Scientifically testing the footballs Pep Guardiola and Mikel Arteta complained about in 2024-25

Walk into the reception of the Sports Technology Institute at Loughborough University and your eyes are drawn immediately to a pristine glass cabinet on the left.

In it, among other things, are basketballs, a cricket helmet, golf club heads, sprinting spikes, a commemorative coin from the London 2012 Olympics, and a middle section dedicated to 12 footballs: one for every men’s European Championship and World Cup since 2002, neatly organised in two rows of six.

Loughborough conducted testing on all of those for Adidas, the details of which are listed on a small plaque next to each one — and go as deep as aerodynamics and performance testing to surface texture modelling.

A display of sporting equipment that has been analysed at Loughborough University (Liam Tharme/The Athletic)

That, if anything, makes them a little overqualified to test our hypothesis: were Mikel Arteta and Pep Guardiola simply displaying a self-serving bias when, respectively, they criticised Puma’s Carabao Cup and Mitre’s FA Cup balls last season?

“It flies and grips differently to a Premier League ball, you have to adapt to it, ” Arteta said after his Arsenal side lost against Newcastle United in the first leg of their semi-final in January.

“We kicked a lot of balls over the bar. It’s tricky that these balls fly a lot.” Ten of their 23 shots were off target and only three were on target.

Arsenal’s Gabriel attempting to control the Puma ball used in the 2024-25 Carabao Cup (Alex Pantling/Getty Images)

Teams, including at academy sides, are given competition-specific balls to train with before matches. Perhaps Arteta’s side needed more shooting practice (their chances are illustrated below).

Guardiola was even more scathing in March when Manchester City came from behind to beat Plymouth Argyle 3-1 in the FA Cup fifth round.

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“The ball in the Champions League is exceptional, the ball in the Premier League is exceptional, this one isn’t,” he said. “It’s difficult to control. ”You know how many shots went over? Normally, the ball goes in.”

“When you lose it (sounds like) you’re complaining, but the ball is not right. It’s the truth. In many years, it’s happened in the FA Cup and Carabao Cup — it’s a business and they come to agreements.”

Armed with a bag of match-replica balls (which are worth more than you might think, in some cases well over £100) and some falsifiable hypotheses, it takes four hours to conduct what Loughborough call “light touch testing”. Fair enough, they have actual studies to be doing.

For context, researcher Ieuan Williams, who has just finished a PhD rooted in testing footballs, explained a project he once carried out for Adidas. With five versions of a model of the brand’s Predator boots, various trial conditions amounted to 350 kicks with their bespoke robot (more on that later), which took him days to complete — and he works quickly.

A mix of player testing and the use of Loughborough’s kicking robot is enough to get a “sense and understanding about why some balls perform differently, and why the perception of players can have a big impact”.

We start outside with three Loughborough University players, all right-footed. Each player has five goes at striking the ball from the same central spot 24 yards out. They can choose the technique and aim for the crossbar. First up is Puma’s Orbita 1 Carabao Cup ball, then the Mitre FA Cup Ultimax Pro, and finally Nike’s 2024-25 Flight, which was used in the Premier League.

Loughborough University players conduct vital analysis (Liam Tharme/The Athletic)

This kind of testing only accounts for one-tenth of their typical research, not because it is unimportant, but because outdoor and human conditions make things harder to control (and thus definitive conclusions more difficult to draw).

Professor Andy Harland, an associate dean for enterprise at Loughborough — frankly, a guru in this field — explains that “the relative time taken for player versus robot testing, allied with the greater natural variation in the player data, is why we tend to favour the robot”.

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“Players can perceive things, and one of our big challenges at the moment is trying to get those minor differences that players feel and try and translate them into mechanical tests,” Williams says. They are working on some tests, but “can’t say anything yet”.

A very expensive and high-speed camera (which records at 1,000 frames per second) is set up perpendicular to the ball, capturing a slow-motion video of each strike. With an in-house algorithm, they use it to quantify velocity and spin.

“You can see some patterns in players,” says Professor Harland. He has conducted tests on every Adidas men’s major tournament ball since 2002.

