Why is retaining the Champions League more difficult than ever?

Barcelona ran into a brick wall on Wednesday as Atletico Madrid dumped the champions out of the Champions League.

A 3-2 aggregate defeat condemned Luis Enrique’s side to a quarter-final exit in their quest of retaining Europe’s flagship cup competition.

No side has ever retained the Champions League – the great AC Milan side were the last to win back-to-back European Cups in 1990 - and you’d be forgiven for thinking that if a side with Luis Suarez, Lionel Messi and Neymar can’t do it then no-one can.

Tiredness is often cited as a factor when great teams lose, though Enrique dismissed this notion in the build-up to the quarter-final second leg at the Calderon:

"As a rule I don't talk about this (tiredness) because I don't like to look for excuses. You have to play and you have to accept the circumstances," said the Barca coach.

Admirable sentiments from Enrique but naïve, given the way Barcelona’s season has crumbled since the return from an international break.

Defeat in a Clasico at the Nou Camp was followed by defeat to Real Sociedad, while the 2-1 first leg win over Atletico had Fernando Torres’ recklessness to thank.

The Barcelona players are clearly jaded. Like Bayern Munich and Real Madrid before them, the European champions have hit a wall just as continental domination swept into view.

The three have been victims of their own success, and principally victims of the annual money-spinning trip to Japan in December for the Club World Cup.

A journey half-way across the world that yields a trophy no-one has ever dreamed of winning adds unnecessary air miles on to an already truncated and arduous season.

Winning cups breeds further games, sponsorship commitments and travelling and the cycle continues ever thus.

Mentally, not just physically, Barca need a break, and their form has collapsed in pressure situations just as Real and Bayern’s has in the past two seasons.

Lionel Messi’s influence has run dry in recent weeks and he has gone five matches without a goal or an assist for the first time in his career.

Spain's Cadena COPE radio station claimed that there was an underlying reason, that the Argentine has been carrying a significant injury for several weeks.

The radio station's "El Partido de las Doce" programme insisted that Messi had been making regular trips to see a specialist in Sacile, Italy, where he had been seeing Dr Giuliano Poser for a "kinesiological" treatment.

"Messi has been suffering serious muscular problems for several weeks," the report claimed, as per Eurosport Spain, adding that he's "carried his injuries in silence" due to his "commitment to Barcelona and the Argentine national team".

Messi apparently travelled to Sacile with Pep Costa, one of the Barcelona coaching staff. The treatment is said to have gone well, but further sessions will be needed before the problems fully disappear.

Despite a two-month injury lay-off earlier in the campaign, the 28-year-old has looked far from his best in recent weeks. And it is no coincidence that his strike-partners Neymar and Luis Suarez have also looked fatigued.

Suarez, with 45 goals this season, has been the standout player for Barca. The Uruguayan’s unquenchable thirst for success drives him on the pitch, but even he is prone to tiredness. During Liverpool’s drive to the top of the Premier League in 2013-14, he scored only three times in the final eight games as Brendan Rodgers’ side fell two points short of the title.

Let it not be forgotten that Suarez has not played a full season of football since 2009-10.

With the great trio of Messi, Suarez and Neymar struggling, critics of Enrique have asked why the Spaniard has not rotated more. But how can you rotate those three?

Arguably the greatest front three world football has ever seen and all playing at, or close to, their peak. They have produced scintillating football and inspired Barca to a level that few imagined so soon after the break-up of Pep Guardiola’s brilliant 2008-12 vintage.

Where Enrique has come unstuck is in the quality of his overall squad, which beyond the squad of 18 is down to its barest bones. That said, Bayern and Real have had stronger, deeper squads in the past two seasons and likewise come unstuck.

Barca’s problems are not isolated, maintaining your level as a European champion proved beyond Guardiola’s side and Enrique has come across the same problems.

In the 26 years since Arrigo Sacchi led AC Milan to a 1-0 win over Benfica and the defence of their 1989 triumph, Manchester United have come closest to retaining the crown before Sir Alex Ferguson’s side ran head into the "carousel" that was Guardiola’s Barca in 2009.

What it shows is the unerring ability of the best coaches to innovate and find new ways to win. As Guardiola’s side were strangled by the ultra-defensive tactics of Inter Milan in 2010 and Chelsea in 2012, so the explosive directness of Enrique’s version was nullified by a supremely effective Atletico.

Diego Simeone did not just set his team out to defend, but to provide such a threat on the break that they made it incredibly difficult for Barcelona to over-commit. In that sense the threat provided by Mourinho’s Inter six years ago, which had no interest in possession or counter-attacking Barcelona, has evolved.

To retain the Champions League has become the holy grail of European football, yet it feels further away from becoming a reality than ever.

Many complain that the competition has become boring, with the same select sides contesting the final stages each season, but the travails of the holders prove it is in ruder health than ever.