The fight-or-flight response was invented by Walter Bradford Cannon, an American physiologist and professor of the Harvard University, he explains it in his books Bodily Changes in Pain, Hunger, Fear and Rage  published in 1929 and The Wisdom of the Body, published in 1932.
Cannon has shown that in the event of an emergency or danger, whether physical or psychological, the body releases the adrenaline hormone into the bloodstream. Adrenaline has several effects on the body (see part About fear below) which allow it to put itself into action : fight or flight. 
The fight-or-flight response is to be considered in any type of dangerous situation, we also talk about an acute reaction to stress. Therefore, the situations can be very varied : an assault in the street by a stranger, a fire in a kitchen, a public speaking in front of many people … etc … In this post, we will focus on the answers to an aggression situation.
“Fear is a defensive reaction without which we would have died a long time ago.” Konstantin Komarov 
Fear is an emotion felt when there is danger or threat. It is a natural reaction that allows us to anticipate the danger or threat and deal with it by choosing an appropriate solution, according to Cannon : fight or flight.
According to the triune brain theory, it is in the limbic system, more specifically the amygdala, that danger is detected through our senses and that fear is generated. What follows is an impulse from the so-called “reptilian” complex, so that the sympathetic nervous system (a subpart of the autonomic nervous system responsible for certain actions of the body beyond control) mobilizes the body to respond to imminent danger, especially through the adrenaline rush that prepares the body for action . The triune brain theory advanced by Paul MacLean in 1949 in The Theory of the Triune Brain has been criticized by a number of scientists. 
Fear and its physiological effects were crystallized in very old times, when survival was a daily affair. Even if an attack by a saber-toothed cat in the Paleolithic is hardly comparable to a contemporary street assault, the mechanisms of fear still condition us in the same way today in all situations of extreme stress.
Let’s take a closer look at the effects of fear on the body  :
- increased heart rate and breathing rate. The body is preparing to move, more blood is needed in the body to oxygenate the muscles responsible for the action, fight or flight.
- tremor of the legs (and possibly other limbs) : in fight-or-flight mode, the blood is reduced in non-vital areas of the body and is concentrated more in areas of the body useful for action, this which causes uncontrollable shaking of the legs.
- trembling voice and difficult speech. In fight-or-flight mode, language is not useful. The cortex complex, part of the brain responsible for processing language, is less supplied with blood and this affects the functioning of language.
- tunnel vision. Peripheral vision decreases as the eye focuses on its goal. Therefore the visual field is markedly reduced.
- hearing exclusion. Just like the eye, the ear focuses on its objective, peripheral noises are attenuated, or even cut off.
- sweaty hands, cold sweats, sweating in general. The body heats up as it prepares for action, sweating helps regulate body temperature. This can therefore lead to the effects of profuse sweating: redness, dry and pasty mouth, smell of sweating …
- slowing or stopping of digestion and nausea or vomiting. The digestion process is slowed down or halted because it consumes too much energy. The food in the stomach represents an unnecessary and cumbersome weight to fight-or-flight, hence nausea or vomiting.
- loss of urine or feces. In the same logic as vomiting ; the body unloads what encumbers it to fight or flee. A second hypothesis can explain this phenomenon (see below).
In cases of extreme fear, terror, the fight-or-flight response cannot be executed, the body chooses different strategies : catalepsy, or “freezing”, and apparent death.
Catalepsy, which is characterized by a loss of voluntary motor skills and muscle rigidity, the person is then motionless like a statue, hence the term “freeze”, can be explained by the fact that the eye of the predator is attracted to movement. To stop would make it possible to deceive the eye of the predator and not to draw attention to oneself.
Apparent death makes the predator think that its prey is dead, so there is no point in continuing its attack. These extreme reactions, of course, made more sense facing a carnivorous predator several thousand years ago.
Other phenomena accompany this state of terror :
- inhibition, distortion or loss of memory. In the face of a threat and extreme fear, it is sometimes necessary to protect your psychic integrity. Disregarding part or all of the event is a defense mechanism that helps prevent post-traumatic stress disorder.
- loss of urine or feces (hypothesis 2). A dead body is no longer under muscular tension, all the muscles relax even those that hold back urine and stool. What is more, this simulation death can discourage the predator who refuses to eat a rotten body.
