Football fans will be talking about Leicester City's Premier League title triumph in 100 years' time, but there's another reason the 2015/16 season will go down in history—one that will have a far longer-lasting impact on the game: For the first time, teams were allowed to use wearable devices during matches.

Unbeknownst to most viewers—and even some opponents—certain players wore tracking devices that recorded every metre they ran, every turn they made and how fast they accelerated. This data was processed and stored so it could be compared with previous matches and training sessions.

Day by day, week by week, coaches built a picture of their players' conditions and performances. They could make tactical decisions based not just on their judgement but also on solid data.

You might not even have noticed. The technology's introduction was low-key; the devices weren't allowed to be visible, and teams couldn't advertise which brand they used. The data also couldn't be used by coaches during games, which is why you didn't see them glued to their tablets.

While the devices themselves might have stayed hidden under shirts, their full effect will soon be felt. As well as being the biggest kit addition since the shin pad, they have the potential to revolutionise the beautiful game not only for players, officials and coaches but also for fans in the stadium and viewers at home.

Give it a few years, and wearable tech could be making stars out of us humble Sunday-leaguers.

Wearable devices themselves aren't that new to football. Premier League teams have used them in training for years. They are officially named electronic performance and tracking systems. They're a little different to the Fitbits and Jawbones you'll see in your local sports shop.

These devices are about the size of a smartphone and usually worn between the shoulder blades. They're held in place by a vest that looks a lot like a sports bra. Unsurprisingly, some players weren't too keen with this arrangement.

"To start with, we put the trackers in the vests the players use in training, but it was a bit invasive asking them to wear an extra layer during a match," says Jonny Northeast, head of sports science at Swansea City, the club that has adopted the technology most keenly. "We asked Adidas, and they found a way to accommodate them in the shirts themselves, which made a big difference."

The devices contain all manner of sensors, such as accelerometers, gyroscopes and compasses. They deploy the same type of GPS chip as your phone, which means they can detect not only where a player is at a certain point in time but also which direction they're facing, how fast they're moving, the impact of events such as jumps and tackles and how quickly they accelerate and decelerate.

This data, combined with a bit of joined-up thinking, helps coaches work out why the game goes a certain way. As Chris Barnes, a sports science consultant who works with Premier League teams, puts it: "If you're wondering why a defender was beaten, you can look at the wearables' data and see which direction they were facing at the time."

But the most important information the devices produce is about player load.

"Over a period of time, a huge amount of pressure is put on a player's body, and if that isn't managed, they'll either get injured or won't be able to perform when you need them to," says Paul Boanas, director of sales for Catapult Sports, one of the world's biggest manufacturers of wearables for professional sports. "By monitoring that loading across the season, you can assess whether a player is going to be fit at the right times or not. And you can gauge how long your training sessions should be based on that.

"Some training sessions might be the equivalent of half a game of football or a full game. If you know that in a matchweek a player shouldn't do more than two-and-a-half games' worth of training, you can adjust your schedule based on how hard they worked in the match and greatly reduce the risk of them getting injured."

Of course, undertraining could be just as dangerous, as players won't be ready for the intensity of a match. Wearable devices can help there too, Northeast says, by giving a team the information it needs to let a player know they need to do more training.

All this data can be calibrated to each player's fitness level, so coaches can individualise the metrics, putting the numbers in context for every member of their team.

The measurements are so precise they can even diagnose injuries before they happen.

"If you usually push off equally from both feet while sprinting but analysis shows that over a few weeks you're favouring more force from one foot than the other, it's likely you've got a weakness somewhere in the body," Boanas says. "It might not be injured yet but might be about to be injured. So you can take that player out of action, get them seen by the physio and try to diagnose the underlying issue before it becomes a problem."

Data can help with rehab too. When a player is recovering from an injury, the coach will know when they're back to full fitness by comparing their data from the most recent training session with that from previous ones. That way, they'll know when is the best time to get a player off the bench and back on the pitch.

Reintroducing a player too early means more risk for re-injury. Too late, and a star can become an expensive benchwarmer.

The decision to allow wearables during match play came out of the blue.

On July 8, 2015, just a month before the start of the 2015/16 Premier League season, FIFA sent this letter to the major leagues around the world. It said that as long as the leagues permitted, teams were allowed to use EPTS devices during matches, providing they adhered to a few stipulations.

Then, in October 2015, FIFA and IFAB—the committee in charge of the rules of the game—announced they were developing a global standard for wearables in football and invited makers of EPTS devices to present their systems for evaluation.

Previously, the only tracking data recorded during Premier League matches was on-the-ball actions, captured by cameras in the stadia. These included passes, crosses and shots but not off-the-ball actions such as acceleration and player loads. While useful for TV pundits who wanted to relay how many passes a player made, the data wasn't all that interesting to coaches, as it was completely different from that gathered in training.

