When that punch lands - and it feels like time stands still for the length of a whole heartbeat - one of two things happens next. The recipient, their face frozen at the point of impact, will crumble to the ground on rubber legs; their body rattling across the canvas until a thundering follow-up blow spikes them into stillness. Or their face won’t freeze at all. It will only wince (or maybe even grin!) as the punch glances off their chin like a hammer sparking off of an anvil.

But why the difference? What is it that makes different fighters able to tolerate different amounts of force to the head without being staggered, knocked down, or knocked all the way out? The answers to these questions can be found in the still-murky world of concussions.

Delving into the science of getting KTFO’d is Dr. Charles Tator, of the Canadian Concussion Centre at Toronto Western Hospital. He’s a neurosurgeon and a leading expert on brain injuries, particularly those sustained by athletes.

To understand exactly what a concussion is, it’s important to understand what it isn’t. For many, the long-held belief is that a concussion is the result of the brain slamming into the inner wall of the skull. But, that isn’t the case.

“That’s a misconception,” said Dr. Tator, as he explained what really causes someone to be concussed. “It’s the movement of the brain within the skull, but it doesn’t really depend on impact; it’s just the jiggle of the brain.”

The technical and biomechanical term for this ‘jiggling’ of the brain is ‘rotational acceleration.’ This movement of the brain is aided by a liquid called cerebrospinal fluid (CSF), which fills the gaps between the brain and the skull. “CSF is considered to have a little bit of a cushioning effect on the brain, but in essence it’s because of this fluid that the brain can rotate within the skull.”

Misconceptions about the brain hitting the inner skull to elicit a concussion are the result of an outdated theory. Some impacts do make brains slam into the skull, though. These types of movements are called ‘linear acceleration’. The injuries sustained from linear acceleration are not concussions, but the results are more immediately dangerous.

“Linear acceleration is when the brain moves at a different rate from the skull and does hit the inside of the skull,” explained Dr. Tator. “And that classical condition causes a ‘coup’ and ‘contrecoup’ injury. And those are the ones that cause bruising of the brain and blood clots in the brain as well, because veins can be torn off during that type of movement to and fro, or front to back, or side to side. But, that’s not what causes concussion. Concussion is due to rotational acceleration.”

“Some people concuss at 50 g’s of force, and some people don’t concuss until the force is much higher and we don’t know why there is that variation.”

Coup and contrecoup injuries like brain bleeds and clots can be fatal in the short-term, and have occurred a number of times across various combat sports. Concussions may not cause those deadly and immediate effects, but any concussion can result in long-term symptoms that can drastically impact the life of a sufferer.

Multiple concussions are likely to bring about a long-lasting condition recently named as Chronic Traumatic Encephalopathy (CTE).

“CTE is a complex biochemical degenerative process,” said Dr. Tator, explaining that an abnormal protein known as pTau, and its effects on the brain, are what causes the symptoms that come with the condition. These symptoms can include aggression, depression, loss of impulse control, and dementia.

“PTau stands for phosphorolated tau,” he continued. “Tau is a normal constituent of all central nervous tissue in the brain and in the spinal cord. And it always has some areas of the protein that becomes phosphorolated. In CTE, the number phosphate molecules that get deposited in the tau protein goes up and that becomes an abnormal form of the protein. And it deposits in nerve cell bodies and ultimately that nerve cell body that has high concentration of pTau dies.”

It is believed that concussions start the process of tau protein gaining phosphate, enough so that it can deform and start killing off parts of the brain. It is not known, however, if just one concussion is enough to start this process, or whether CTE requires multiple traumatic brain injuries.

Diagnosing a single concussion is relatively simple. Especially in fighters.

Dr. Tator stated that ‘seeing stars’, as the result of a blow to the head, was a clear sign that a concussion had been sustained. Also, if a fighter has experienced any loss of consciousness, Dr. Tator said that “absolutely” was sign of a concussion. “Even if the loss of consciousness is for a split second,” he added. He also mentioned that loss of motor control (for example a fighter’s legs going out from under them) was “suggestive of a concussion.”

Every time a fighter gets knocked down or out (from a blow to the head) they have suffered a concussion. But does rotational acceleration impact all brains in the same way? Or do different brains concuss easier than others?

“There’s a huge variation in the tolerance of the brain to concussion,” answered Dr. Tator. “Some people concuss at 50 g’s of force, and some people don’t concuss until the force is much higher and we don’t know why there is that variation.”

“You don’t go back to square one each time you recover from concussion, the brain does retain some of that change due to the concussion.”

Dr. Tator stated that it’s likely there is a gene responsible for why some brains concuss easier than others. However, science has yet to identify it. He also explained that brains vary quite a lot in size, person to person, but there’s no link between brain size and tolerance to concussion. CSF, the goop that bathes our brain, doesn’t change much from one person to another either, and is thus unlikely to be a factor.

So, brains do differ quite a lot between people in their ability to withstand force. Fighter X may take a punch better than fighter Y, purely because they have a ‘tougher’ brain. But, today’s science is a bit baffled as to why this is the case.