- loss of consciousness. Reflex syncope can occur in cases of extreme fear. Two consequences : for the duration of the syncope there will be no memory of the event and lying on the ground, often very pale in color, it evokes the apparent death.
In summary, fear triggers an adrenaline rush in the body which allows you to take action according to one of two responses : fight or flight. The positive effects of adrenaline are to gain speed and power. This then implies an action of the victim, fight or flight. however, if the victim remains passive, panic sets in and the effects of adrenaline will become harmful and counterproductive. 
Now let’s look at fight and flight responses to an aggression situation.
This response is the object of all martial methods ; martial arts, combat sports and self-defense methods. However, the reality of aggression is very different from a sporting context. Very often in a aggression situation, fighting is not a choice, but a necessity. It is in this way that the methods of self-defense are taught ; one partner plays the role of the aggressor and launches an attack, the other partner, in the role of the victim, responds with an appropriate defense or counterattack which is intended to be decisive. The combat is envisaged in an asymmetric way, by a lightning assault which does not leave time for the aggressor to organize his attacks. The techniques used aim to hit the weak points of the body because the objective is to stop the aggression as quickly as possible. When an opening is possible, the leak then becomes possible (again).
This response implies mobility, movement. It also implies taking initiative before the aggression. In the animal world, a too late flight of the prey is often sanctioned by the killing of the predator. In an aggression situation, it may be otherwise, flight may also be considered once the confrontation has started.
In general, flight is always preferable to fight. Avoid conflict, “Flight first” is the first of the ten basic concepts in Protegor , book that I have already mentioned in my previous posts. Take for example the situation of an attacker with a knife, facing an unarmed victim. Any good coach must hammer home to his students that such a situation must be avoided at all costs. Run is the best option. This is the first thing I learned in Cadena de Mano (Filipino martial art for knife fighting).
“Never stand and fight if there is a possibility of ‘flight’.” Geoff Thompson 
Alternatives through communication
Walter Bradford Cannon’s fight-or-flight concept is still widely used today in the world of self-defense. However, this concept has been criticized because it appears to be too simplistic. Indeed, other responses are used in the animal world to deal with dangerous situations. Some examples ; camouflage (change in color of the chameleon or octopus), intimidation (swelling for fish of the diodontidae and tetraodontidae type) or defense (the shell of the turtle).
For humans, there appear two additional responses that they have in common with all other mammals of the animal world : intimidation and submission . Indeed, communication strategies more complex than the fight-or-flight response can be used to respond to a situation of danger. When we talk about communication, it is not necessarily verbal language. On the contrary, it is more about body language. During a conversation, depending on the nature of the message sent, body language can play a very important role in understanding this message . In the animal world, body language has of course a preponderant place. Some mammals use it in the rituals of domination, which sometimes avoids the confrontation within the same species to establish its ascendancy over the others.
Human has also developed a fifth response that only he is capable of implementing, negotiation, made possible by his preeminent language. 
Intimidate an aggressor is not easy. All the more so, that an aggressor chooses his victim, even unconsciously, according to his psychological and physical advantages that he projects on her. Intimidation is based on the threat, whether verbalized or not, to go on the attack. In my opinion, this can only be considered if you have the resources (confidence in yourself, in your technique, and being able to take action). The aggressor must understand that he may have chosen his victim badly and that he runs the risk of getting its feathers torn out. 
Intimidation is also part of the aggressor’s ritual of domination and approach. The aggressor’s body language is decisive in this ritual . We are talking about a type of aggressor who is not a predator, in which case there is no approach ritual.
When the threat and/or aggression is too great, submission becomes the adequate response. It can manifest itself in several ways. For example, being knocked down by an attacker and suffering a flurry of blow puts you in a particularly dangerous and restrictive position ; curl up, fetal position, protecting your most vulnerable organs and wait for the storm to pass. Another example, being threatened by an attacker with a knife for the theft of a valuable object, the threat being too great, it is better to cooperate, submit and give the attacker what he wants in the hope that it will end to the threat. 
