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Monday, June 14, 2010

Football and fatigue discovered

Fatigue in football:  Physiology of performance

A few days ago, I posted on the physiological demand of playing football, and what exactly goes into a 90-minute match.  To refresh your memories, here is a summary graph that shows distances covered, and time spent in different activities:

Quite clearly, football cannot be treated as a continuous endurance activity.  A match may last 50% longer than an elite half-marathon, but the activity profile is so different that if we wish to discuss fatigue, we have to appreciate the intermittent nature of the sport.

And the crux is that a footballer will attempt an average of 100 sprints per match, each lasting somewhere between 2 and 5 seconds.  Recovery time is minimal - a 1:2 work-rest ratio means that the most important requirement of conditioning is to prepare the players to recover from repeated sprints.  Speed, acceleration and ability to change direction - all of which are impaired when a player is 'tired' - are the difference between good and great players.  But they are meaningless if a player only possesses them for 20 minutes of a 90 minute match!

Understanding fatigue and its implications for football

There are many components to fatigue in soccer - studies have found, for example, that a footballer leaves the field with near maximal glycogen depletion.  In other words, just as a marathon runner is liable to "hit-the-wall" if they fail to replace energy, a footballer is 'running on empty' by the end of a match.  Similarly, the intensity of football raises body temperature to close to 40 degrees, which we know to be a limit for performance.  By any measure, the 90 minutes is a challenge to the physiology of a player.  And as we'll see later in this post, the higher the level of play, the more demanding the game.  So elite performance, at the elite level of the World Cup, puts physical conditioning at a premium.

Repeat sprint fatigue - the game "opens up" at the end

So, given the above, it will come as no surprise to you to learn that even the very best footballers do display some fatigue during a match.  The graph below is reproduced from a study (Krustrup 2006) where players were asked to perform five 30m sprints with a 30 second recovery, either during the first half, the second half, or at the end of the game. So you're looking at a 4 second sprint, 30 second recovery (1:7 work-rest) at different phases of the match.

Quite clearly, there is an accumulated effect of repeat sprints on performance ability, as shown by the blue line (for the first half) and the red line (second half).  Let's apply this practically - a player is making a 30 m sprint, and by the very end of the match, he is covering the 30m about 8% slower than at the start of the match.  This means that a 30m sprint that might take 3.8 seconds at the start of the match will now take 4.1 seconds.  At the speeds we're talking, that's about 2 m that a player 'concedes' as a result of fatigue, compared to in the first half.

So now let's imagine that two players are sprinting for a through ball in the 81st minute of a match.  If the defender is fatigued, but the striker is not, then then striker has a 2m advantage and that is easily enough to allow him away from the tackle, onto goal and perhaps, a match-winning moment.

So there are two implications of this.  First, when you hear commentators saying that the "game has opened up" in the second half, PART of the reason is fatigue.  There are others - teams figure one another out, they start to work out how to create space through movement, their mindset changes and the weighting of risk to reward changes (especially if a goal is scored).  But a big reason the game opens up is that players start to fatigue, often at different rates.  Suddenly, a run into space that would have been closed down is not, and the game seems much more open.

The second implication is that of substitutions to manage the game.  The implication of the above graph is that a player who is 5% slower than the opponent will still outperform them at the end of the match, provided he is fresh.  The point is that fatigue may have a greater impact on performance than the natural differences between players, and so this is why clever substitutions can either control matches, or open them up.  

From good to great:  Different demands depending on level of play

Now, an even more interesting implication of understanding these physiological demands and fatigue is comes from comparing different levels of football.  A study by Mohr (2003) compared the physiological demands in two different leagues.  One was the Italian Serie A, where most of the players analysed where playing Champions League, and at a very high international level (top 10 ranked teams).  The other was the Danish league, where no Champions League players were analysed, and the international level was a notch down (Top 20).  So you have this comparison between great, top-level players, and good, second-level players.

And this is what was found:

  • Top level players (shown in Blue) jog LESS than good players during a match - 16 vs 19 minutes
  • Top level players spend more time doing medium-paced running (12 km/h to 15km/h), high paced running and sprinting.  They also run backwards more.
  • The number of sprints attempted is also greater in top level players - 108 vs 75
  • Consequently, top-level players cover more distance at high speed (2.4km vs 1.9km, 28% higher), sprinting (650m vs 410m, 43% more) and a 5% greater total distance covered per match
So how do we interpret this?

The most likely is that when you play in the company of other top players, you are forced to cover more distance, sprint more, run faster.  The overall level of the match demands that you perform at a higher physiological level, and "drags" you up to that level.  There are other studies, for example, that confirm this, by showing that when players from these "lesser" leagues play against players from top leagues, they must run more and faster than they are accustomed to.