What Professor Harland does not know about ball testing isn’t worth knowing. For instance, he explains Diego Forlan’s success with the infamously unpredictable Jabulani ball at the 2010 South Africa World Cup owes to different ball-striking techniques — with his laces at altitude (where reduced air resistance makes objects fly faster) and curled finishes at sea level (where the atmosphere makes them move more).

Professor Harland tells me that each World Cup match can have up to 30 balls specifically provided for it, all stamped with the date and teams playing.

Then, he explains how the iconic star designs on the Champions League balls, a mix of pentagons and hexagons, make the flight more readable in the air because they turn into straight dark lines as the ball spins.

“Players adjust their kicks differently,” Professor Harland explains of the testing done with our three balls. One player “showed a higher and tighter launch angle”, another “showed greater variation”.

Unique physiology means everyone kicks differently from a biomechanical perspective, and thus players interact with different balls in unique ways. One player, after four kicks with one ball, said he needed to change his run-up, and there were different opinions as to which balls had bigger “sweet spots”.

“Two of the players clearly chose to apply higher spin rates to certain balls. But the balls they did this with were different,” Professor Harland explains. One spun the Mitre FA Cup and Nike Premier League balls “significantly higher” than the Puma Carabao Cup one.

“Some players show decent repeatability with repeated kicks of the same ball, occasionally a player will show a large variation with a particular ball — for reasons we can only speculate about.” One player in our test demonstrated more consistency with Mitre’s FA Cup ball, for example.

(Liam Tharme/The Athletic)

Professor Harland says he would need a bigger sample to make confident conclusions but a trend from the player testing came from two players kicking the Nike Premier League ball more consistently. It “gave some curious results”.

“Overall, the Mitre ball generated greater spin than the Puma and Nike. These differences were not statistically significant, but of greater significance than the differences in speed.”

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Interestingly, Nike’s marketing for the Flight 2024-25 Premier League ball talks about how its “moulded grooves disrupt airflow to reduce drag and keep the ball stable in the air”, with four fused panels intending to increase the size of the sweet spot.

So, to conclude from the first part of the testing procedure, Arteta and Guardiola’s claims seem to have some scientific support.

Follow the Club World Cup on The Athletic this summer…

Williams apologises for the mess as he walks into the lab. It is not quite at the level of a mad scientist’s lair but there are machines under repair, whiteboards displaying various scribbles, equations and stats, and stockpiles of miscellaneous sporting equipment. Even at the end of the academic year, they are busy.

He asks if I want a guided tour. Of course I want a guided tour.

After showing me two world-leading golf-ball testers (giant robotic arms to which club heads are attached), Williams pops into an office to retrieve what he calls a “more-authentic human head”, featuring a realistic brain for concussion research, before we return to the kicking robot.

Next to it is a giant trolley filled with 30 footballs. He speaks about the “generations” of balls and how they have changed.

A selection of footballs covering many eras of the game (Liam Tharme/The Athletic)

They start with the “old leather generation, which would be the uncoated one with laces. Then they moved to being laceless with coatings — they were more water repellent, but still hand-stitched. Then they moved to synthetic, hand-stitched, 32/18 panels. At the turn of the millennium, Adidas moved towards — and others have followed — thermal bonding.”

Adidas’ 2004 Roteiro was the first ball to be thermally bonded rather than hand-stitched, and Loughborough helped test its impact. From there, Williams says, “people started to go, ‘Well, these don’t have to be regular shapes any longer. We can do crazy things with panels’.” Puma’s Carabao Cup ball has eight panels and the Mitre FA Cup has just four panels.

Adidas’ 2004 Roteiro ball introduced thermal-bonding to the industry (Tom Shaw/Getty Images)

“Classically, you’d have an 18- or 32-panel ball,” Williams explains, noting the evolution towards single-digit panels.

“There’s been a bit of a readjustment and now we need to make sure that the ball flies properly again, which has made a load of investment in that. So the Al-Rihla (World Cup 2022 ball) was 20 panels — it’s getting back to the 18 to 32 range again.”