This is about showing diplomacy. Find common ground to relieve tension and ensure that neither party is harmed. Whenever I discuss this topic with my students, it is this response that is offered by the majority. We try to discuss, to “calm down”. Even if it is sometimes difficult to set up a negotiation, it is certainly the most intelligent and benevolent solution because it does not affect either party, neither physically nor psychologically. Verbal and body language are decisive for a successful negotiation. However, we have seen above that fear affects our language skills. Self-control and the management of emotions are essential.
We also talk about the de-escalation of violence. Peyton Quinn, an American expert in self-defense, stated 4 rules allowing this de-escalation, they involve finding a balance between oneself and the aggressor  :
- don’t ignore the aggressor.
- don’t insult the aggressor.
- don’t provoke the aggressor, nor to accept his provocations.
- give the aggressor a face saving exit.
“Most fight are avoidable.” Peyton Quinn
The notion of self-defense certainly finds its origins in the beginnings of humanity and life in society. This notion is universal and we can qualify it as an imprescriptible natural right. Men have gradually delegated this natural right to the State, one of the fundamental roles of which is to ensure the security of its people. 
In our modern society, the notion of self-defense is a legal concept. This notion, or at least its interpretation, evolves at the same time as society. Nowadays, we live in a non-violent culture, Europe has been at peace for over 70 years. Certainly crises have been crossed and episodes of violence have taken place, but these events are of a much smaller extent compared to a war situation. Therefore, not committing acts of violence is deeply rooted in our ideology. 
The laws on self-defense of most countries are in this direction. Violence should only be used as a last resort. Any other alternative is preferable to physical confrontation. There is consensus on the criteria for the legitimate use of violence in the event of an assault. Faced with an aggression, the physical response can only be done if it is immediate, necessary and reciprocal/proportional.
Our ideology of non-violence has been confronted in recent years with extreme violence : terrorism. To deal with this, governments have developed protocols for citizens in the event of terrorist attacks of the active shooter type.
On December 4, 2015, three weeks after the terrorist attacks of November 11, 2015 in Paris, the French government published a handbook to be adopted in the event of a terrorist attack : s’échapper, se cacher, alerter (run, hide, alert). This French protocol is in fact a copy/paste of the English protocol published 1 year earlier, in 2014 in the United Kingdom : Run, Hide, Tell. (see poster below).
On the other hand, his English vade-mecum is reminiscent of another protocol published in 2012 in the United States. Indeed, the city of Houston in Texas, has set up a guide Run, Hide, Fight to survive in the face of an active shooter type terrorist attack. This initiative by the state of Texas was echoed by several other US states and the FBI.
We have just gone through more than 70 years of peace, since the end of the Second World War (except for certain major social crises). So that’s approximately 3 generations of people who have not experienced violence, who have not fought . This is reflected in this “s’échapper, se cacher et alerter” vade-mecum. There is no question of “fighting” to survive in the face of this extreme violence, as the American “run, hide, fight” advocates. And yet, as a last resort, if confrontation is no longer a choice, but a necessity, combat can become the one and only solution. It is not about heroism, but about survival which falls within the scope of the law on self-defense. Remember the foiled Thalys train attack of August 21, 2015. That day, four passengers chose the fight option to face this terrorist attack … and the massacre was avoided.
“Fight, run, and hide are the three classic survival strategies.” Rory Miller 
The 5 responses presented in this article should not be considered as choices, but as self-imposed solutions depending on the context. Several criteria are taken into account to determine, often unconsciously, the solution to be adopted. Responses can also combine and change in the same aggression situation. For example, you can run away, get caught by your attacker and be forced to fight until an opportunity to escape presents itself again.
The combat response, although innate, instinctive, can be difficult to implement, even if it is justified. This response should always be considered as a last resort. It should not be a choice, but a necessity. Keep in mind that in court, it is up to the person invoking self-defense, a priori the victim of the assault, to provide proof. For those who practice martial disciplines, the appreciation will be less tolerant of the use of violence. It will be a question of proving that recourse to violence was the only possible solution and that there was no belligerent spirit on the part of the victim invoking self-defense.
Ultimately, in order to be better prepared to face an aggression situation, it is necessary to be interested in what happens before the aggression (prevention), during the aggression (management of the aggression) and after the assault (psychological, legal aftermaths, etc…).
 Youtube >Conference about self-defense lawyer Thibaut de Montréal