Yet, the brain alone isn’t the only reasons why some fighters have better ‘chins’ or ‘beards’ than others. A logical thought may be that fighters with thicker or harder skulls would be better equipped to brush off a right hook or a spinning head kick. But that’s simply not the case.

“I don’t believe skull thickness matters. Because it’s the jiggle inside that causes the concussion. So it doesn’t really matter how thick the skull is and, in fact, that’s why helmets don’t really protect from concussions. No matter how thick the helmet is, the brain is still going to jiggle inside,” explained Dr. Tator, noting that helmets do protect against linear acceleration based injuries (brain bleeds, bruises, etc.).

While skull density may not matter, what about the mandible (where the jaw and chin reside)? In the language of fighting the default descriptor for a fighter’s toughness is their iron - or rock-hard - chin, but does science back this up? Nope.

“Generally a blow to the jaw, that sets the head in motion, is a very effective way of knocking someone out,” confirmed Dr. Tator. “But [receiving a concussion] doesn’t seem to depend on the actual structure or thickness of the jaw, but the way the blow is delivered to the jaw.”

No more saying, ‘that girl has a hell of a chin!’ or ‘man, that dude’s chin is shot,’ - it seems.

What about diet? Does what someone puts into their body affect their chances of getting knocked out? Apparently not. “There are a lot of dietary supplements being advertised to protect against concussion, but there’s no evidence that they work,” cautioned Dr. Tator.

And dehydration? Stories linger in combat sports about dehydrated fighters increasing their risk of brain damage from brutal weight cuts. “I’m not sure about that,” said Dr. Tator. He reasoned that if the volume of the brain reduced during dehydration, it would jiggle more violently within the CSF, but he’s not sure if the brain would shrink at all.

Age has to be a factor, right? Well, At certain ages human beings are way more susceptible to concussions. But, it turns out that those ages are your late teens - when the brain is expanding dramatically. There is also an increased risk factor in the elderly. But beyond those extremes, brains are more or less the same.

Though age may not affect a brain and it’s ability to be concussed, wear-and-tear most definitely does. After receiving your first concussion, the chances of sustaining a second rises. After the second, it rises further, and so on.

“The stronger the neck muscles, the less jiggle of the brain within the head, from a blow to anywhere on the head or jaw.”

“If a person’s number of concussions goes up, the susceptibility to concussion goes up,” said Dr. Tator. “The amount of force necessary to cause a concussion goes down as well.”

Longtime MMA watchers know this story all too well. They may have squinted through it, but they’ve seen it nonetheless.

The reason fighters who’ve received multiple concussions get knocked out more often, and with greater ease, is because the brain does not heal like a lot of other body parts. “It carries what we call ‘memory of concussion’,” said Dr. Tator. “You don’t go back to square one each time you recover from concussion, the brain does retain some of that change due to the concussion. So there is a definite cumulative and carry-over effect.”

The cumulative damage of concussions may be the single biggest factor deciding whether or not a fighter will sustain subsequent concussions. But, outside of indecipherable differences in the brain itself, there is another part of the body that plays a huge role in determining whether or not a fighter will receive their first concussion - the neck.

“Yes, precisely!” exclaimed Dr. Tator. “The stronger the neck muscles, the less jiggle of the brain within the head, from a blow to anywhere on the head or jaw.”

It's not how thick your skull is, or the shape of your jaw. It's not what you eat, or how old you are. The stronger the neck muscles, the less the brain jiggles, the less likely a concussion will be suffered.

This suggests that if a fighter, or any other person, wants to stand a better chance of avoiding concussion they should exercise and strengthen their neck muscles. Dr. Tator’s clinic has advised hockey and football players to do just that. He also enforced that the length, thickness, shape, etc. of a neck means nothing, it’s all about how strong the muscles are within the neck.

Along with strengthening neck muscles it seems there may be another step that fighters could take to make themselves more resistant to getting knocked out. Dr. Tator suggested that tensing the neck muscles, in preparation for a blow, can also help avoid a concussion. ‘Tuck your chin’, is not just a thing that gets barked out, it’s scientifically and medically prudent advice.

And while the shape, size, and thickness of the chin or jawbone doesn’t make a blind bit of difference, according to Dr. Tator, aligning the jaw does - just like tensing the neck. Not only is keeping your ‘chin tucked’ strong advice, so too is the command to ‘bite down’ on a mouthpiece.

That’s why some fighters get knocked out a lot and others don’t. Partly it’s their brains, which can vary a great deal from fighter to fighter. Some may have brains that shut off easily, others not so much. Fighter age doesn’t really matter, but how many concussions they’ve already sustained matters a great deal. The thickness of the bones in their skull means nothing. Neither does the structure of their jaws and chins. But how they hold their head and jaw can mean a lot.

Ultimately, according to Dr. Tator, If a fighter wishes to guard themselves from concussions, the best advice seems to be to get as few of them as possible, to keep their chin tucked, and to bite down on the mouthpiece. But most importantly, they may be able to protect their brains by keeping their necks strong.

So next time, when you see fighters take that punch and walk right through it, as if it were just a stiff wind, You’ll know the reasons why. It’s not down to their ‘iron chins,’ but their ‘iron necks’ instead.