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Thursday, June 17, 2010

Altitude performance implications

Altitude: From great to good, and the impact on the game

Yesterday I did a post on altitude and the potential impact it might have on performance at the 2010 Football World Cup.  What I didn't do in that altitude post is discuss how the altitude might affect performance - the application of the physiology.  And so here is a follow-up to share some thoughts on whether the altitude will affect what you see during this tournament.

A big question mark: Research in short supply

The first point is that research on this is in short supply, so the best we can do is to apply physiological principles, and then infer their effect.  Quite why the research hasn't been done is difficult to explain, but mostly, I suspect it's because there is not really a huge need.  As I said yesterday, how often do elite sports events face this particular problem?  In the USA, teams going to Denver have to face it, but that happens once or twice a year in NFL, maybe half a dozen (out of 72) time in the NBA.  In South American football, altitude is an issue, but again, the frequency of games is so low that I think it's been overlooked.  So suddenly, the world arrives in South Africa, and there are great big question marks.

Physiological effects

But here are my thoughts around what would happen in a match:
  • Distance covered per player per match would be reduced.  This would be due to two things: One is the decrease in the overall intensity of the game, because players would adjust their "pacing strategy" to conserve energy.  Pacing in a team sport is complex, but I've no doubt it exists.  Technology will one day provide ways to study it very effectively.  The second reason for a drop is that I suspect that the game will slow down more than normal at altitude, as a result of increased levels of fatigue.  Which brings me to the second theory:
  • Matches will fail to "ignite" in the second half, remaining low tempo.  The drop-off in running distances (at various intensities - jogging, medium, high and sprint) will be greater between first and second halves.  So if matches fail to "come alive" in the second half, and games seem to be meandering along at a low tempo, this may explain part of the reason
  • Reduced number of sprints attempted per match.  This is related to the pacing issue, but also, players will not recover between sprints.  As we saw in the graph yesterday, if the rest period between sprints is increased, then the effects of altitude are negated.  Shorter rests means worse performance per sprint.  Therefore, at altitude, players will maximize recovery and sprint less, so that the performance per sprint will be maintained. 
  • A drop in the number of sprints means a reduction in the distance covered at high speeds - you may recall that on average, players run 2.4km at high speed, and about 600m at sprint speeds.  This would fall at altitude, primarily because fewer sprints would be attempted
  • Tactical changes would also occur - because the ball flies faster at altitude, it will be more difficult to control, both for outfielders and goalkeepers.  Therefore, ball control skills will be affected
  • Players will shoot more from long distances - this is actually not a hypothesis, it has already been shown that at altitude, players tend to take more shots from further away.  Is this coaching?  I doubt it.  I think players figure out very quickly that they can't control the ball, and deduce that their chances are increased from further out
So now, let's go back to that really interesting study by Mohr that I looked at the other day.  To refresh your memory, below is a graph that compares "great" players at a high level of competition to "good" players at a level lower (see the post for definitions).  What it shows is that the high level players (blue bars) do less jogging, but more high intensity and sprint running than lower level players (orange).  Great players also sprint more, and cover 28% and 43% more distance at high speeds and sprint speeds, respectively.

Altitude - going from "great" to "good"

Now, let's look at altitude.  For the purposes of illustrating the concept, assume that in the graph above, the blue bars now represent elite players at SEA-LEVEL, while the orange bars represent the elite players at ALTITUDE.  If the above bullet list of hypotheses was accurate, then the overall impact of altitude on the tournament you're watching would be to cause a drop in high intensity running, efforts made and distances covered.  This might be best be summed up as:
Altitude turns "great" players into "good" players because it changes their activity profile in more or less the same direction as we see when comparing the highest level of football to a level below it.
Because the impact is the same for both teams (notwithstanding that a few teams have not based themselves at altitude), the overall "dynamic" of the game would not change too much.  Which is why it's unlikely to be decisive, as mentioned yesterday.
Of course, this is just a theory.  It requires proof.  And proving this is enormously complex.  Even analysing matches at the World Cup probably doesn't do it, because there are "only" 64 matches, and perhaps 40 of them are at altitude.  This is not a large enough sample, because there are too many other factors that impact on the game and the activity of players in it.

For example, take the France v Uruguay match, which was played in Cape Town.  In that match, France covered 101.5km, an average of 9,228m per player.  Uruguay were even lower - 9,201m per player.  Compare this to South Africa's match against Mexico.  This match, at altitude in Johannesburg, saw Mexico covering 10,562m per player, and South Africa 10,805m per player.  A huge difference - about 1.5km PER PLAYER.

So now we see how a match at sea-level can have reduced running, a match at altitude increased distances.  This is simply because of the way the teams played - France v Uruguay was a conservative, tight match, much in the middle of the field.  SA v Mexico was, well, frantic. End to end, a lot of movement, a lot of space, and that probably has nothing to do with altitude.

So the point is that a football match is the result of so many factors, that isolating the impact of altitude is nearly impossible.  Of course, this doesn't mean I'm not going to try, and we (UCT) are going to look hard at this question and see what can be found in the data on the 2010 World Cup.

As always, you'll be the first to know if we do find anything!


