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The Right Angle

Forward in all directions!
    -- Slogan of musical group 3 Mustaphas 3

Why do Goalkeepers Dive Backwards?

As a goalkeeper, I was always taught to dive forward at an angle for balls. This is also what I coach. However, many goalkeepers have a strong tendency to dive backwards. You even see this at the international level. Why is this so common when supposedly the coaches have taught them otherwise?

I encountered a fellow on rec.sport.soccer who claimed a goalkeeper should always dive backwards, since it gave them more time to react to the ball (he was obviously not a goalkeeper coach!). But there had to be some reason why the tendency was so strong. I decided to write a short program to simulate the problem and see what it could tell me.

Modeling the Situation

I modeled the situation with some simplifying assumptions (see below), varying the position of the shooter and the goalkeeper, the direction of the shot, speed of the goalkeeper and the ball, and the delay of the goalkeeper's reaction time. This last turned out to be the key. If the keeper always reacts at the same instant the ball is struck, the angle of interception can never be less than square (90 degrees). However, if there is a delay - which there almost always is - the optimum angle of intercept can be backwards (greater than 90 degrees)! The longer the delay and the slower the keeper, the more "backwards" the angle can become.

I put the program into a Java applet so you can look at it graphically and play around with it. The blue spots are the shooter and the final ball position; the black circle is the goalkeeper. The lines show the path of the ball and the angle between the shooter and the goalkeeper. If the lines are green, the ball was saved and the angle of intersection is shown. If the lines are red, a goal was scored and the keeper is shown at the point they got closest to the ball and their angle at that point.

Shooter and GK Y distances are measured in feet from the goal line; X distances and the "Shoot at" spot are measured in feet from the center of the goal, with negative numbers being to the left of center and positive numbers to the right.

So, why do goalkeepers dive backwards? The answer is that sometimes that's the best angle to intercept the ball. I think instinctively, athletes will try to make the most efficient movements; in this case, get to the ball by expending the least amount of energy, at the lowest speed possible. As we can see, slower speed means a more backwards angle. And consider the case of a hard, close-in shot: by the time the keeper reacts, the ball is practically past them and their only hope is a backwards sprawl.

Why Dive Forwards?

So given that the best angle to dive at is often backwards, why coach goalkeepers to dive forwards? There are still four very good reasons, three of which don't show up in this simulation. They are listed in roughly their order of importance:
  • Greater acceleration and speed
    Backwards dive angles may be optimum at a certain speed, but in any situation the dive angle decreases as the goalkeeper speed increases. A forward step allows a more explosive power step and greater acceleration and speed on the dive. The legs can't generate nearly as much power with a step backwards as with a step forwards. In a close case, a few extra fractions of a foot per second of extra speed generated might turn a "dive backwards and miss" into a "dive forwards and make the save". You can easily see this in the applet above. The faster the goalkeeper speed is set, the better the forward angle.
  • Better angle of deflection
    If the goalkeeper gets to the ball but does not make a clean catch, they have a much better chance of knocking the ball away from the goal if they are diving forwards. A goalkeeper diving backwards often will manage only to knock the ball into the side netting.
  • Better catching position
    It is easier to catch a ball that is coming straight at you - not rising or falling. As a goalkeeper dives backwards, the angle the ball approaches the keeper's hands becomes greater, seeming to travel "upwards" from the palms towards the fingertips. To get a "straight on" hand position, the keeper would have to dive at the angle perpendicular to the flight of the ball - and this angle is always forwards if the keeper is properly positioned.
  • Better coverage of the goal with the body
    Diving square means that the length of your body is covering the largest portion of the goal. This can be crucial if the dive is misjudged or the ball takes a bad hop. As the dive turns backwards, the keeper gets more and more "feet on" to the shooter and the body covers less of the goal.

To conclude, the goalkeeper response of diving backwards is to be expected, since that's where the easiest interception point may be. The human mind is pretty good at instinctively judging angles (as a friend of mine put it, "You don't have to know much about trigonometry to figure out that bus is going to hit you if you step off the curb."). Our challenge as coaches is to overcome the instinct in order to teach the players how better to keep the ball out of the net.

Some assumptions and simplifications used to create the above applet:

  • Both goalkeeper and ball start their movement instantaneously and at a constant speed. This somewhat approximates a driven soccer ball, but obviously is not even close to actually simulating a goalkeeper's dive.
  • I have no idea what a realistic number is for the goalkeeper's dive speed. You can put it close to zero to approximate some of us. :-)
  • Goalkeeper and ball are both represented as points. Again, this approximates the soccer ball much more closely than the goalkeeper.
  • The problem is kept strictly two-dimensional. Height (up/down) aspects, as for a lofted ball, are ignored.
  • Some situations can be pretty unrealistic because I have not limited the distance the goalkeeper can dive. The keeper simply keeps going until the ball is saved or it's in the net.

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© 2003 Jeff Benjamin, all rights reserved