Penalty rates: the science of World Cup football

The biggest sporting event of 2018, the FIFA World Cup, is under way and penalty kicks have been a prominent feature of the matches played so far.

Notably, superstar Cristiano Ronaldo scored the first of a hat-trick from the penalty spot as he led Portugal to a lively 3-3 draw against neighbours Spain. Meanwhile, his arch-rival Lionel Messi fluffed his lines from 10 metres in front of the goal line, his tame effort easily saved as Argentina struggled to a 1-1 draw against Iceland.

Australia’s 2-1 defeat at the hands of a disjointed and somewhat lucky French side featured a penalty for each side, both successfully converted.  The high incidence of fouls given in the penalty (or “18-yard”) box – dubious or otherwise – suggests there will be many more. In the later knock-out rounds, penalty shoot-outs will settle tied contests.

It is reasonable to wonder, therefore, about the science that influences successful spot kick.

This is where the British consultancy Quintic aim to step in – their director Paul Hurrion is an expert in the field of biomechanical analysis and counts the aforementioned Portugal frontman Ronaldo among his clients.

Speaking to Imaging and Machine Vision Europe magazine, Hurrion notes that while his firm’s sports imaging services are popular in track and field disciplines, football coaches are more subjective in their judgements. 

“Football coaches tend to let the players work it out for themselves. What we’re trying to do is highlight that there is a more scientific way to analyse human movement,” he says.

Hurrion has applied his biomechanics experience to putting techniques for golf players, using a high-speed camera to track the putter and ball, capturing between 360 and 1080 frames per second (fps) and measuring a range of parameters, including the angle of the putter head at the moment of contact and the speed of the ball when it stops skidding. 

A similar set-up is used to analyse penalty kicks. Fewer frames per second are needed to capture a penalty kick for analysis – the speed, spin and trajectory of the ball can be measured at 250 fps, along with the point at which the ball crosses the goal line, the placement of the kicker’s feet and their posture.

The goalkeeper’s stance, positioning and the direction of dive is also recorded. Multiple captures of this process are then layered on top of each other, allowing the kicks to be compared.

The aim of the technology is to improve a player’s confidence and composure ¬– high pressure comes with the territory of a penalty-taker, but as Hurrion explains, “if you know your technique can deliver and you know what you’re trying to achieve, it makes the mental side so much easier”. 

While Hurrion’s work focusses on the attackers tasked with scoring penalties, a recent study led by John van der Kamp of the Vrije Universiteit Amsterdam in the Netherlands aims to make life easier for the goalkeepers aiming to thwart their efforts.

Conventional wisdom dictates that the principal factor in saving a penalty is deciding which side of the goal to dive towards, with much emphasis placed on “reading” the taker’s run-up, kick and even gaze. Van der Kamp and his colleagues assert, however, that this is insufficient, and the timing of a dive is also paramount.

A powerfully struck spot kick can reach the goal in as little as 0.4 seconds, while an average goalkeeper – often the largest player on their team – takes roughly 0.6 seconds to reach either side of the goal, and as much as one full second to complete a dive covering either of the goal’s uppermost corners. This effectively leaves no time for a goalie to absorb the flight of the ball, and means even a dive in the right direction can often be made too late to reach a well-placed shot.

Conversely, a decision to commit to an early dive can be based on cues from the penalty-taker that can be misleading, often intentionally so. These conditions place the goalkeeper in what the study refers to as “a situation of the barely possible”.

Van der Kamp and his colleagues aim to open discussion of this issue by advocating for an approach based on the theory of affordance-based control. The notion of affordance was coined by psychologist James Gibson, and has come to be defined in human terms as the range of possibilities readily perceivable by any actor.

Thus, the study states that goalkeepers “must act in ways that sustain the perception of the ball being stoppable”. This means walking a very fine line or, as the study puts it, acting “critically close to their maximum action boundaries”.

This statement covers two components of goalkeeping. According to the rules of soccer, in a penalty situation the goalkeeper cannot move until the ball is actually struck.

The first component, thus, is timing – neither committing too early (or more pertinently, betraying their intentions to their opponent) and thus affording a penalty-taker the opportunity to pick a side, nor waiting for too long and leaving themselves too little time to reach the eventual shot.

The second is what might be called mind-games: the looks, the lunges, the fake-outs. With the time constraints goalkeepers work within, the complex interactions between custodian, kicker, and the strategies that motivate them take on greater importance.

The study suggests that successful penalty stoppers are distinguished from less capable goalkeepers in part by their physical aptitude, but also by their ability to separate useful from misleading information when analysing the actions of the kicker.

A 2014 study conducted by University College London in the UK and reported in the journal Current Biology supports this finding, suggesting goalkeepers fail to make saves in penalty situations because they perceive trends that don’t exist.

In short, the best goalkeepers don’t simply read their opponents, they make more sound decisions in choosing what to read.

The Vrije study is published in the German Journal of Exercise and Sport Research.

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