"When we put the wearables on the boys for the first time during matches, we saw a discrepancy between their data and that from the matchday cameras," Northeast says. "By using the same metrics in training and during games, we get much more consistent data."

The upside of cameras? They're less invasive than wearables. Cameras are still used during Premier League matches. Ryan Bahia, marketing manager at OptaPro, which helps analyse the camera data, says the two technologies can work together to give a greater understanding of what players do.

"We certainly don't see wearables as a threat," he says. "We'll try to work with these companies where we can. We understand that teams work with a variety of different data sources that work in completely different areas, so it's important we make sure our data is compatible with those and integrates with those. The technologies complement each other."

Proponents of wearable technology argue that the more data you can get, the more you can game your strategy. You just have to look at other sports to see the evidence. In 2003, Michael Lewis' book Moneyball: The Art of Winning an Unfair Game showed how a baseball team could use statistical analysis to gain an advantage—instead of buying players who hit home runs, they were better off opting for those with high on-base percentages. Not only did this translate to better results, but it also saved the team a lot of money.

All kinds of sports have embraced the power of big data. Rugby, American football, basketball, tennis, cricket, ice hockey and even rowing have incorporated wearable technology into training and/or matches.

For example, in cricket at this summer's ICC Champions Trophy some batsmen added a device to the top of their bats which captured data on how hard they hit the ball, the angle of stroke and follow-through speed to allow a a profile to be built of a player's strengths and weaknesses.

According to analysts IHS Technology, global revenues for sports, fitness and activity monitors will grow from $1.9 billion in 2013 to $2.8 billion in 2019. So while football is getting on board, other sports have a decent head start.

But football has never been the most innovative when it comes to new technology. The Premier League only started using Hawk-Eye in 2013, a full 11 years after the same technology revolutionised cricket. Though if football can embrace the power of wearables, the impact could be massive.

As the technology improves, these devices will become capable of amazing things. At the moment, they're focused on helping players avoid injury, but they could soon help them improve their technique too.

The data is already detailed enough to show whether they're pushing off more with one foot than another when running, so it wouldn't be a huge step to incorporate that into fine-tuning players' body positions when they're striking the ball.

"Injury prevention and technique are inextricably linked," Boanas says. "As they become more advanced, I'm sure we'll see wearables used more as an aid for technique."

The units will also become more accurate thanks to local positioning systems, which will replace the standard GPS as the best way to pinpoint a player's location. Instead of using satellites to locate a player, as GPS devices do, the wearable will communicate with a series of nodes around the stadium via infrared, in the same way your remote control interacts with your TV.

This will provide much more accurate positioning data. LPS also uses smaller components, meaning the wearables themselves can be smaller and less invasive. According to Barnes, LPS is a "game-changer" and will cause the adoption rate of wearables among coaches and players to skyrocket.

One day, devices will be taken out of the equation, as shirts themselves will capture data. Smart fabrics are already being developed with this very purpose in mind—at the 2014 U.S. Open Tennis Championships, Ralph Lauren showed off a T-shirt that can record the wearer's heart rate, breathing depth and more. These fabrics are nowhere near as sophisticated as a full-blown tracking device. But given technology's rate of progress, it's only a matter of time.

This is all well and good for players and coaches, but it'll also mean big things for fans. Imagine being able to bet on which player had the highest heart rate or who ran the farthest in the second half. That could soon be a reality.

"The fan engagement side is going to be huge," Boanas says. "If they can commercialise it in any way, shape or form, fan engagement is key because that's ultimately where the money is going to be."

Clubs are already using technology to offer fans exclusive content. In March 2016, the English Football League announced that clubs in the Championship, League One and League Two would offer free in-stadium Wi-Fi to fans to access official team apps and services. Such as betting, for one. Since almost every fan in the stadium owns a smartphone, and with a whole range of metrics to bet on, the clubs' owners must have pound signs in their eyes.

More betting could also transform the viewing experience at home.

"Look at the discussion a couple of years ago about the second screen," Boanas says, referring to viewers who use a mobile or tablet to bet or monitor social media or live blogs while watching the game on the TV.

"People thought it would never happen," he says. "Now there are discussions around having the third and fourth screen. A lot of people are watching the game on the TV, they're checking their mobile phone and they've got a laptop or tablet in front of them to bet. And that's normal for a lot of people. That's just a Saturday afternoon."

TV broadcasters will be keen to get involved, just as they are with new platforms such as catch-up viewing and Facebook Live. Once one starts offering extra-rich data, you can expect the others to follow suit.

As Boanas puts it: "All these screens need content, and that's what we're able to provide."

But isn't there a danger of swamping viewers with meaningless statistics? Surely we don't want Premier League coverage to resemble a business news screen, with ticker tapes running riot.

