“Fatigue is the number one injury risk factor”

Tim Harkness is a sports and performance psychologist with extensive experience in psychology, physiology, data, and elite European football, including work at Chelsea FC. He now advises organizations in elite sport and business on complex performance challenges. In this interview, Tim discusses why fatigue is often misunderstood in football, how data should inform injury decisions, and why effective sports science frequently remains unrecognized.

Fatigue is one of those words everyone uses and assumes they understand. Based on your experience, what do people get wrong about what fatigue actually is?

Tim Harkness: I think, for whatever reason, fatigue isn’t an integral part of the conversation around football. People are happy to talk about talent, motivation or form, and every now and again someone will mention an overloaded fixture list. But I don’t see many educated conversations about fatigue.

Clearly, there’s a connection between fatigue and injury risk. In my view, fatigue is the number one injury risk factor. But there’s another consequence that’s just as important: a decline in performance. When I worked in professional football, we had some players who became a severe injury risk the moment they moved outside their loading window. Others weren’t at much greater risk of injury, but their performance simply got worse and worse. Both situations are something you want to avoid.

Another thing people often misunderstand is that there isn’t just one kind of fatigue. You can have muscle fatigue, nutritional fatigue or aerobic fatigue, and they all affect performance differently.

Fatigue also happens over different time scales. You can become fatigued over seconds, minutes or a single match. But you can also become fatigued over days, weeks or even months. A player might not be fatigued over 90 minutes, but could be completely fatigued over a 28-day period.

One of the most interesting—and least understood—things in football is long-term fatigue. Some players become fatigued over 12, 24 or even 36 months.

I think you sometimes see it in teams that perform brilliantly for a season or two before the whole club drops off. We had one world-class player at Chelsea who believed he’d become fatigued over seven years. His attitude completely changed. He said, “I’ve worked hard for seven years. I’m looking after myself now.”

That, for me, highlights one of the biggest responsibilities of working in football. The football calendar is built around entertainment, not athlete health. It’s the club’s job to protect the athlete.

Let’s look at a real-world example. A coach wants twenty runs in behind the defence, but the player only has six in him. How do you bridge what the tactics board wants with what the body can actually give?

Tim Harkness: Cycling has a concept called “burning a match.” The idea is that you can only produce a certain number of maximal efforts during a race. Once you’ve done one, you’ve burned a match. Cycling understands fatigue very well because fatigue is the sport. One thing it teaches you is that motivation can only take you so far. Being more than fully motivated doesn’t allow you to overcome fatigue.

Football is different, but players are still burning matches. If you’re asking a player to accelerate into a full sprint behind the defence, that’s a significant physical output. Most players only produce around 50 metres of sprinting above 30 km/h during a match. That gives them roughly six to eight maximal sprints.

So if a coach wants players making runs in behind, they need to understand that those actions are limited. If you can only do it six or seven times in a game, that’s roughly once every fifteen minutes. The tactical plan has to reflect that reality.

A good example was England playing Mexico at altitude. England knew they weren’t acclimatized, so they changed the way they played. Instead of pressing aggressively throughout the game, they stood off much more and reduced their physical output. But when the right moment came, they pressed aggressively, won the ball and scored. That’s what good coaching looks like—understanding that physical capacity is limited and choosing the right moments to use it.

You once mentioned that your opponent’s fatigue data would be golden information. Was there ever a moment when reading the other team’s tiredness changed a tactical decision?

Tim Harkness: We never had that data, so we couldn’t make decisions based on it. But it’s exactly the kind of information I’d want.

At Chelsea we tracked things like accelerations, decelerations, sprint speed and work rate, then linked those moments to video. The players loved watching clips of high-speed runs because they usually led to something positive—a goal, a recovery run or a great tackle.

What really interested us, though, was work rate. When players were working deep into the red, the video often showed mistakes or fouls. At that point, they’re simply trying to survive.

