Tilting At Windmills: Tracking Fatigue In Team Sport

Battling fatigue in sport is a science. How we can use tracking data to help?

By Prof Rob Aughey

How Tired Is That Athlete?

Tracking energy expenditure in team sport is a whole new ball game.

We live in an exhausting world, but for professional athletes (and those that manage them) battling fatigue is a science. Over the last century, physiologists have been fascinated by what causes fatigue. Giants of the field like Nobel Laureate Archibald Hill designed elegant experiments to understand what exactly happens when we move, but testing isolated muscles in a bath was a long way from being able to fully grasp the intricacies of human functioning. So what do we know about fatigue all these years later and how is it measured today?

Early Attempts to Understand Fatigue

Hill was a pioneer of sport science, well before the discipline truly existed. In 1926, he applied his newfound understanding to athletic performance in a paper titled ‘The Scientific Study of Athletics’. In it he stated “our bodies are machines, whose energy expenditures may be closely measured”. Since then, Hill’s work has given rise to a thriving industry of fatigue investigation, and still today the science is primarily concerned with a single question: how much energy can the body produce when working at a constant intensity for a long period of time? The reason we even care about this is simple – if we understand the limitations of performance, perhaps we can work out how to eradicate them. From the perspective of competitive sport, it’s not hard to see why this is such a juicy proposition. 

As sports science developed into a serious discipline around 40 years ago, scientists entering the field brought with them deep knowledge from fields such as physiology. It’s only natural that concepts of fatigue followed physiologists from benchtop studies in the lab to application on the field, and today these notions inform the decisions of trainers, coaches and athletes across the world. Early concepts fit really nicely with our modern understanding of prolonged, relatively constant efforts – such as those you see in cycling, running, rowing or swimming. Time trials, where an athlete needs to cover a certain distance in the shortest period of time, inform us that there’s a certain strength of effort athletes can sustain and that they will ‘pace’ their efforts to get them over the finish line. 

We know that pacing is a skill, and like any skill it can be improved upon with practice. Put simply, an athlete can learn to adjust their effort levels higher or lower depending on how fatigued they feel and how long is left to go. Time trials are a particularly pure form of measurement, where physiology largely rules over skill, tactics and strategy. The athlete gives their greatest sustainable performance, without the need to react to their opponents or the scoreboard. These studies give us fascinating and insightful information about the human body, but do they really apply to professional, competitive sport?

What We Think We Know (And What We Definitely Don’t)

Of course, as we often find in science (especially the science of human performance), a little bit of knowledge can be a dangerous thing. What we think we know might merely reflect the things we can measure – leaving out vital factors in a sort of blissful ignorance. As sport scientists, we can sometimes be guilty of applying what we think we know to a new area, without first understanding all their differences, and finding ourselves in a 2+2=19 scenario. In fact, the one thing we do know is that hardly any knowledge on any given topic is complete, and rarely can it be freely applied to new situations. Nuance is crucial, but difficult to manage in the high pressure environment of professional sport. 

Fatigue is not a fixed state. A tired athlete doesn’t stay tired. Wherever intensity fluctuates, there are key opportunities for recovery. We know these periods exist in actual sporting competition, yet hardly any laboratory studies account for them. And so, despite the best efforts of researchers, our knowledge of how fatigue plays out during competition is poor. To truly understand fatigue in this arena, we need to be able to directly measure the physical output of players during competition. 

There’s No ‘I’ In Team

Team sport is about more than the individual. Despite how fans might sometimes feel, games are won and lost by the team, not by a single player. Performance relates to much more than one player’s level of running or effort. 

In sports such as cycling and rowing, we can clearly measure how hard an athlete pushes on the pedals or pulls on an oar. Recently, we used power meters to measure the important factors in rowing performance. From there, we can start to examine the effects of fatigue and design training programs to combat it. In team sport, however, there are many more factors to consider. We can measure the result of a player’s physical output via electronic and performance tracking systems (EPTS), which gauge position and speed of movement up to 25 times per second. Instead of measuring the specific force being applied, as we do in cycling or rowing, we measure the result of that effort. But if we want to come close to truly understanding fatigue in team sport, we need ways to more effectively measure the force applied by players as they battle it out as a group.

When playing on a team, an individual’s movement is related to any number of possible factors: the position and movement of others on the field, strategies for scoring and defending, or the location of the ball and how they want to maneuver it. A team player doesn’t pace themselves in the same way a marathon runner might. Protecting your energy for the end of the game is not the most viable way to win. Teams win by scoring, and that can happen at any moment. At the same time, in most team sports, players can be pulled off the field and substituted with rested replacements. With no need to compete for the full duration of a match, there’s no imperative to pace the effort.

You cannot draw a straight line from how much effort a player puts in, to the outcome of a match. But, although it doesn’t guarantee a better result, fitness does matter. When all else is equal, a fitter player can make important efforts in matches during the parts that really count. And so, we need a way to measure the energy expended by players to maximise their physical conditioning and ensure they can rise to the challenge.  

Some EPTS involve players wearing a special device to track their movement. For example, global navigation satellite systems (GNSS) – like what you find in your smartphone – communicate with satellites to determine the location of the player at regular intervals. Many of these GNSS also have accelerometers built in. These nifty devices measure all the things that require energy expenditure by a player, like a foot hitting the ground with every stride, collisions with opponents, kicking, throwing or tackling. We can see them at work, effectively measuring energy expenditure in Australian football (see here). We know that they are more effective than GNSS alone at identifying differences in physical output in rugby forwards who expend a lot of energy, but don’t really run that much (see here). And we know that when a player expends a higher amount of energy in a football match, it takes them longer to recover (see here). 

So, whilst we may not have reached the holy grail of measuring fatigue in all its complexity just yet, we do have the tools at our disposal to measure energy expended. And that is no small feat. Understanding this offers context for far better insights into the movement profiles of players. Energy expenditure becomes one more constraint to understand in the complex system that is team player performance. We got here almost accidentally, thanks to manufacturers including accelerometers in wearable devices for other purposes. What accidental developments will lead to future breakthroughs? We now have real tools that just may aid in our future understanding of fatigue and performance in team sports. Archibald Hill would be proud.