What's more likely is broadcasters will introduce the data gradually.

"It's like peeling away the layers of an onion," Barnes says. "You don't want to bombard the viewer with too many stats right away or you'll turn everyone off. Rather, you start with quite superficial information, and pretty soon, they'll be comfortable with that. Then you can take them on a journey where you unravel exactly what it is that defines performance."

Soon we could see how hard a player kicked the ball, how quickly they made a decision and how hard they were fouled. And that's just the beginning.

As Barnes says, "With the kind of data these devices produce, the possibilities are endless."

And there's no slowing down in the pushing of the boundaries. At the start of June, while Real Madrid were plotting the downfall of Juventus in the Champions League final in the Welsh capital, Cardiff University was partnering the HYPE Foundation in a "Dragon's Den-style" competition.

Per Inside The Games, the pitches by startup companies was pushing the boundaries yet further.

"The quality of the startups is outstanding," Bernd Wahler, ex-Adidas chief marketing officer, VfB StuttgartPresident who will chair the jury, told Inside The Games. "We all want to maintain the beauty of the fascinating game and at the same time welcome meaningful and exciting innovations. That’s what this event is all about: showing the world the future of football."

With startups joining established players in bringing new products to market, the winner will be football.

The category of wearables encompasses much more than just tracking devices. It also includes virtual reality headsets such as Oculus Rift and Samsung Gear VR, and augmented reality devices such as Microsoft HoloLens and Magic Leap. And these could make watching the match at home even better than being in the stadium.

"In some ways, watching football at home is already preferable to going to the game," says James Wallman, founder of The Future Is Here and author of Stuffocation: Living More with Less. "You get every camera angle, you're comfortable, it's on a 50-inch screen. The only thing missing is the atmosphere. So if wearables could recreate that at home, that would be incredible."

The question is: How? Wallman sees putting the viewer into the action. It could be like Secret Cinema, the immersive cinema experience company, which gives every attendee to a film screening a character to play so they feel more invested in the experience.

And just as Secret Cinema gives its customers minor roles—be it a shop owner in Back to the Future or a farmer in Star Wars—so too would this more immersive form of football viewing.

"You couldn't be one of the main players because then you could influence the outcome," Wallman says. "In order for it to work, the main action needs to remain intact so everyone can watch it. But you'd still be part of that universe and feel more immersed in it. You wouldn't be on the pitch with the players, but you could be closer than anyone in the stadium."

It would be entirely possible to see what the players see, as this video shows. And as Wallman points out, it would be handy for us amateurs to jump into Harry Kane's boots and see how he scores his goals. But you could also see the match from the referee's point of view, or the goalkeeper's. Who knows, you might even feel some sympathy for the poor goalie when you see the speed of some of the shots.

Wearables could even add some star power to the average Sunday league kickabout.

"Once they've perfected the technology for the pros, us amateurs will get a version too," Wallman says. "If everyone in a Sunday league or five-a-side game is wearing a tag on their shorts and there's one in the ball, you can record not only their stats but their movement too. Afterward, you could feed the data into, say, FIFA 2030 on the PlayStation 7, and it would recreate the highlights.

"The system would recognise when the goals were scored and count the 10 passes before that so you could watch the highlights in video game form. You could be played by your player of choice in the game, so you could watch Lionel Messi score your goal."

There are a few obstacles to overcome if wearables are going to predominate. Despite being given the green light for use in matches, they're not used by every Premier League team.

"The only club wearing them every match is Swansea," Barnes says.

Northeast, the club's head of sports science, says about 80 per cent of their players use the wearables on matchday. The other 20 per cent find them too invasive.

Barnes says some coaches are also sceptical of new technology.

"If I were at a club, I'd want proof I would get quality data," he says. "There are some terrifically experienced coaches who rely on their coaching eye to make decisions. And they're more reluctant to adopt the newer ways of thinking."

Some players have sacred matchday routines too. Anything that disrupts getting ready, even if it's something as simple as slipping a tracking device into their shirt, is an instant no-no.

However, Barnes reckons we're not far from the day when all players in the league wear them.

What with coaches studying data on tablets before making decisions, you could be forgiven for thinking the future of football looks a bit like a real-life Championship Manager. But the experts disagree.

"It needs to be a balance between the art and the science," Boanas says. "We can't make every decision purely on data. The decision is always made by a human being, and we just want to provide as much data as possible so they can make the most informed decision possible."

Adds Wallman: "Data enhances the game. The sport will always be humans versus humans. The data just makes it more exciting. If someone is overwhelmed by the data, he's not doing his job very well."

Randomness also plays a big part in football. It's the nature of the game—it only takes one break to change the outcome of a match. It's why Leicester had the season they had last year. No amount of data can account for that.