Remember when Ashley Cole was lauded as the best left-back in the world? Of course you do. It was last year, around the time of his 100th England cap. And the year before that. And that. Say what you like about Cole's cock of the walk demeanour – opposition fans certainly have – few have ever questioned his talent.

And with good reason. For nearly a decade, from Cole's first international swap-meet with Cristiano Ronaldo, whose shuffling feet briefly forgot how to dance when Portugal faced England at Euro 2004, he has hung an invisible "thou shalt not pass" sign around his neck. He also speed-reads the game adroitly, covers vast tranches of turf, and smothers the best, time and again, like a lumpy but effective footballing memory pillow. There is a lot to like, at least on the pitch.

Yet, in recent months, what seemed so deep-rooted has been ripped up. Leighton Baines has slipped past him in the England pecking order. José Mourinho says he is no longer an automatic choice at Chelsea. And just like that, without much comment or ceremony, Cole made the transition from standing at the pinnacle to slipping down the side.

He is entitled to ask, as the Smiths did on The Queen is Dead, has the world changed or I have I changed? – although he might not like the answer.

So what happened? One reason is that during the past decade the full-back role broadened, and not always to Cole's advantage. The widespread bandwagoning to the formation de jour, 4–2–3–1, has led to greater overcrowding in central areas, freeing space on the flanks and placing added demands on full-backs – who are expected not only to bust lungs more frequently but to create more too.

The additional physical demands are clear from Prozone's data. In 2003-04 Premier League full-backs made an average of 29.5 sprints – any movement greater than seven metres a second – over a game. This season that figure is exactly 50. A decade ago the average recovery time for a full-back between high-intensity activities – any movement greater than 5.5m/s, or a three-quarters speed run – was 56.4sec. Now it is 40.4sec.

As Prozone's Omar Chaudhuri points out, no other position in the last 10 years can match full-backs' percentage increase in high-intensity activities or sprints. "The increase in their physical demands has been above and beyond the increased demands of the Premier League as a whole," he adds. "And full‑backs continue to cover more ground than any other position except wide midfielders."

In the past Cole has revelled in the physical demands of the job. He has always been more athlete than artist. But in the past two seasons the box-to-box shuttle service has run with less frequency and dynamism. That is understandable. He is 33 now. Injuries don't just niggle any more, they linger too.

This has had a knock-on effect. During his peak, from 2008-11, Cole created around a chance a game, according to Opta's data – a smidgeon higher than Patrice Evra, whose attacking prowess has perhaps masked defensive lapses. Indeed in 2010-11, Cole's best attacking year, he created 44 chances in 38 games. But in the past 18 months that figure has sunk to just over one in every two matches.

Baines, by contrast, averaged more than two chances a game between 2009‑10 and 2011-12, and in his break-out season last year created 116 chances from 38 Premier League matches – as well as scoring five times, just 11 fewer than Cole's total goal tally in his 14-year career. Certainly Cole's declining attacking contribution goes against the grain. In 2006-07 full-backs made 27.2% of their teams' crosses in the Premier League. This season it is 34.2%. Over the same period, the percentage of passes played by full-backs in their opponents' half has gone up from 42.1% to 48.1%.

Unsurprisingly, full-backs are also scoring more goals. There were 52 from them in the Premier League last season compared to 35 in 2006-07 and 34 in 2009-10; this season another half century is in sight.

Creation has never been Cole's strength. But elsewhere he continues to do a large amount right. His pass completion – an underrated strength – has, astonishingly, ranged between 87-89% in every season since 2007-08, despite numerous changes in managers and tactics. This season his figure of 88% is equal or better than any full-back in the Premier League.

True he was poor against Newcastle last month. But there is no vast video library of bloopers ready to be scrutinised on Match of the Day – and nor has there been a reputation‑shredding evisceration of the kind Maicon underwent at the feet of Gareth Bale. It's just that the demands of the position, along with an inevitable decline in his vast physical abilities, means that Cole is not quite as good as he once was.

Other things, however, stubbornly resist change. Full‑backs remain as unappreciated as ever, with only one, Philipp Lahm, making the 23-man longlist for Monday night's 2013 Fifa Ballon D'Or awards. No full-back has ever won the trophy in its 58-year history either. Perhaps attacking players will always get the glory; but it is about time the full-backs' vital role got more recognition.