If I knew an opponent’s heart rate was 180 or 200 beats per minute, that’s exactly where I’d attack. That’s the player I’d isolate and go after. If I could have one piece of information in real time, it would be: Who’s burning right now? Because that’s where I’m going with the ball.

How much of the injury story is really just a fatigue story that nobody recognized in time?

Tim Harkness: I think it’s a huge part of it—70, 75, maybe even 80 percent.

One thing we never did was routinely test athletes with blood, saliva or physical screening tests to measure fatigue. I never believed in that approach. Those tests are noisy, and if the results tell you an athlete has a problem while the athlete says, “I feel great,” there’s no agreed way of deciding which one you should trust.

For us, the least noisy measurement was what the athlete had actually done. We measured how far they’d run, how fast they’d run, how much they’d accelerated and how hard they’d worked. Then we compared that with what the same athlete had done before.

That gave us a simple but relatively noise-free way of assessing fatigue. From there, our job as sports scientists was to assess risk. The coach’s job was to decide whether to take that risk. We might tell the coach a player has a 30% chance of injury. If it’s a squad player in a routine game, you probably don’t take that risk. If it’s a key player in a crucial match, maybe you do. That’s not the sports scientist’s decision. The sports scientist’s responsibility is to explain the level of risk. The coach’s responsibility is to decide whether it’s worth taking.

Injury prediction is a bit of a paradox. If you predict an injury and it never happens, you can’t prove you were right. How does that affect communication between sports scientists and coaches?

Tim Harkness: That’s exactly the challenge. The injuries that are easy to predict often never happen because you intervene. The difficult ones are the ones you don’t predict.

One thing we always wanted to understand better was whether our forecasts were accurate. If we said a player had a 30% chance of injury and the coach decided to play him anyway, did those players actually get injured around 30% of the time? That’s how you improve your forecasting. But until you’re completely confident in your forecasting model, the whole system depends on trust between the sports science team and the coach.

I remember one of the first conversations I had with Thomas Tuchel. After I explained our injury forecasting methodology, he said something I’ve never forgotten: “I will never, with hindsight, criticize a decision you’ve made. Every decision we make, we make together.”

That gave me the confidence to express my opinion honestly. He’d shown that he was trustworthy, and that’s probably the most important ingredient in this whole process. People will only speak openly if they know they won’t be judged by the outcome alone.

In many ways, sports scientists are the people telling athletes to slow down while everyone else wants them to push harder. What kind of person does it take to do that job well?

Tim Harkness: I don’t actually think our job was simply to slow players down. I remember talking to Eden Hazard once. He didn’t like wearing his GPS device in training because he didn’t like being measured.

I said to him, “We’re not asking you to wear a GPS because we want you to do more. Sometimes it’s because we don’t want you to do too much.” He looked at me and said, “I never do too much.”

That was an Eden Hazard joke. In matches he’d give everything, but in training he knew exactly how to manage himself.

The important thing is that overloaded players are at risk, but underloaded players are as well. One of the best ways to avoid injury is not to overload players. Another is to prepare them to cope with load—and the only way to do that is by loading them appropriately. Our job wasn’t simply to slow people down. It was to make sure every player was loaded appropriately across all the different qualities they needed to perform. Sometimes that meant holding players back. Sometimes it meant pushing them harder.

Sports science departments often have some of the smallest budgets in European football. Do you think clubs are actually making the most of the data they already have?

Tim Harkness: I think it’s an incredibly exciting time. Better data around match performance can lead to breakthroughs that simply haven’t happened in football yet. For years we’ve been able to track where players are on the pitch. But the next step is understanding how they’re moving.

Once you know how a player moves, you can compare those movement patterns against that player’s normal profile. You might spot changes that tell you something about fatigue, emotion or awareness before anyone else does.

When I was at Chelsea, people kept telling me that kind of information was coming. It never really arrived. That’s why I find this area so exciting today. I think it will provide the next generation of information for sports scientists and coaches—especially if it can be delivered live.