Newton and the Goalkeeper

In this article we present some basic concepts that we will apply to the Goalkeeper in future articles on biomechanics.

From the physics standpoint, the movements of the Goalkeeper and the ball are largely explained by three principles known as Newton’s Laws of Motion. The first, the Law of inertia, indicates that the goalkeeper or the ball will tend to remain in its state of motion or rest until a new force modifies that state. For example, the ball will remain still until a player kicks it. Once kicked,it will remain in motion until it is stopped by friction with the ground, the hands of the goalkeeper, the net, the posts, etc.

The second, the Fundamental Law of Dynamics, states that the change in the movement of an object is proportional to the force acting on it. That is, if the player kicks more strongly, that is, with more force, the ball will move farther.

The third, the Law of action and reaction, indicates that for each action between two objects in contact there is a reaction in the opposite direction. In the previous example, the ball will exert an equal force on the kicker’s foot. This law also applies, though not as obviously but very significant, when the goalkeeper’s foot exerts force on the ground to move towards the ball. In do so, he or she receives from the Earth a force equal to that produced by the Goalkeeper on the Earth. This force has a vertical component that counteracts the goalkeeper’s weight and a horizontal one that generates the momentum towards the ball.

The ball will remain static until it receives the blow of the foot (Law of inertia).
The greater the force it receives, the greater the distance it will travel (Law of dynamics).
At the moment of impact, the ball will exert an equal force on the foot (Law of action and reaction)

In physics, force is expressed as the mass of the body multiplied by acceleration, as in the following equation:

F = m x a

Mass is the amount of matter that constitutes objects, such as the Goalkeeper, the ball, the net, etc., and is measured in kilograms. We can imagine that all the mass is concentrated at one point, the center of mass. In practice, the center of mass coincides with the center of gravity, the point at which the force that gravity exerts on different parts of the body is summarized. In the human body, its position can vary depending on the way in which body weight is distributed, age, sex, etc. In general, in a standing Goalkeeper it is in the pelvis, below the navel, in front of the sacrum. Its location changes slightly with changes in body shape when moving.

So, when considering the “force” of the goalkeeper or the ball, its mass and speed must be taken into account. Since the mass is invariable during the actions that involve the goalkeeper and the ball, the performance variable that the goalkeeper can control is his or her own velocity.

Different parts of a Goalkeeper’s body can be considered as lever systems. These levers are organized as kinematic chains, that is to say series of parts that move and allow a greater sum of forces. Bones are the links that join at the joints and move due to the force generated by the muscular contractions applied to the tendons that surround the joints. The Goalkeeper’s kinetic chain includes the joints of the ankles, knees, and hips, and those of the shoulders, elbows and wrists.

The Goalkeeper as a kinetic chain

In future articles we will apply these notions to the understanding of different technical movements of the Goalkeeper.