“Making sure the ball flies properly again” brings us neatly to the kicking robot. Behind a robust plastic screen stands the 640kg (1,400lb) machine that amounts to a mechanical arm with a foot last and boot attached.

Loughborough University’s kicking robot, ready to take aim (Liam Tharme/The Athletic)

Slightly unusually, only left-footed boots can be applied — they use it to test boots, too — that are a size U.S. 9 or UK 8.5. Typically, it is set up for lace strikes. They have tested more wrapped, curled finishes, which is why there are marks on the glass from the ball hitting it.

Cinematography lights shine bright, with a similar camera system set up outside, plus a TV screen on which they can broadcast the slow-motion footage live. The robot contraption all hooks up to a motor that Williams controls from a nearby desk.

A crash mat is on the far wall, behind some green netting, into which the ball is smashed. The robot produces so much force — and is so loud — that, for safety reasons, Williams calls out “three, two, one… FIRING” before every kick.

For more robust testing, they nail it into the floor to improve consistency, which is why there are holes in the ground. Williams explains how it “rattles and shakes”.

The robot is not as cutting-edge as it was in 2006 when Professor Harland and other researchers, with Adidas’ support, developed it. Notwithstanding, Williams says this is one of only three such models anywhere in the world: Nike has one in Portland, Oregon, and Adidas another at its HQ in Germany.

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The benefit, Professor Harland explains, is they can set the robot to a “fixed leg speed and fixed ball position. This should give a near-identical kick each time.”

The five-kick test for each ball is repeated, with the robot set at 20 metres a second to replicate kicking speeds of players outside.

Between kicks, Williams has to unlock and open the door, retrieve the ball and locate the tee it sits on (it ends up in some obscure places) and then reset it for the next one. It is rather menial work that can define what version of a ball millions see at a World Cup or European Championship.

Professor Harland’s top-line from the data: “The results show some clustering and marginal differences between balls in terms of speed and launch angle — but nothing statistically significant.”

A bigger sample and more varied kicking conditions, such as changing the valve position or launch angle, could change results, “but nothing immediately obvious” presents itself.

“Robot testing confirmed very clear uniformity between balls, and much tighter consistency of kick — as you would expect.

“This is consistent with what we would expect to see from top-level FIFA-approved match balls. They satisfy the standard that constrains size, shape, mass and rebound, among other things.”

So while the players showed individual differences in how they kicked the balls (and these are important), on a more objective level, they were remarkably similar.

A pristine Mitre Ultimax at Ewood Park in 1997 (Graham Chadwick /Allsport)

Bizarrely, in the early codified laws of the game, there were no rules for balls. These only came in from 1872.

Now, balls must pass FIFA’s ‘Quality Programme for Footballs’ test, which gives about a 10 per cent leeway for balls in circumference, weight and pressure.

FIFA Quality Programme for Footballs Feature Lower limit Upper limit Circumference (cm) 68 70 Weight (g) 410 450 Pressure (psi) 8.5 15.6

The tolerance lines are even tighter at the top level, with FIFA introducing testing from 1995 onwards. It takes place at the EMPA lab in St Gallen, Switzerland, and six prototypes of a ball have to be sent for each batch of tests.

“We do some slightly more advanced tests (than FIFA)”, Williams says. These include a shooter test: firing a ball repeatedly to see how quickly (or not) it degrades; a rebound/height drop test; stiffness testing; and, amusingly, a test of sphericity — just how round is it?

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“There’s so much market competitiveness now and so many other things going on, like sensors in the ball, the invention of fuse welding as a joining mechanism. There’s such a broad range of balls on the market currently, each of which players quite like.

“The aerodynamic designs have been really important but, in the broad picture, that doesn’t make a massive difference to the ball — that’s small percentages.”

With Puma replacing Nike as the Premier League’s match-ball provider from next season, meaning a different ball manufacturer and inevitable managerial complaints, Williams makes a wry, but evidence-backed summation: “A football broadly is a football.”

And, as Arteta himself pointed out in January, players will just have to adapt to it.

(Top photos: Getty Images; design: Kelsea Petersen/The Athletic)

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