Almost without exception, all great players at the FIFA World Cup this summer—the likes of Lionel Messi, Cristiano Ronaldo and Neymar—will have endured a grueling season at their respective clubs before arriving in Brazil.
Neymar's final La Liga game for Barcelona already sparked controversy with the Confederacao Brasileira de Futebol, which denied rumours it requested he was rested, as reported by FourFourTwo.
So, will match fatigue plague some squads this summer?
Can an active club schedule really impact a player's performance, and is there such a thing as "match fatigue"?
Or is Brazil coach Luiz Felipe Scolari right when he says he wants his players to join up with the national squad having played many minutes and experienced much success on the domestic front?
The first thing to consider is a concept known in strength and conditioning circles as "periodisation."
Research published in the Strength and Conditioning Journal states this is "a logical phasic method of manipulating training variables in order to increase the potential for achieving specific performance goals."
Put more simply, this just means you plan and adjust your training intensity, sometimes resting, so you can peak at just the right time of the season.
This method of training has been used in other sports for years yet still remains quite alien to most footballers. Usain Bolt in athletics is perhaps the best example: Back in 2011-12, Bolt displayed worrying signs of form and lost to training partner Yohan Blake in the Jamaican Olympic trials.
Completely unfazed, Bolt confidently told the Daily Mail: "We always train to peak at the major championships. My coach sets out the program with this as the goal."
Bolt later won gold in the 100-meter sprint, 200-meter sprint and 4x100-meter relay at the London 2012 Olympics, peaking at just the right time.
Of course, the massive popularity of football around the globe—and the economics of it as a business—calls for players to fulfill a busier work schedule compared to athletics stars.
For this reason, the idea of periodisation may not be so easily implemented in football, which is why it's even more important for coaches to be aware of the dangers of match fatigue and how it can affect their squad.
But how exactly does match fatigue occur?
Put simply, research from the American Journal of Physiology shows that during a game, your oxygen usage skyrockets. This, in turn, causes an increase in lactic acid accumulation in the muscles, which then causes your body to pull alkaline reserves from bones and other mineral dense sources. Not to mention the fact that muscle tissue can be torn and adenosine triphosphate levels in the muscles become depleted.
All things considered, a player's body has a lot to cope with, and if there isn't sufficient time to recover, it can result in match fatigue—in the form of a suppressed immune system. Your immune system is your body's defence against disease, and if this is affected, you can dramatically increase your chances of becoming ill.
At best, this affects performance; at worst, it puts a stop to a player's tournament altogether.
Whilst this is purely speculative, could this have played a factor in Messi's mysterious illness when he was sick on the pitch during a goalless draw against Romania in March earlier this year?
After the game, Messi played down his on-pitch illness and was quoted in the Daily Mail as saying: "My vomiting has always happened. It happened a few times with Barca. It’s nothing to me."
This could, of course, be true. But a study published in the International Journal of Sports Science shows a suppressed immune system certainly wouldn't have helped matters.
Finally, something to consider for any fixtures scheduled closely together is post-match muscle fatigue. Scientists at the Human Performance Laboratory at the MAPEI Sport Research Centre in Varese, Italy, set out to determine the level of fatigue experienced by elite players after a 90-minute match.
Sprint and maximal voluntary contraction in the legs were tested before, immediately after and at 24-hour and 48-hour post-game intervals. Results revealed that although recovery rates were higher in well-trained players, sprint performance and other indicators didn't fully recover until at least 48 hours afterwards.
Now, granted, most group matches are scheduled five days apart to allow for sufficient rest. But according to the World Cup schedule on the BBC website, the second semi-final loser will play for third place only 72 hours after their semi-final match.
That recovery time is only slightly more than the limit proposed by Italian sports scientists, whilst in comparison the first semi-final loser will have 96 hours to recover. Therefore it's possible the team that wins may not be the best team but just the team that's better able to recover in time.
Ultimately, because of all of the above, it's not surprising when certain footballers don't quite seem to perform on the international stage like they do at club level.
Wayne Rooney is a player who has received some criticism for his past performances in an England shirt, and to his credit, he’s also very self-aware of this. He told the Daily Mail (h/t The Guardian): "I really want to be 100 percent fit this time and, barring an injury in training before we go, I will be totally ready."
But looking at the evidence above, asking any athlete to perform at their optimal level and peak 365 days of the year is an almost impossible task. Bolt and other world-class athletes can't achieve it, so why should footballers be any different?
There could, however, be good news for England fans. It might be argued Rooney's past success and renowned work rate at club level has hindered his fitness on the international stage previously.
However, following a lacklustre 2013-14 season at Manchester United, he could have inadvertently periodised his training and now be ready to shine in Brazil.