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Almost every weekend in footy season brings a fresh reel of high-impact collisions between players, replayed in slow motion and high definition on our television screens.
And while technology now lets us see — in often sickening detail — the exact moment of impact between shoulder and chin, or player and turf, concern is rising about the potential health consequences of concussion in sport.
Football players from across the codes are also voicing their concerns about the short and long-term health effects of concussion, with some players putting their careers on hold, and others even leaving the profession for good as a result.
In the last few years, and following greater awareness of head injuries in NFL players in the US, Australian sporting codes have made rules changes around dealing with concussed players, and earlier this year multiple medical and sporting institutions released the Concussion in Sport Australia Position Statement.
But how much do we really know about concussion and what it does to the brain and body in the short and long term?
What is concussion?
Most people will associate concussion with a knock to the head, followed by feelings of grogginess and confusion that clear up after a few days.
While that's not necessarily wrong, there is a lot more going on in our brains when we experience a concussive head injury.
A concussion is a temporary disturbance to brain function resulting from a direct, or sometimes indirect impact to the head that affects the brain, according to Alan Pearce, a neuroscientist at La Trobe University and the Australian Sports Brain Bank.
"If you get an impact that strikes the head and then you get nauseous, confused or dizzy, or you lose your balance and you're seeing stars — that's generally a concussion," Dr Pearce said.
It can also happen from an impact that's not directly to the head, and concussion doesn't always cause loss of consciousness.
"Sometimes you can get concussed when your head is moved violently forwards and backwards or side to side so that the brain is being shaken," Dr Pearce said.
"That can happen, especially on the sporting field where someone might 'clean you up' without actually hitting your head," he said.
"Loss of consciousness only occurs in about ten per cent of cases."
It's the movement of the brain within the bony confines of the skull that causes the symptoms of concussion, said neuroscientist Frances Corrigan from the University of Adelaide.
"This can stretch the axons — the part of the neuron, or brain cell, that sends information," Dr Corrigan said.
"Typically with milder injuries there is no permanent damage to the neurons, it just alters the way the neurons communicate with each other, causing temporary symptoms," she said.
Brain cells working overtime
In order for our brain cells to send messages to each other they create an electrical signal, and it's this signal which can be stopped following a concussion.
Brain function is affected when the brain cells get stretched or sheared to the point where they can't effectively send their signals, interfering with the brain's internal messaging.
Concussion can also cause a reduction in blood flow to the brain, making the brain cells work even harder because of the low-oxygen environment.
"The brain is funnelling energy into restoring this signalling, so people may have symptoms like difficulty concentrating, insomnia and irritability as result," Dr Corrigan said.
Researchers don't yet know whether the direction of impact to the head can alter the outcome of concussion, but Dr Pearce said whether you get hit front-on or from the side may be a factor.
"What generally happens is that shearing and stretching [of the brain cells] is because of rotational forces on the brain," he said.
"So if someone gets hit straight-on they may not actually get concussed.
"But if someone gets hit from the side and their head spins, that puts a lot more stretch and strain on the brain cells that can then disrupt function."
Dr Pearce said that, over time, brain cells can recover and resume normal functioning, but blood flow to the brain can take up to 10 days to return to normal.
"So there's that window in which the brain is vulnerable and if you get a second concussion or second hit to the head it could be quite catastrophic," he said.
This window of time after a concussion has taken place — on the sporting field or elsewhere — is critical for recovery.
Rest to avoid worse injury
There is currently no specific treatment for concussion besides physical and mental rest — meaning no sport, school, or work until you've fully recovered.
The key to reducing short and long-term health consequences is to rest as soon as the concussion has occurred, and for sportspeople, to carefully manage the return to play, Dr Pearce said.
In sport, players with suspected concussion are often also treated with the first-aid principles for protecting the spine and, while this is very important, it won't make the concussion outcome better.
And wearing head protection on the sporting field may prevent skull fractures and abrasions, but it does not prevent concussions, both Dr Pearce and Dr Corrigan said.
Rest and recovery is just as important for non-sporting related concussions, and seeking medical attention is recommended, especially if symptoms persist for several days or longer.
Dr Pearce has also found that, just because a football player's concussion symptoms have cleared up, it doesn't necessarily mean their brain has recovered from the impact.
"They might be back to normal on the memory and reaction-time tests, but when I do more subtle physiological testing [of the brain] I can see it hasn't returned to baseline by 10 days," he said.
"So they are potentially playing the following week and putting themselves in danger of another injury, because their brain isn't recovered."
Rugby league players found with neurodegeneration
While it's the knock-out hits in boxing and head-thumping footy tackles that get attention in the media, there is another type of dangerous head impact that flies under the radar.
Subconcussions occur when the head is directly or indirectly impacted, but is not followed by symptoms of concussion — and they can be particularly dangerous long-term because people don't realise they've happened.
That means they don't take necessary precautions after the impact, and may experience repeated subconcussions following the first.
Dr Pearce said when these subconcussive head knocks happen repeatedly it may lead to long-term health issues, like what is being seen in the NFL.
"Eventually the brain may no longer be able to function properly and you start to see mental-health issues and cognitive problems."
For some people — particularly those who have had repeated head impacts — the inflammation around the brain can lead to a chronic neuroinflammatory state which results in neurodegeneration, said Dr Corrigan.
"There is evidence to suggest that some former NFL players show signs of neurodegeneration that may be associated with repeated impacts," she said.
This is referred to as Chronic Traumatic Encephalopathy or CTE, and was detected in the brains of two former rugby league players as reported in the journal Acta Neuropathologica Communications at the end of June.
"I was shocked but not surprised," Dr Pearce said.
"For me it was like a confirmation that Australian sport isn't immune from this. It's not an overseas disease or an American sport issue, this can affect any sport anywhere."
There are now reports of two separate class actions on behalf of former players against both the AFL and the NRL, similar to legal action by former NFL players in America that has resulted in $US500 million in claims to date.
Scientists don't know why some people experience concussion symptoms more severely than others, or why the symptoms can persist for months in some cases — known as post-concussion syndrome.
"We also don't really know if there's a certain threshold of force required to cause a concussion because everyone is so different," Dr Pearce said.
"It also depends on their age, their prior concussion history, even their sex."
But for many people concussion symptoms will come and go with no long-term consequences.