The Science of Robbie Keane

How many muscles does Robbie Keane use to kick a ball and do the boots he wears make a difference? CLAIRE O'CONNELLfinds out

KICKING

Robbie Keane has put the ball in the back of the net more than 50 times while playing for his country. You don’t notch that up without some impressive legwork: and plenty of studies have looked at what’s involved in a soccer kick.

Let’s think about kicking a stationary ball: perhaps taking a free to pass to a team member. The kicking player will often take a short run up and strike the ball with the instep, or inner side of the foot.

A soccer kick is actually a swinging motion, and it involves more than just the kicking leg, it’s about the whole body, with various muscles working in concert to stabilise or to move body parts in a co-ordinated way.

In the build up, the kicking leg generally swings back and the pelvis rotates to bring the leg forward, the knee extends and the instep of the foot collides with the ball. Using the relatively large surface area of the instep can allow a skilled player to control where and how the ball flies.

So when the kicking foot eventually makes contact with the stationary ball, it takes just milliseconds for momentum to be transferred and the ball hopefully heads off in the desired direction.

GOAL CELEBRATION

When Robbie Keane scores a goal, whether it’s for Ireland or (currently) LA Galaxy, he has a goal celebration where he runs towards the side of the pitch then does a quick cartwheel followed by a head-over-heels to land and face the crowd.

So what kind of physics lie behind his signature gymnastics mini-routine? Keane first accelerates to build up linear momentum, which is determined by his mass (which he can’t change quickly) and velocity (which he can increase, by running).

Then he leans over and literally falls off balance towards the ground.

Keane’s body rotates into the cartwheel and when his hands hit the pitch his legs swing up and over. Again the velocity or speed is important in keeping this angular momentum. Then his feet land back down on the ground, he curls and falls into the head-over-heels motion. Hurray!

PUTTING THE BOOT IN

The original soccer boots were hard leather work boots worn by English factory workers, but today soccer footwear is specifically designed to be light and keep good contact or traction with the playing surface.

So can a well-designed shoe help performance? One study measured the impact of a lighter or heavier shoe on the all-important ball velocity or speed of the ball after the kick.

“We did not find an increase of maximum ball velocity with a lower shoe mass,” wrote the researchers in a paper this year in Research in Sports Medicine.

“Apparently, a higher shoe mass compensates for the lower foot velocities and thus creates a similar momentum.” They also found that barefoot kickers could make the ball move quickly, but there was a sacrifice on accuracy.

The researchers point out that the design and arrangement of cleats on the soles of soccer shoes can allow for fast accelerations and stops, rapid cuts, and turns.

So overall, yes, it seems the nature of the boot can make a difference: “Soccer shoe design can influence shooting speed and, even more important for the game of soccer, kicking accuracy,” they concluded.

SWEATING AND HYDRATION

Soccer players can perspire a lot over the course of a match – running around the pitch, especially in a warm or hot environment means the body tries to keep cool by producing sweat.

Many players reduce their body mass the equivalent of over 2 per cent through such sweating, according to a paper last year in the Scandinavian Journal of Medicine Science in Sports.

And another paper in the Journal of Sports Medicine and Physical Fitness gave details of how players competing in the 2008 European championship experienced fluid loss and a decrease in body weight.

Our bodies need to keep enough water on board to function well, and with all that fluid making a hasty exit it’s important for soccer players (and any other sweaty athletes) to keep well hydrated.