THERE IS a very evident subtext to the Champions League semi-finals. Manchester United, 56 games in all competitions, and Chelsea, 59, have played considerably more this season than AC Milan, 52, and Liverpool, 53. Compounding this exertion, United and Chelsea have also rotated their squads less than Milan and Liverpool: the latter two clubs have 22 and 20 players respectively who have appeared in at least one third of the matches this season. Chelsea and United both have 18, which shows Jose Mourinho and Alex Ferguson have not only played more games, they've done it with fewer men.

So much for the physical fatigue.

If you look at the mental side of it, the edge to Milan and Liverpool becomes even more obvious. Chelsea and United, of course, are still chasing three competitions. Liverpool found themselves with a double-digit deficit in the Premiership, out of both domestic cups and with a top-four finish virtually assured as early as mid-January. It all means that for the past 15 weeks they've focused on one thing only: the Champions League.

Similarly, Milan knew by Christmas they would not be in the Serie A race. While it took them a bit longer to secure their top-four finish - due to a horrible start and the eight-point penalty they were slapped with for their role in Italy's influence-peddling scandal last summer - they too have been under very little pressure domestically.

If we see a repeat of the Istanbul final, expect the pundits and commentators to trot out the above arguments with glee and, perhaps, rightly so.

You can also bet that, should Chelsea and United advance, the fatigue/fixture pile-up school of thought will swiftly evaporate, as if it simply wasn't a factor. Why? Because, for all the fitness trainers and sports science types in football, we still don't fully understand how playing tons of high-pressure matches in a short space of time affects players, both physically and mentally.

"I've been fortunate that, on a number of occasions, I've gone right down to the wire in more than one competition," says Gianluca Vialli. "And, as long as you keep winning, you don't really feel tired at this stage of the season, perhaps because the adrenalin is so high."

Like a woman who goes through 18 hours of labour then manages to stay up for another 24 hours cradling her newborn, the combination of adrenalin, sheer joy and satisfaction can push the human body to new boundaries. It's a belief echoed by many footballers until things go wrong. Then, fatigue becomes very real.

At the tail-end of the 1988-89 season, Liverpool were chasing the domestic double. Due to a fixture pile-up earlier in the year, they had to play seven games in 19 days. They won the first six then, famously, lost at Anfield to Arsenal, 2-0, with Michael Thomas' late goal handing George Graham's side the title.

"We were exhausted," admits Ray Houghton. "We had played so many games in such a short period of time that we had nothing left in the tank. To this day, I'm convinced with a normal run-in things would have been different."

Both explanations have merit, of course. And yet, in many ways, they are mutually exclusive. The adrenalin argument is based on the fact that fatigue is somehow psychosomatic.

A stressed and tired mind affects how the body works. But if the mind is, somehow, fresh, the body will function at peak potential, no matter how exhausted it is. Except it's not true.

"No matter how good you feel, you still need recovery time," says Duane Ladejo, the former European 400m champion and British Olympian who now works as a fitness consultant. "Every time you exert yourself, you lose something and you have to give yourself time to build it back up."

Ladejo, like many with a swimming or athletics background, is probably tempted to sneer at what passes for "training" in football. At most Premiership clubs, players do no more than three weeks of heavy training during pre-season, aimed at increasing strength - for sprinting - and stamina. During the season, they might get in two more days of tough training, usually Tuesday and Wednesday, in those weeks when they don't have to play in midweek. This allows them to "top-up" their fitness to some degree. But that's it. And, at the highest level, it's very rare for clubs not to have a midweek game.

Take Chelsea. If you exclude international breaks (which since there are so many internationals at Stamford Bridge aren't really breaks at all), Mourinho's players will have gone more than four days without a game just twice since September.

"With a four-day break, it's very difficult," says Antonio Pintus, Chelsea's former fitness coach now at Juventus. "With a three-day break, it's simply impossible. You play Saturday, then you might give the boys Sunday off. Then you've got Monday, but you can't do fitness work because there isn't enough recovery time if they're playing again on Wednesday. And Tuesday is the day before a match, which means it's normally a lighter tactical session.

"There really is no time to train, particularly when you factor in travel times as well. As a fitness coach, I'd love it if we trained more and more heavily. But it's a football club, not an athletics club and, beyond physical fitness, they also need to work on technique and tactics."

This is the rationale usually trotted out by the football fraternity to explain their relatively undemanding fitness regimen, at least compared to swimming and athletics. In those sports, you might compete twice a month at most, which means you can work hard between events - hence the stories of Ukrainian Olympic swimmers spending 12 hours a day in the pool. But this doesn't mean football can't do more on the fitness front, according to Ladejo.

"Obviously it's tricky to do things in the second half of the season, but you can make up for it by working more in the summer," says Ladejo. "A few years ago, an English club asked me to draw up a fitness plan for their pre-season training. I told them I would have liked to have eight weeks, they said it was impossible and we compromised on six. I designed the plan and then, at the last minute, they pulled out. The players simply revolted, they didn't want to cut into their summer holidays."

There's the rub. The six-week summer holiday is simply non-negotiable to players. As is training for no more than two hours a day. And that's why their performances inevitably tail off at this stage of the season, even if they claim they feel fine, which they are bound to do if they win. Amazing what a piece of silverware and a bonus cheque can do.