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
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!