Soccer players tackle animal performance

By Hugo Dutel, Muséum national d’Histoire naturelle, Paris, France

Every soccer team supporter can remember the thrilling moment when the outcome of a world cup match depends on a penalty shootout. But for Andrew Hunter, Ph.D. student in the University of Queensland, Australia, these intense seconds turn into an unexpected occasion to study how animals can balance competing demands on their actions.

Whether it is predation, fighting rivals, or even mating, all moving animals need to perform complex motor tasks that require power and accuracy. To study how animals adapt in a certain context, biologists usually measure athletic performance including physical traits such as bite force or running speed. However, little is known about the influence of individual motor skills, i.e. the complex combination of cognitive and physical abilities that determine how well we can control our actions. “We assume that every individual would select their optimal performance to success”, Hunter said, but added, “How do power and accuracy interact? How do individuals select the best combination of power and accuracy?”

With a better understanding of what influences motor skills doctors could design customized rehabilitation training for people suffering from disabilities after serious injuries. In a broader context, assessing motor skills in animals is necessary to get a more precise picture of the overall physical capacities of organisms, and on the evolutionary pressures that influence their adaptation to the environment they are living in. “Everything we do is a balance between how fast we should be doing it, and the level of control we should be doing it. We try to optimize, to reduce the error involved”, said Robbie Wilson, Senior lecturer at the University of Queensland, Australia, and Hunter’s Ph.D. advisor.

Surprisingly, one way to tease apart the questions addressed by Hunter and Wilson is to open the doors of a soccer club. Measuring motor skills in humans is in fact easier than in other animals, as people can be asked to perform specific tasks; and a soccer game is an ideal model that combines power and accuracy. Wilson started this research program three years ago, with the goal of predicting a soccer player’s performance based on athletic traits and motors skills using the same quantitative approaches as in ecology. “I was lucky enough to have a good friend who was coaching a semi professional team”, says Wilson.

The first results already challenged common beliefs. Whereas players are mainly selected based on physical and athletic criteria, such as running speed or stamina, he found that what makes a player good in a soccer match has little to do with how fit he is or how fast he runs. Of course, a player must have a good physical condition, but what seemed most important was what he could do with the ball; how he balances his power and accuracy, for instance in dribbling or shooting. “That’s what separates the good players from the absolutely great players! Look at what Messi can do with the ball, no one else can!”, Wilson said.

The first step of Hunter and Wilson’s work was to understand the relationship between power and accuracy. “We wanted to look at a very simplified part of the game. And what part of the game can be simpler than the penalty shoot?” Wilson said. “Penalty is a neat system”, Hunter explained. “It is all about power and accuracy”. Hunter first asked players to accurately kick the ball towards a target placed in a soccer net and filmed them with 3D high-speed cameras. By analyzing the recordings, image-by-image, he was able to measure the speed of the soccer ball. Hunter also quantified the accuracy of each player by measuring the distance between each shot and the center of the target. By plotting the accuracy measured at each kicking speed and the ball speed, he found that overall the kick accuracy decreases as the kick power increases. At the same time, Hunter was able to determine the maximum kicking power for each player, and to predict the optimal balance between kicking force and accuracy of each player.

He then led the same experiment, this time including a goalkeeper. In this scenario the players were challenged to accurately kick the ball into the goal, but with enough speed to beat the goalkeeper. Interestingly, players never kicked at their maximum capacities, but they adapted their shots so that they balanced power and accuracy to hit the target while defeating the goalkeeper.

However, did each player actually reach the optimal trade-off between power and accuracy? To answer this question, Hunter compared the predicted optimum kick power with the self-chosen power of each player when they had to challenge the goalkeeper. Some players actually selected their optimal trade-off; they kicked strong enough to beat the goalkeeper, but not too hard so that they avoid a decrease in their accuracy. On the contrary, other players were kicking the ball too hard; although they defeated the goalkeeper, they had a low accuracy.

Thanks to these data, Hunter and Wilson were able to advise each player on how to reach the best tradeoff between power and accuracy during the penalty kick, and to give feedback on how to adjust kick direction. Such information helps to design training adapted to the individual skills of each player, as well as refining the selection criteria for young players by designing skill tests. “We try to figure out what’s the best way to train”, says Wilson, “And we can show that in a quantitative framework. It’s testable!”.

A soccer fan knows well that a penalty kick is more than simply kicking the ball towards the net; it’s a thrilling face-to-face encounter between a player and the goalkeeper. During the few seconds that drown an entire stadium in silence, the player tries to fool the goalkeeper in the direction he wants to kick by adopting different techniques. How would these different strategies affect the power and accuracy of the penalty kick? That is what Hunter and Wilson plan to investigate in further work.

I am currently PhD candidate at the Muséum national d’Histoire naturelle in Paris, France. I am interested in morphological evolution of vertebrates and the relationships between forms and functions.

As a model research topic I am interested in lobe-finned fishes. My PhD project aims at studying the evolutionary history of Mesozoic coelacanth, as well as the functional morphology of prey capture in the extant coelacanth Latimeria chalumnae.

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