these hormones to facilitate physiological responses, but in excess, they can cause our downfall," warns Gordon Lynch. Chronic stress, associated with sustained and high secretions of adrenaline, noradrenaline, and cortisol, "can leave lasting effects on the organs and systems of the body and lead to harmful physiological outcomes," explains Gordon Lynch. High blood pressure, sleep disorders, diabetes, obesity, and cardiovascular diseases are among the undesirable side effects of chronic stress. Stress can also affect the brain region where memories are stored. "While stress spectacularly affects your short-term memory and especially in fight-or-flight situations, probably because your brain wants to remember how to avoid such a situation in the future, in the long term, chronic stress can greatly impair your memory," warns Andrew Huberman. Furthermore, the extreme end of the stress response continuum, the one concerning superhuman power and fight-or-flight, can lead to particularly concerning outcomes. "By their very nature, the physiological cascades leading to the manifestation of 'superhuman power' abolish safety barriers and can therefore be extraordinarily dangerous," warns E. Paul Zehr. "If we were at maximum all the time, we would not live very long." According to Holly Blake, even the incidental secretion of an excessive amount of adrenaline, such as when the body anticipates a threat that never materializes, "can lead to symptoms such as dizziness, insomnia, nervousness, and restlessness, and in the most severe cases, cause damage to the heart." In cases where a trauma is added to the experience of an extreme end of the stress response continuum, a person risks suffering from post-traumatic stress disorder (PTSD) and being affected for a long time. Even without a PTSD diagnosis, the experience of a vivid stress reaction can be emotionally difficult to overcome for many people. Andrew Huberman explains that the fight-or-flight mechanisms activate very rapidly, "but the deactivation of these reactions tends to take much longer and some people continue to ruminate about what they experienced for hours, or even days." These people are likely to have difficulty concentrating, experience changes in appetite, and have difficulty falling asleep at night. "We are human," concludes Andrew Huberman. "And sometimes, when we are stressed, we cannot prevent [these reactions]."