Snipers and Navy Seals would appear to have little in common with Premier League footballers, but techniques used by the US Special Forces to perform better under pressure are helping world-class athletes gain a mental advantage over their rivals.
Technology used to train the elite military to stay calm as they pull the trigger has been adopted by an Olympic gold medal-winning Team GB athlete, while several clubs in English football's top flight have bought a brain-training device normally used to help improve the peripheral vision of marksmen in battle.
Scanning the brains of Navy Seals has also revealed the power of meditation in developing the mental muscle of both combatants and athletes, and a new piece of naval-funded research could help solve the mystery of unfulfilled talent in sport in the years ahead.
The battle for marginal gains has never been as fierce.
The brain war
For the past six years, sports psychologist and applied sports scientist Dr John Sullivan has assisted America's elite military and law enforcement to optimise the performance of the brain through advanced training to gain an advantage over the enemy.
One aspect of Sullivan's approach is aimed at helping a soldier to track several targets at the same time.
"It's the same on the pitch; we're processing multiple pieces of information at once," Sullivan told BBC Sport.
"Where is the ball going to end up? Where are my team-mates? Where is the opponent? Where am I going to play the ball? This is what Lionel Messi does incredibly well - he has a quicker processing speed than everyone else. But we can train it."
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Sullivan, who has also worked for an unnamed NFL team for the past 15 years and as a consultant with the Football Association and several Premier League football clubs and Premiership rugby union sides, uses a tool called Neurotracker to enhance this skill with soldiers and athletes.
The training system requires the user to sit in front of a screen while wearing a pair of black 3D glasses and track four balls among a moving pack of eight for 30 seconds. The speed is then adjusted dependant on the individual's score, with a typical training session lasting eight minutes and performed a minimum of twice a week.
Manchester United bought the software towards the end of Sir Alex Ferguson's time in charge of the club, where Park Ji-sung recorded the single highest score, although Paul Scholes was consistently the best performer on the device. Southampton have also used Neurotracker as part of a wider assessment to evaluate the mental skills of their players.
Staying calm like a sniper
Much of Sullivan's work with the US Special Forces involves training snipers to stay calm after each round of fire. The techniques he uses have helped Team GB skeleton star Lizzy Yarnold to keep her nerves in check, and assess the temperament of some of Chelsea's biggest names.
"We start by teaching them to train their mind, because their mind is their biggest weapon," Sullivan added.
"Sensors are attached to a sniper's head to read brain activity in a training situation using live fire. The signals are then compared to the ideal patterns required during combat - a level of arousal that ensures an individual is alert without becoming anxious - with specific sounds indicating when a sniper is in this mental state."
In 2009, Frank Lampard, John Terry and the rest of Chelsea's first-team squad underwent a similar procedure inside a laboratory called the Mind Room at the club's training ground.
Each player took part in a number of tests, including solving a mathematical problem under time restrictions, to measure how they performed under stress, with the results detecting greater nervousness in youngsters compared to senior players.
With the military, Sullivan attaches sensors to a sniper's body to measure the length of time between each heart beat in real time. Stressful situations can result in erratic changes but can be controlled through a technique called tactical breathing.
"I teach snipers to drop their heart rate in two breaths," said Sullivan. "The recoil on a weapon hits back. If you're on that system for an hour, you'll be hit multiple times. It's just like taking a hit in rugby or American football, so the operator has to recover after every shot and drop their heart rate to conserve energy to improve decision making."
In the build-up to the Sochi Winter Olympics in 2014, Yarnold integrated the same approach into her training regime. "I wanted complete control of my nerves and how my heart responded under pressure so we used it multiple times every training session," she told BBC Sport.
"In skeleton you have to be really aggressive like a sprinter to get your speed up. You need a high heart rate, but then you have to become a completely different person who thinks very carefully and makes quick decisions.
"My heart rate is about 140 beats per minute during the race, but I've learnt to control it even as I'm sprinting which allows me to think very clearly in stressful situations." Yarnold won gold in Sochi before becoming world and European champion.
Clive Woodward's war room
A key member of Yarnold's support team is sports psychologist Charlie Unwin. He served as a platoon commander in the Royal Horse Artillery in Iraq before becoming a Team GB pentathlete and now uses his experiences to help performers operate under pressure.
"One thing the Army do really well is teach you about mental rehearsal," he said. "They don't call it that - they call it priming - but you're constantly encouraged to play out possible scenarios in your head.
"The problem-solving doesn't necessarily happen in the moment - it happens in training because you have no idea when a roadside bomb might go off or what resources you'll have available."
Unwin passed on those lessons of war to Yarnold. "Visualisation was very powerful," she said. "You do a certain amount of practice runs, but I do hundreds of runs in my head prior to competing. It's true that when the mind leads the body follows."
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Former England rugby union coach Sir Clive Woodward prepared his players for the 2003 World Cup by creating a 'war room' - a term borrowed from the military for a command centre where battles are planned.
The squad were gathered inside the dressing room with a map laid out detailing possible positions of both sets of players. A clock displayed how much time had elapsed in the fictional game, with the scoreboard also set to replicate a possible scoreline.
Woodward then selected a player at random to stand up and say exactly what they would do in that situation and what they would expect from their team-mates. The process was repeated multiple times a week over several years to ensure the players thought and solved problems quickly in the same way under pressure. England won the tournament with a dramatic 20-17 victory over Australia in the final.
Training athletes like Navy Seals
The US Special Forces are continuing to invest in identifying and improving areas of the brain that are vital for performance - and their findings have already made an impact on sport.
In 2009, the professor of psychiatry at the University of San Diego in California, Dr Martin Paulus, and his team began a piece of naval-funded research which involved scanning the brains of Navy Seals, elite adventure racers and normal civilians during a restricted breathing test.
Each participant lay down inside a brain scanner before being told their breathing would be interfered with through masks they were wearing. They were only told this between eight and 12 seconds before the interference.
Paulus said: "With the Seals and racers, there was a lot of activity in the area of the brain which is important for resilience and is essentially the link between the brain and the body.
"During and after the test, there was a lot less activity. They were able to anticipate the danger very well and then return to normal quickly. This is an elite response to difficult conditions."
However, a number of civilian participants panicked and had to be removed from the scanner. It prompted Paulus to question whether this part of the brain could be trained like other muscles in the body.
To test this, he prescribed a course of meditation - 20-minute sessions twice a week for 12 weeks - to trainee Royal Marines, who were also subjected to the same breathing test as the Navy Seals.
Paulus added: "We scanned their brains before and after the training and found they modified in the direction we'd seen with the Seals."
Paulus was contacted by the coach of the USA's Olympic BMX team, James Herrera, who wanted to see if meditation could have the same effect on his riders. After winning three medals at the 2008 Beijing Olympics, they won none at 2012, which Herrera thought might be attributed to a psychological issue.
"We did some scanning before and after the mindfulness training and the changes to their brains were pretty profound," said Paulus. Weeks afterwards, members of the team finished first, second and third at the 2014 USA Cycling Elite BMX National Championships - an annual competition contested by the nation's top 20 riders.
Paulus' vision for the future is an exciting one: "We want to use brain scanning to predict someone's future performance within the military and sport and we're beginning research and setting up experiments around that.
"We hope to be able to scan the brains of athletes and see how their brains react under various conditions and then follow them up to see how their performances relate back to what we see in the scanner.
"There are examples of brilliant junior athletes who don't do well as professionals. We want to solve that mystery."