The NY Times reported this,
The United States team will arrive at its base camp, at about 4,755 feet, or 1,450 meters, between Johannesburg and Pretoria. The Americans will have nearly two weeks to adjust to the altitude — the minimum period required for effective acclimating, said Randy Wilber, a senior physiologist with the U.S.O.C. and an altitude expert who is advising the team.
In a soccer match, players can run from six to eight and a half miles. The effects of altitude lead to faster heart rates, less oxygen in the bloodstream and reduced power. An athlete participating in the United States-England match without first adjusting to altitude could experience a reduction of performance of 10 percent, according to the London Altitude Centre.
Arriving in South Africa early should allow a player to perform more efficiently in thinner air as the body begins to produce more oxygen-carrying red blood cells and as changes occur in respiratory capacity, Wilber said.
Eurosport documented this,
Oxford University researcher Patrick McSharry trawled through the scores of 1,460 international matches played at different altitudes in 10 countries in South America spanning more than a century. Altitude difference had a major impact on performance, McSharry found. Teams that were used to playing at altitude scored more and conceded fewer goals as the height progressively increased.
Each additional 1,000 metres (3,250 feet) increased the goal difference by half a goal. McSharry found that in the case of two teams from the same altitude, the probability of the home side winning averages 53 percent. But this rose to an astonishing 82 percent for an altitude difference of 3,695 metres (12,008 feet). But it fell to just 21 percent when the altitude difference was minus 3,695 metres (minus 12,000 feet). Coaches can help their side by factoring in a player's individual susceptibility to altitude sickness when making their selection, says McSharry.
The Confederations Cup held last year in South Africa will have given Italy, Brazil, New Zealand, USA and Spain a taste of what to expect playing at high altitude. Italy seemed distinctly affected by the conditions as they bowed out meekly at the group stages.
As well as both mental and physical fatigue, altitude and the atmospheric pressure will have an impact on the footballs and their varying flight speeds. The ball speeds up at higher altitudes but there is also a distinct lack of curl on the football, making free kicks and crosses harder to execute.
A recent study suggested the Adidas Jabulani football could potentially move five percent quicker in Johannesburg than in Durban because the lower air resistance changes the flight path. Let’s hope whoever emerges as Capello’s number one between now and the start of the World Cup is sufficiently alert to the prospect of having to contend with the altitude as well as blistering strikes from Lionel Messi, Kaka, Ronaldo and David Villa.
Mexico will be looking to capitalise on their familiarity playing at high altitude. They reached the quarterfinals in 1970 and 1986 when the World Cup were held in Mexico 7,400 feet above sea level.