Football teams from high altitude countries have a significant advantage when playing at both low and high altitudes, finds a study in this week's Christmas issue of the BMJ.
In contrast, lowland teams are unable to acclimatise to high altitude, reducing physiological performance.
At altitude, lack of oxygen (hypoxia), cold and dehydration can lead to breathlessness, headaches, nausea, dizziness and fatigue, and possibly altitude sickness. Activities such as football can make symptoms worse, preventing players from performing at full capacity.
In May 2007, football's governing body, the Federation of International Football Associations (FIFA), banned international matches from being played at more than 2500 m above sea level. So Patrick McSharry, a research fellow at the University of Oxford, set out to assess the effect of altitude on match results and physiological performance of a large and diverse sample of professional footballers.
He analysed the scores and results of 1,460 international football matches played at different altitudes in 10 countries in South America spanning over 100 years.
Four variables were used to calculate the effect of altitude and to control for differences in team ability (probability of a win, goals scored and conceded, and altitude difference between home and away team venues).
Altitude difference had a significant negative impact on performance. High altitude teams scored more and conceded fewer goals as altitude difference increased. Each additional 1,000m of altitude difference increased the goal difference by about half of a goal.
For example, in the case of two teams from the same altitude, the probability of the home team winning is 0.537. This rises to 0.825 for an altitude difference of 3,695m (such as high altitude Bolivia versus a sea level opponent Brazil) and falls to 0.213 when the altitude difference is -3,695m (Brazil versus Bolivia).
The surprising result is that the high altitude teams also had an advantage when playing at low altitude, so benefiting from a significant advantage over their low altitude opponents at all locations.
There is still some debate over the best strategy for low altitude teams to employ when playing at high altitude to deal with this disadvantage.
He suggests that assessing individual susceptibility to altitude illness would facilitate team selection.