Editorial: Soccer hits the heights
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FEBRUARY 2007. Brazilian football team Flamengo are playing a South American cup match in Bolivia. Their opponents, Real Potosi, are based in the high Andes and the stadium is nearly 4000 metres above sea level. In lashing rain, Flamengo fall 2-0 behind. Many of their players need bottled oxygen to alleviate the effects of altitude. Though they eventually fight back for a 2-2 draw, Flamengo announce after the game that they will no longer play matches at altitude.
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So began football’s “high altitude controversy”. Flamengo’s case was taken up by the Brazilian Football Confederation, which complained to the world governing body FIFA that venues in the high Andes were not suitable for football. In May 2007, “in the interests of player health”, international matches could no longer be played above 2500 metres.
If Brazil thought that meant victory, they were not reckoning on a comeback by Bolivia, Ecuador and Colombia, who complained to FIFA that this would put a stop to international matches in their national stadiums. In response, FIFA suspended the ban pending further studies.
Fast-forward to June 2010 and altitude is again an issue in football. The will be the first for 24 years to stage games at venues significantly above sea level. The main stadium, , is at 1701 metres. That’s not quite the high Andes, but it is still high enough to have an effect. Six other venues are at altitude (see map). Will it have a bearing on the tournament?
In the wake of the South American controversy, FIFA invited leading medical scientists to a conference in its home city of Zurich, Switzerland, in October 2007 to discuss what was known about the effects of altitude on football. The delegates quickly established that there are few well-controlled studies on football at altitude, so they would have to make inferences from research on other sports, including running, skiing and climbing.
The first thing they looked at was physical performance. They concluded that below 500 metres there are no effects. Above 500 metres, negative effects such as increased heart rate, breathlessness and reduced stamina become noticeable and get progressively worse the higher you go – though some people are more badly affected than others. At 2000 metres altitude sickness becomes a problem and acclimatisation is essential. Above 3000 metres there are substantial hits on performance.
The physiological effects of altitude are mainly caused by a reduction in the amount of oxygen in the air, which in turn limits oxygen levels in the blood. The resulting decrease in an individual’s physical performance can be quantified by a measure called VO2max – the maximum rate of oxygen uptake in litres per minute per kilogram.
Research on endurance athletes has shown that above 300 metres, VO2max falls by around 6 per cent for each 1000 metres in elevation, while the time before exhaustion sets in drops by around 14 per cent per 1000 metres ().
The standard way of minimising this decline in performance is a few days of acclimatisation. The FIFA team recommends spending three to five days at altitude, although it is never possible to recover full sea-level fitness levels in this way. Even Bolivian footballers who live at altitudes of 3600 metres have a VO2max around 12 per cent lower than footballers living at sea level ().
So are the physiological effects of altitude likely to skew the results of matches at the World Cup? Probably not. None of the venues is above 2000 metres, and most of the 32 teams will be living at altitude during the tournament. Those that are not are certain to use altitude chambers to prepare.
Flight of the ball
There is, however, a wild card. Some researchers at the meeting noted that there are reports of athletes who are acclimatised to altitude suffering a decline in performance after suddenly descending to sea level. This effect may put them at a corresponding disadvantage when they come down to play a team acclimatised to sea level (). This could influence the latter stages of the tournament, as both semi-finals will be played at sea level between a team that won its quarter final at altitude and a team that won at sea level.
“Athletes acclimatised to altitude can suffer a drop in performance after suddenly descending to sea level”
It’s not just physical performance that is affected. The FIFA team also concluded that altitude is likely to alter the aerodynamics of the ball in a way that could catch players out.
The key to this is the reduced density of the atmosphere, which affects how fast the ball moves through the air and also the bend of a spinning ball. Every 1000 metres increase in altitude reduces atmospheric pressure – and hence the density of the air – by about 11 per cent (though the precise formula is ). Other things being equal, Johannesburg has an atmospheric pressure around 81 per cent that of Cape Town.
Temperature also has an effect, with air density falling 3 per cent for every 10° C rise. So the difference in air density during a chilly winter evening in Cape Town (7° C) and a relatively balmy winter afternoon in Johannesburg (11° C) could be over 20 per cent. Such differences are quite possible during the World Cup.
