The first signs may come before kickoff. A faster heartbeat. Deeper breaths. Perhaps a restless night’s sleep.

As England prepare for Sunday’s World Cup last-16 clash with Mexico at the Azteca Stadium, they are confronting the reality of playing at 2,200 metres above sea level with little time for their bodies to adjust.

Thomas Tuchel’s side face a different opponent in Sunday’s clash – thin air – which Tim Meyer, Germany’s team doctor at six World Cups and a leading expert on sports medicine, said was “quite a decisive advantage” for Mexico.

Mexico City’s elevation impairs aerobic capacity because less oxygen is physically dissolved in the blood, making it harder for the body to push oxygen into the cells and delaying recovery.

In a paper published in the journal Sports Medicine in March, lead author Chris Esh of Loughborough University and colleagues including Meyer constructed the worst possible situation during a tournament.

“It’s exactly this one,” Meyer said. “You can do a lot in preparation, but what you can hardly do is acclimatise, or even sufficiently adapt, to altitude before you are at altitude.

“When you don’t have enough time, it’s really, really difficult. It will probably be the most strenuous game for the English team.”

Tuchel has acknowledged the problem.

“My understanding is that we cannot adapt to the altitude,” he said after Wednesday’s 2-1 last-32 win over DR Congo. “That is just a huge advantage that Mexico will have. It just takes too much time. We have only three days in between this match. It’s physically just not possible to adapt to the altitude, which is quite high.”

Altitude Impact

Altitude’s impact on sport is well established. Meyer pointed to the 1968 Olympics in Mexico City as perhaps the clearest illustration, where 30 world and Olympic records were shattered in athletics power events – sprints, jumps and throws.

“Whereas the long distances were run up to 10 per cent slower than the world record at the time,” he said.

The reason is straightforward. In power events, reduced air density lowers aerodynamic drag while the body’s anaerobic energy system is unaffected.

Endurance events rely on aerobic energy production. Footballers spend 90 minutes constantly accelerating, recovering and moving around the pitch. Those efforts depend on oxygen being transported efficiently by red blood cells.

“If you play high-level football or you run a marathon, that makes a huge difference … your performance is impaired,” Meyer said.

The challenge is compounded by the tournament schedule.

Meyer calculated that England will have had 102 hours between Wednesday’s match in Atlanta and taking the field in Mexico City. About half of that time will have been spent at their base camp in Kansas City.

It is far too little time, Meyer said, for meaningful acclimatisation as the body adjusts to altitude by producing more red blood cells, which increases the blood’s oxygen-carrying capacity.

“But red blood cell production takes days to weeks. Full acclimatisation is impossible in such a short time frame,” said Meyer, a sports and preventive medicine professor at Saarland University in Germany and chair of both UEFA’s and the German Football Association’s medical committees.

Mexico Base

The Mexican team, by contrast, are based in Mexico City for the tournament and have played three of their four World Cup games at the Azteca.

“I think the advantage for Mexico is quite huge,” Meyer said. Altitude can also affect players in less obvious ways. A lack of oxygen immediately triggers the sympathetic nervous system.

“You feel it because your heart beats faster and your breathing becomes faster and deeper,” Meyer said. “It is not good for sleep. If you arrive in Mexico City and your heart rate goes up by 20 or 30 beats (per minute), this can be incompatible with sleep.”

There is also the matter of the ball itself. The thinner air means less resistance, changing the speed and flight of passes and shots.

“The ball flies a bit easier, longer and faster,” Meyer said. “The South American teams, who are more used to this situation because there are a number of stadiums at altitude, typically use lighter balls when preparing for matches to simulate the altitude.”

The scenario echoes aspects of the 2010 World Cup in South Africa, where six of the 10 stadiums were at 1,200 metres or more above sea level.

But Meyer said England’s predicament was significantly more difficult because teams in 2010 had prepared for the altitude by choosing their base camp locations accordingly.

For England, then, the challenge is not only a Mexico team who have cruised unbeaten through the tournament without conceding a goal.

They are also coping with conditions that favour the hosts before a ball has even been kicked.