So now, the implication should be clear - if you are playing in the World Cup, against some of the greatest players in the world, at the highest level of competition, the physiological demand is maximal (as it would be for Champions League, I'd imagine).  Under these circumstances, the risk of fatigue is greater than ever - you take a player who is accustomed to running 2km fast, with 400m sprinting in 75 sprints, and you force them to run 2.5km fast, and sprint 100 times to cover 650m, and that player would struggle over 90 minutes.  The fatigue effect, the drop off in sprint performance is thus likely to be even greater.  It is the same as saying to a 10km runner who is accustomed to running 3:00/km that they have to start at 2:50/km.  By 7km, the effects will be clear!

And this is why physical conditioning is so vital to elite teams.  Ultimately, I would be overstating the value of sports science (I am biased, after all) if I said this was decisive to the outcome of matches.  It's not, and there are so many other factors that determine the result.  Physical conditioning is but one of them.  But what I can say is that if players are NOT conditioned for the demands of the match, then their decline in performance may cost them.

And finally, remember that it doesn't take much to be shown up by an elite player - if you concede even 1m over a 20 m sprint (5%), then you look like a carthorse alongside a thoroughbred!  And fatigue will cost you that 5%!  So next time you are watching a match, and you suddenly start seeing players leaving others behind (whereas at the start, it was always an equal contest), you may realise that this could be due to a shift of even 1m over 20m, 5%, and a goal that wins the game may be the result!

Looking ahead - altitude and performance

The stage is now set to discuss altitude and its potential effect on the World Cup.  But that is for another time!

Three great matches today, beginning with the Netherlands, many people's pick for overall glory.  Enjoy the action - I'll do my best to follow on Twitter!



ramachandran said...

Early days yet, but quite clearly, the month long duration of the world cup 2010 will definitely lead to viewer fatigue and boredom, if this kind of play continues. Where are the attacking players?? Where are the forwards?? So far all matches have been dull and insipid (i don't watch the third match in a day, which comes to me at an unearthly hour of 12 a.m.). The overwhelming feeling that i get is that radical surgery is needed to get more goals into the goalpost. Probably, something like in hockey where they have rolling substitution could be tried out. Anyway, players are always feigning illness in order to take a few minutes break. So why not give them a few minutes break in order to get fresh legs into the ground. Otherwise, it is quite pathetic to watch these matches bereft of goals.

ramachandran said...

Early days yet, but quite clearly, the month long duration of the world cup 2010 will definitely lead to viewer fatigue and boredom, if this kind of play continues. Where are the attacking players?? Where are the forwards?? So far all matches have been dull and insipid (i don't watch the third match in a day, which comes to me at an unearthly hour of 12 a.m.). The overwhelming feeling that i get is that radical surgery is needed to get more goals into the goalpost. Probably, something like in hockey where they have rolling substitution could be tried out. Anyway, players are always feigning illness in order to take a few minutes break. So why not give them a few minutes break in order to get fresh legs into the ground. Otherwise, it is quite pathetic to watch these matches bereft of goals.

Harald Solheim said...

Reading this post, I'm reminded of Ole Gunnar Solskjær - deemed a super-sub - when playing for Manchester United. He was not particularly big and strong, and not as pacey as the best strikers around, but he had a lethal finish. As suggested by this post, playing him only in the last 15-20 minutes of a game would give him the extra space needed to take full advantage of his superb finishing inside the box. Combining this with his ability to analyze the game from the bench, you have a striker that although though not necessarily very effective when in the starting 11 against top teams, often had huge impact when coming in late in the game.

Ross Tucker and Jonathan Dugas said...

To Ramachandran:

Thanks for the comments. You're not alone in your views. I was actually just writing something on my Twitter feed yesterday, because only 8 goals were scored in the first 6 matches. IN the last 3 World Cups, the average has been 2.4 goals per match.

So something clearly is up. My impression has been that:
a) The finishing has been poor. The Argentina v Nigeria match was actually very good, a lot of chances. But no conversion. That match could easily have had 5 or 6 goals.
b) The strategy has been too conservative. I think teams are playing scared. Scared of losing is a more powerful force than the desire to win. My observation is that every team, barring Germany, has not committed numbers to attack. They are always outnumbered, and I think it's because it's the first match, and they're playing conservatively.

I have a feeling it will improve, as the stakes get higher. Teams that have drawn or lost their first match HAVE to win their second match. So you can expect Denmark, Algeria, Australia etc to be much more attacking in their second games.

That should in turn open the matches up. I certainly hope so, because at one point, it seemed like we might get more red cards than goals! Well, I'm exaggerating, but the scoring potential has been low so far.

As for feigning injuries, to me, the solution is simple - every time a player is "injured" and needs the game to stop for him, he must spend a compulsory 5 minutes off the field - for treatment. After all, he is injured. We'd soon find out whether players enjoy being off for 5 minutes as a result of their cheating.


Anonymous said...

Any thoughts or comments on the most effective way to train for such a stochastic effort?

Although clearly not a steady-state effort, aerobic fitness is arguably as, if not more, important than V02 or peak sprinting ability.

Does one focus on aerobic endurance, in order to flatten the decline curve? Does one focus on max sprinting ability, so as to start higher on the curve?