To see what this difference would mean in practice, consider a ball struck from directly in front of goal, 18 metres out – that’s just outside the penalty area – and aimed at the top left-hand corner of the goal. Say that at sea level, the shot travels at an average speed of 22.8 metres per second and crosses the goal line after 0.817 seconds ().
What happens at higher altitude? Since drag force is proportional to air density, an identical shot at 1700 metres travels faster than at sea level, reaching the goal line after 0.801 seconds – about 2 ball diameters ahead of where it would be at sea level. This gives it less time to dip, and it hits the crossbar.
The upshot of this is that players must also acclimatise to the flight of the ball. To get the ball under the bar, they must learn to aim slightly lower than normal, reducing the ball’s take-off angle by around half a degree.
Defensive players will need to adapt too. Goalkeepers who are accustomed to the behaviour of the ball at sea level will need to react faster than normal or else see it fly past their outstretched fingers into the net.
Now consider a shot taken from the same spot but this time struck to bend around a wall of defensive players lined up in front of the goal. This is done by applying side spin, which generates a force – called the Magnus force – that causes the ball’s trajectory to bend.
At sea level, the shot travels at 20 metres per second, bends around the right-hand end of the wall and comes back round to enter the top left corner of the goal after 1.114 seconds. In Johannesburg, the exact same shot would either fly over the crossbar or hit the wall, as the lower air density reduces both drag and the effect of spin. With acclimatisation, players will learn to hit the ball slightly lower while applying more spin to get the ball around the wall and into the top corner. The shot now crosses the goal line 0.030 seconds (or more than 2 ball diameters) ahead of its equivalent at sea level. If the goalkeeper reacts as he would at sea level, the ball would be in the back of the net before he gets to it.
“If a goalkeeper reacts at altitude as he would at sea level, the ball would be in the back of the net before he can get to it”
So far so theoretical. Is there any evidence that altitude actually affects the results of football matches? To investigate, engineer Patrick McSharry of the University of Oxford looked at 1460 international matches played in South America between 1900 and 2004. What he found is that it is not altitude per se but changes in altitude that matter. After taking differences in skill into account, teams from altitude playing at home against teams from sea level have an increased probability of winning. The converse is also true: when high-altitude teams descend to play at sea level they are less likely to win (, vol 18 (supplement 1), p 85).
Tactical switch
The reasons for this are likely to be both physiological and aerodynamic. Tactics could play a role too, with teams changing the way they play to compensate for the changed flight of the ball. To put this hypothesis to the test, we studied data from eight matches played by Mexico in their campaign to qualify for the 2010 World Cup, including four home games at the Azteca Stadium in Mexico City, which is 2288 metres above sea level. According to the data, provided by UK-based football performance analysts , the number of shots from outside the penalty area increased with altitude while the number of shots from inside the area decreased. The implication is that, at altitude, players might be coached to have a go at goal from longer distances to take advantage of the straighter, faster trajectories.
Similar tactical calculations might come into play in South Africa. Imagine a match in Johannesburg where a direct free kick has been awarded 18 metres from goal. As the defending team lines up a four-man wall, the captain of the attacking team discusses the options with his two main free-kick takers. One is renowned for his hard, straight shot while the other is an expert at curling the ball around walls. Whom should he choose?
The physics points towards a curler: if the keeper isn’t fully acclimatised to the flight of the ball then the bending shot gives a bigger time advantage. The only problem is that it needs to be supremely accurate, since the margin for error on a curling shot is much smaller at altitude.
It is worth noting that in the , the last time altitude was a factor, eventual winners Argentina played all their matches above 2000 metres. All the other leading contenders had to change altitude. Did that give Argentina an advantage? If it did, it won’t happen this year. Whoever wins the tournament will have switched from altitude to sea level and back again at some point.
The key message for the World Cup, then, is that teams need to take into account transition, acclimatisation and tactics. Players will need to adapt to changes in altitude, especially the effect this has on the flight of the ball. Teams that use high altitude to their advantage – or that are already used to switching from low to high altitude – will profit. That points to a win for a South American team.
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