What Happens When A Fighter Gets Knocked Out?

In combat sports such as boxing or MMA, knockouts are fairly common, and are often applauded by appreciative fans. And while you may know that a knockout results in a loss of consciousness, you may be wondering more about the details regarding a fighter getting knocked out. So what exactly happens when a fighter gets knocked out?

In short, when a fighter gets knocked out, a part of the brain known as the Reticular Activating System is disrupted, as this part of the brain is susceptible to rotational forces. The Reticular Activating System is responsible for maintaining one’s consciousness, and so disruption to it caused by a punch results in a loss of consciousness, known as a knockout.

This is according to an article written by UK neurologist JMS Pearce, which I have referenced previously in the article How Long Does A Knockout Last? This is the article I will be referencing in the rest of this post.

In the rest of this post, I’ll go more into detail on how exactly a punch disturbs the processes of the Reticular Activating System, as well as covering other things that happen during a knockout, such as tension of the muscles.

How Does A Knockout Occur?

So as mentioned in the short answer above, a knockout occurs when a fighter takes a strike to the head, usually on the chin. This causes a disruption in the Reticular Activating System (RAS), which is responsible for maintaining consciousness. As a result, the fighter loses consciousness, which we call a knockout.

A knockout usually occurs following a strike that jolts the chin, such as this left hook from Jan Blachowicz.

But why does the RAS get affected by strikes, and how does it result in the loss of consciousness?

To understand why a knockout occurs, we first have to understand the RAS, and its role in maintaining consciousness. First, we have to address the physical portion of the brain that we’re addressing, which is the brainstem. The brainstem is located at the lowest part of the brain (at the base of the neck), and contains what is known as the Reticular Formation.

The Reticular Formation is a system of neurons that have ascending and descending pathways. The ascending pathways make up what we know as the Reticular Activating System. The Reticular Formation as a whole is responsible for perception and consciousness, while the RAS is responsible for wakefulness.

The Reticular Activating System is located in the brainstem, and is responsible for wakefulness.

Ok, so now that we know exactly what the RAS does, let’s talk about what causes the knockout.

As you already know, strikes to the head are what causes a knockout, and more often than not, those strikes are landed on the chin. Why does a strike to the chin cause a knockout?

If we look at the position of the brainstem, you’ll see that the joint that connects the jaw to the skull, known as the Temporomandibular Joint (TMJ), is located right next to it.

The brainstem is located almost directly between the two temporomandibular joints.

Because the brainstem is closely positioned to the TMJ, when the chin gets jolted by a heavy strike (like a hook for example), the brainstem is put under a lot of stress. As a result the RAS is also put under stress, resulting in its disruption and inability to function properly. Without the systems necessary to control wakefulness, the body loses consciousness.

This is backed up by Pearce’s research on concussions. In his article he states:

“Current opinion is that concussion results from the disruption of the electrophysiological and subcellular activities of the neurons of the reticular activating system situated in the midbrain and diencephalon, where maximal rotational forces are exerted.”

JMS Pearce, Observations on Concussion

I should note that in his quote, when he talks about concussions, he is referring to a loss of consciousness. It was formerly believed that a concussion only occurred when consciousness was lost, although we now know that you can be concussed without getting knocked out.

UFC Lightweight Kevin Lee knocks out Gregor Gillespie with a head kick.

This explains why the majority of knockouts come from angled strikes, such as hooks and roundhouse kicks. Because these strikes hit the jaw from a sideways angle, they have a better chance of causing rotational force on the RAS. In contrast, a straight punch has little to no rotational force, making it less likely to cause a knockout.

It is also worth noting that the RAS is activated during tasks requiring alertness and attention. The midbrain has increased regional bloodflow when performing tasks with attentiveness. As fighting is likely a task that might cause this increase in bloodflow, I’d imagine it also has some correlation with being the region of the brain responsible for a loss of consciousness.

But I’ve talked enough about the brainstem and consciousness. I want to get into an interesting response seen in people moments after they get knocked out, often seen in more severe cases. And that is, why do fighters tense up after a knockout?

Why Do Fighters Get Tense After Getting Knocked Out?

So sometimes when a fighter gets knocked out badly, their body will tense up in a weird way. A perfect example of this occurred when UFC Bantamweight Marlon Moraes knocked out Aljamain Sterling with a knee during their fight.

This image was taken seconds after Moraes (yellow trunks) hit Sterling with his knee.

During their fight, they ended up on the ground for a while, after which the referee allowed Aljamain to stand up. As soon as the fight was restarted, Aljamain went for takedown, while Moraes simultaneously went for a switch kick with his left leg.

The force of Moraes’s knee was so great, that it resulted in a devastating knockout of Sterling. The force of the knockout causes Sterling’s body to tense up in what is known as the “Fencing Response”. Below you can see Sterling’s body reacting in this way.

Sterling’s body is experiencing the fencing response, where different muscles are contracting and extending simultaneously.

Because the force that causes a fencing response is so great, the resulting knockout also tends to last longer. While a normal knockout may result in a few seconds of unconsciousness, Sterling remained unconscious for several minutes, and was carried out of the arena on a stretcher.

Fencing Response is caused by damage to the Lateral Vestibular Nucleus (LVN), which is responsible for maintaining balance. The fencing response is a result of overstimulation of the LVN, which results in a reflex of extending the arm. In other words, the LVN is reacting in the way someone would to break a fall, by reflexively extending the arm.

The Fencing Response is often used as an indicator of moderate Traumatic Brain Injury, as these knockouts are a lot more damaging than mild concussions.


That’s pretty much all there is to the inner workings of a knockout. Here’s a quick summarry of what happens when a fighter gets knocked out:

According to neurologist JMS Pearce, a knockout occurs when a strike exerts enough force on the chin to affect the brainstem. This disrupts a part of the brainstem known as the Reticular Activating System, which is responsible for maintaining consciousness, resulting in a fighter being knocked out.

I hope this post helped you learn about the science behind a fighter getting knocked out. If it did, please consider checking out similar posts on the Fan Questions page. Thanks for reading!