Of course then when get into confounding factors like how often the player plays, and whether endurance or sprinting is their natural strong suit.

Fascinating stuff. I may be way off, but either way would be interested in the conditioning and training theories and modes.

Gene said...

Please excuse me for repeating what I said in the other post's comments last night. I'm assuming that the 5% you keep mentioning is a convenient figure to make a (good) point. My sense is that in endurance, and presumably other, sports a 1%-4% difference - and sometimes less than 1% - typically separates the very best from the also-rans. This is most obvious when average relative finish times (e.g., over the course of a season) are used as a surrogate for "being at the level they need to be." Breaking it down further, some other sports have been studied more closely, with measures such as stride length and turnover in runners and glide length and turnover in x-c skiing used. In golf, a start and stop sport if there ever was one, the current difference in scoring average between the PGA Tour's leader and Tour average is 3.07%; 5% puts one at about 160th. I wonder if the same doesn't apply to football (soccer).

Ross Tucker and Jonathan Dugas said...

HI Harald

Yes, you get a few of these guys who come on late and seem to find space, and you're right. Part of it is that they have this great awareness, plus the skills, which express themselves best when the game has opened up. It's a vital part of a team's weaponry, the ability to change games from the bench.

To Gene:

Good points. The reason I keep saying 5% is partly convenience, but also because that research on soccer shows about an 8 to 10% drop in performance, and if a player could cut this by half, then they'd be 5% better off. So it was somewhat guided by the finding, but you're absolutely right, that 5% is a massive difference, and so a non-fatigued footballer compared to a fatigued player is the same comparison as your 1st vs 160th!

Obviously, there are a lot of factors that confound this - anticipation, for example, would allow a player to manage space even if they're 10% off the pace. But yes, I think the concept still applies, that the margins are so small that even 1% makes a significant difference to performance!


leagz said...

Following on Harald's point about OGS, I think you are generally underestimating, at least in your coverage so far, the importance of the tactical use of substitutes. Originally these may have been to respond to injuries, but planned substitution allows a player to burn out, say after 60-75 mins, and be replaced by someone fresh who can carve out a devastating impact against the oppositions tired legs.

aluchko said...

Since they do so much running I'm curious about how well these players do in straight running, ie running a 100m or 10K?

I know in North American football many of the receivers tend to be strong 100m runners. It would be interesting to gauge the importance of speed and endurance to socc^H^Hfootball by gauging their abilities in pure running events.

Ian said...

What about doing a piece on the physiological profile and game demands of referees and their assistants?

Ross Tucker and Jonathan Dugas said...

Hi Aluchko

Good question, and I must say I'm not sure. I have never seen performance testing on the very best elite footballers that would give us an idea of their ability compared to say, NFL players or basketballers. There are a few studies on junior players, or club level players, and I'll try to find more on elite players.

To Ian:

Will do. The plan was to look at the physiology of the game, then the fatigue issue, and then to move onto altitude, since it fits naturally in that slot. But there are some good studies on referees, and I'll certainly post on or two times on them. So might be later this week, maybe next, but I will!

Thanks for the comments!

Farid said...

The fact that "lesser" players get dragged up in terms of running performance cannot be ignored when looking only at elite level players; because there is no higher level than "the elite level" (World Cup, for example), how do we know that the very best can't be pushed to greater physiological feats of power and/or endurance.
I apologize for unearthing the mental v. physical debate, but when speaking of physiological limits of football, how can we be sure what we're seeing is really fatigue and not just an ability to "dig deep/push harder/give it all you've got". This is not to say fatigue is not a factor.
FATIGUE HAPPENS IN FOOTBALL, there is absolutely no doubt. But when a match comes down to the aforementioned 10m sprint for a ball between two "equal" players, is it fatigue or the "mental" effect that lets one win out.
(I can just see Dr. Noakes asking himself "is there a difference?").

Football is arguably a tactical and talent based sport with a lot of running in it; teams can mask their apparent fatigue with anticipation, clever ball work, and of course substitutions.

Short of testing every player to their maximum physiological potential, is there any way of teasing out whether certain results are attributable to fatigue, tactics, talent, or just a greater mental control of one's output?

Anonymous said...

I second Ian's idea to do a post on the physical demands for on field officials. It looks that they are in great shape, especially at the world cup level.

Popey said...

You talk about this 5% however is this influenced by the position of a player? What players were tested in the research that you have included (strikers/defenders or both?)

I believe position could have a large influence on fatigue. For example a left back has to do more running to overlap and support play whereas a right midflielder would have to make less sprints. (Not sure if this is the case but using this as an example)

Would this then influence the fatigue towards the end of a game if they were to sprint for the same ball?

So surely it is not as simple as you have discussed in this post.

Ross Tucker and Jonathan Dugas said...

Hi Popey

This is part of a series, as I said in the very first sentence of the post.

So to take this one alone is to take it out of context.

Read http://www.sportsscientists.com/2010/06/physiology-of-football-profile-of-game.html

For more on the positional demands of the game.