P.S. Comments on altitude and the ball

In all these discussions on altitude, there is a huge effect that I haven't covered yet - the impact of altitude on the flight of the ball.  The Jabulani ball has been slammed left, right and centre (unfortunately, it has not been slammed into the goal often enough!) by coaches and players.  Part of this may be the ball - it's certainly different.  However, I really do believe that a big part of it is the effect of altitude on ball flight.

For example, a free-kick struck from 30m out with spin, would be expected to curve a total distance of 4m when playing at sea-level.  At altitude, because the air density is reduced (in Johannesburg, on a cold night, it would be around 20% lower), the forces acting on the ball are different.  The end result is that a ball will fly faster and further, and also deviate LESS than at sea-level. 

How much less?  Some calculations show that the ball may move 60cm less in Johannesburg than at sea-level.  It will also "dip" less, which is why it would be so much more difficult to get up and over the wall, but down in time for the goals.  So when you see yet another free kick fly over by a meter, partly blame the altitude

So consider a striker who tries to bend the ball around the wall and into the goals from a free-kick - he misses by 50cm to the right.  That was a goal at sea-level.  At altitude, he blames the ball...

Meanwhile, the goalkeepers are complaining at the erratic movement of the ball, while strikers are complaining that the ball doesn't move enough.  Apart from the obvious contradiction of these complaints, I feel that the keepers are judging the reduced reaction time - a shot from 18m out will get to the goal about two ball diameters earlier in Johannesburg than in Cape Town.  That's a significant distance, and it explains why keepers are floundering - their reaction times are 0.1% too slow!  Tiny, but enough of a difference.

There's more to be said about this, but it's a post by itself.  It's one I'm a little reluctant to tackle, because I'm not an engineer and feel a little out of my depth explaining the details (I'd find an engineer's explanations of physiology frustrating, so I expect they'd frown upon mine!).  But I'll certainly discuss it at some point!  In the meantime, this article explains it really well.


Jamie CD said...

What about designing the ball to compensate for the effects of high altitude on aerodynamics? Tennis has a specific type of (lower-bouncing) ball for use above 1,219 m.

Increasing the drag coefficient of the ball would make its behaviour closer to that at sea-level, to which most players are accustomed. It is not limited by FIFA, and could possibly be achieved by increasing the surface roughness.

I don’t have any statistical evidence, but my guess is that the success rate of crosses, aerial passes and free-kicks during the first week of the World Cup has been lower than it might have been had all games been played at near-sea-level.

renchix said...

Altitude doesn't change "great" sportsmen to "good" sportsmen. They are still "great" in comparison with others on that atlitude.

MJ said...

I would be curious as to what you would recommend to the teams stationed at sea level or altitude and have to travel vice versa.

There is some old data suggesting that two weeks at altitude seems to be necessary for basic adaptation (blood volume shifts, pH normalizing, etc), not considering all the other "long term" adaptations happening. As such I have heard that being at altitude for 3-5 days is about the worst time to try and compete. Together these accounts have led to the idea that one should either arrive to an altitude venue well in advance or as close to the event as possible.

Do you agree? and if so do you know of any teams that have taken this approach?

Ross Tucker and Jonathan Dugas said...

Hi Jamie

I suspect you're right regarding the free-kicks and corners. Certainly direct free-kicks have been poor. Can't even think of an occasion where a keeper has had to make a save.

I guess re the first part, it would be difficult to design that ball. Even now, the makers and the testers of the ball say it behaves normally (and of course they must say this), so to do the testing to ensure that it behaves the same might be a bridge too far.

To Renchix;

No, it doesn't, but you've missed the point of the post entirely. The point is that at altitude, the physiological level of players "MAY" drop down in the same way that research has shown great players have different activity profiles compared to "good" players.

Nobody is saying they're average all of a sudden. So you're right, but it's a moot point, because that wasn't the message.

To mj:

I'll cover this in a future post. Having now explained the impact of altitude on physiology, the logical next step is to discuss adaptation. And so there's a whole post on that, because you're right, there are different approaches to it, exactly as you say. And there is a fairly clear (in terms of research) answer done.

But, to save me time (and you the burden of reading massive posts!), I'm splitting it up, so that is a subject for another post!


Anonymous said...

One of the reasons that the British press found to be optimistic about England's chances was that the World Cup would be played in wintery temperatures rather than the usual high summer — with the idea that the English players and the English playing style are not well suited to hot conditions.

Whether or not that's true, how do you think the effects of playing at altitude compare to the effects of playing in hot, humid conditions?

Wiredchop said...

Very interesting stuff on the effect of altitude on the game, thanks for the link to the article we wrote for the New Scientist. As you say, we were very wary of adding too much regarding physiology, it's great to see a really authoritative treatment of it. I hope you don't mind me posting a link, if anyone wants to read a bit more regarding the engineering of sport, we have a few posts about the world cup and will try to get some more stuff up there soon. Visit Engineering Sport for more.


Simon Choppin

Sports Engineering Research Group
Sheffield Hallam University

Ross Tucker and Jonathan Dugas said...

Hi Simon

Thanks for the post and the feedback! I'm glad I haven't misinformed people!

Thank you for the link - no problem at all putting it up. In fact, when I do a post on the engineering side, I'll put a link in the main post too.

Thanks very much!