As the two-hour marathon barrier is breached and with records tumbling every year, James Tapper explores sport’s science, skill and sheer guts.

There are a handful of numbers that are imbued with meaning. Roger Bannister's 3min 59.4sec. Usain Bolt's 9.58sec. And now Eliud Kipchoge's 1hr 59min 40sec.

Kipchoge's extraordinary sub-two-hour marathon in Vienna on Saturday is one of the greatest sporting achievements - recording a time that has never been achieved before and pushing the limits of human ability, again. It is a time on the fringes of what scientists believe is humanly possible.

And yet record-breaking events such as Kipchoge's seem increasingly common. On Sunday Simone Biles became the first female gymnast to win a fifth world all-around title, using two signature moves that no other woman has ever achieved.

Sarah Thomas swam the English Channel four times in a row, 215km in open seas in 54 hours. Dalilah Muhammad broke the 400m hurdles world record twice this year and Sifan Hassan did the same for the 5km and one mile events. In men's athletics, Geoffrey Kamworor is waiting to hear if his half-marathon time of 58min 1sec will be ratified, while Julien Wanders set a 5km record in February. And don't forget the nine men's swimming world records Adam Peaty and others have set this year. So far.

"It is a great feeling to make history in sport after Sir Roger Bannister in 1954,'' Kipchoge said afterwards, predicting that others would repeat the feat. ``I am the happiest man in the world to be the first human to run under two hours and I can tell people that no human is limited.''

Is he right? Where are the limits of human ability? And how close are we to reaching them?

Sports scientists generally agree there are theoretical limits. Michael Joyner, a marathon runner and physiologist, published a paper in 1991 examining the three defining elements of a distance runner: VO2max, the maximum amount of oxygen a body can take on; running economy, the rate at which the body uses energy; and lactate threshold, the amount of effort a body can maintain before it releases lactic acid - the burn.

Joyner's calculations predicted that the fastest time anyone would ever be able to run a marathon would be 1hr 57min 58sec. And a paper this year estimated that human endurance for the fittest athletes was ultimately limited by their metabolisms. An amateur sprint relay team ran a marathon last year in 200m bursts, and they took 1hr 30sec.

Raph Brandon, head of science for England cricket, distinguishes between feats which are constrained by human anatomy, and those which require human determination or skill.

"When Bolt ran 9.58 in Berlin 10 years ago, if you analyse the split times, it's very hard to imagine where the improvement comes from,'' said Brandon, who worked with Team GB through three Olympic games until 2014. "The Usain Bolt 100m or the two-hour marathon, they're in that category.''

Multi-day, ultra-endurance events, such as Thomas' cross-Channel swim, are different, Brandon says.

"They need grit, psychology and bloody-mindedness to go that little bit further. Those people will continue to do unique things because you're not really taking the body to its anatomical limit - it's more a question of how much you're prepared to deplete and fatigue yourself.''

And there's a third category, those sporting endeavours that rely on hand-eye co-ordination: the goal tallies of Cristiano Ronaldo and Lionel Messi, the tennis grand slams of Roger Federer and Serena Williams, and the batting of Virat Kohli, Steve Smith or Don Bradman, who trained by hitting a golf ball with a stump against a wall to become the best batsman ever to play test cricket.

"You've got to put the right kind of hours in,'' Brandon said. "Bradman's [golf ball is] a nice example. It's something that overloads your perception and movement control so you can cope with more and more types of delivery bowler and pitch condition. But there are physiological limits to that as well.''

In some sports, far from setting records, the limit seems to have been reached. The fastest baseball pitch was tracked at 169kmh in 2010, and hasn't been beaten. Then, only a few pitchers threw the ball so fast; now it's commonplace - but no-one has thrown faster. "I'm not convinced there is a big increase in world records,'' said Gary Brickley, a sports scientist at Brighton University.

"What you see is little peak points which might be related to some intervention, whether that's equipment or drugs or coaching, or some technological means of making someone go faster.''

Equipment has been a factor for many sports. NFL receivers wear gloves that enable them to make improbable one-handed catches. Football boots have been designed to help players put exactly the right amount of spin on the ball ever since adidas launched the Predator boot.

The line between what is fair and unfair is blurry. Kipchoge's sub-two-hour run will not be officially recognised - he still holds the official record of 2hr 1min 39sec - because he had so much help. The Kenyan ran behind a car which beamed a green laser on to the ground in front of him. Teams of pacemakers, 41 in total, ran in a V-formation to shield him from headwinds. He wore specially designed shoes and the time and date of the event were picked - by the organiser Ineos, the chemical company owned by Sir Jim Ratcliffe, Britain's richest man - only after detailed weather forecasting.

Yet Bannister also trod a fine line between fair and unfair assistance. His first attempt in 1953 was ruled out because his pacemaker, Chris Brasher - who went on to found the London Marathon - ran the first two laps slowly so that he could set a fast pace in the final half of the race. His record-breaking race might have gone the same way, according to Bannister's other pacemaker, Chris Chataway,who wrote in 2003 that the publicity after the event meant the athletics authorities "did not have the temerity to question whether Brasher and I had been bona fide competitors. Of course we were not. But we did not flaunt the fact.''

The question of assistance is not simply nit-picking, according to Ross Tucker, a sports scientist, who said when he first heard about Kipchoge's attempt he wondered if they should run downhill, followed by a truck with giant fans to generate a tailwind.

Perhaps the final limit is inside athlete's heads. Jo Davies, a sport psychologist, says recent studies have shown athletes can push themselves harder because of their perception of exhaustion. "It's how we interpret pain or discomfort - does this mean I need to slow down, this is too much? Or does it mean - this tells me I'm on track - it's meant to be hard?''

Other research published this year which looked at super-elite athletes - who had won multiple gold medals - found that they were different in several important ways. They had often had a traumatic life experience and had suffered significant setbacks in their performance during their careers, as well as personality traits of ruthlessness, perseverance and perfectionism.

So whether or not those limits have been reached, there will be no shortage of people prepared to try to go beyond them. - Guardian News