ACCELERATION OF PARTS OF THE BODY IN SIDE DIVES

When teaching dive techniques, Goalkeeper Trainers usually refer only to the impulse provided by the legs to reach greater distances, without considering the contribution of other parts of the body that do not push against the ground. However, accelerating the hands directly towards the ball helps to increase the general momentum of the body (Figure 1) and, consequently, the linear distance traveled during the dive. Although the arms and hands have relatively little mass in relation to the total body mass (3.85% arm, 1.97% forearm, 0.65% hand), since the force they can generate is proportional to theirs acceleration, propelling them with speed adds thrust to that provided by the legs. A small increase in the accelerations towards the ball can make the difference between saving a goal or not. Boxers and martial artists acknowledge the importance of accelerating the arms to provide more power in their blows.

Figure 1. The resulting force (yellow arrow) of the acceleration of the arms towards the ball can be analyzed by decomposing it into a vertical component (red arrow) and a horizontal component (green arrow).

Some coaches favor accelerating the knee of the far leg in the direction of the ball once the near one has started to accelerate the body. This action usually occurs in situations of maximum demand, even for elite goalkeepers. The leg’s mass comprises about 17% of the total body, but if its acceleration is greater than that of the body, its contribution to the total thrust could be significant. Since the line of action of the acceleration of the center of mass of the leg does not pass directly through the center of mass of the body, it may generate a rotation around it that could move the body away from the shortest direction towards the ball. This rotation would be backward if the knee is raised frontally, or downward if it is raised laterally (Figure 2). To avoid these vicious deviations, the goalkeeper performs postural compensations.

Figure 2. Since the force resulting (yellow arrow) from knee acceleration does not pass through the goalkeeper’s center of gravity, it produces a rotation. If the acceleration is toward the front, the rotation is backward and requires compensation trough the anterior flexion of the trunk. If it is upward, the rotation would raise the legs and lower the front of the trunk, so the goalkeeper flexed the trunk laterally to compensate it.

There are also goalkeepers who, before or during diving to intercepts the trajectory of the ball, accelerate one or both heels behind the body. The accelerated mass is less than the total of the two legs, but it could still add something to the total force. In an analogous situation to knee acceleration, it would produce a rotation, in this case forward, which these goalkeepers usually compensate with a backwards flexion of the spine (Figure 3).

Figure 3. Acceleration of the heels produces forward rotation, which is compensated by dorsal flexion of the trunk.

Many goalkeepers, especially those who adopt a set position with their hands hanging at the sides of the body, during a lateral dive accelerate their hands towards the ball with a pendulum movement of the arms. Generally, these goalkeepers practice a previous countermovement by bringing their arms back before propelling them towards the ball. By accelerating the outstretched arms toward the ball with a pendulum motion, the hands make a curved path with a radial (outward) and a tangential to the curve components (Figure 4).

Although this counter-movement generates much momentum, when the shot is very fast it can be less efficient than directing the arms directly to the ball. First, the distance traveled by the hands is longer, which implies that the arms should move faster and overcome greater resistance. In many sports activities for a limb to travel a distance faster, the farthest part of the limb is first picked up and then extended. Consider, for example, the mechanics of the leg when running or the movement of the arms of a dancer or skater when making fast turns. This reduces resistance to rotation and increases the angular speed, that is, the speed in degrees per seconds that the limb travels.

Figure 4. In the pendulum movement of the arms, the resulting force (yellow arrow) has a radial component (green arrow; from the center of the curve, outwards) and a tangential component (red arrow; touches the curve, but does not cut it). At moment 3, the resultant points directly at the ball, the radial component contributes to the horizontal thrust and the tagential tends to prolong the trajectory of the hands towards a plane posterior to that of the goalkeeper’s body. If the movement were prolonged (moment 4) the tangential component and the resulting component would tend to move the hands away from the ball.

In Figure 4 it is seen the pendulum trajectory of the extended arms. At each instant of the curve the total force developed consists of a radial force (in the direction of the arms) that pulls the curve outwards (as when taking a curve with the car at high speed) and a tangential force, perpendicular to the radial, which accompanies the direction of the curve at that time.

When the hands align with the ball and the body (Figure 4; moment 3), the radial force has its maximum efficiency to add to the general momentum. Instead, at that moment the tangential component gives the arms an impulse in a different direction, backwards, wasting energy and forcing the hands to continue the rotation; thus it is necessary to slow down the movement to achieve effective contact with the ball. In addition, it would produce a backward rotation around the center of gravity. After the moment when the hands align with the ball, both the tangential and the resulting force would contribute to the backward rotation (Figure 4; moment 4).

It is common to observe goalkeepers who produce an impulse with their legs with a trajectory perpendicular to that of the ball, but due to the pendulum movement of the arms, the hands do not reach the ball in time or end up taking it behind the path considered most efficient.

REFERENCES

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Rathee, N. K., Magnes, J. y Davis J. (2014). Kinematics of board breaking in karate using video analysis –a dynamic model of applied physics and human performance. European Scientific Journal, 10 (12), 338-348.

Vizcaíno, S.F. y Cortizo, L.H. (2017). Análisis biomecánico cualitativo del vuelo del portero de fútbol. RED 37(2): 1-8.

Vizcaíno, S.F y L.H. Cortizo. 2020. Caídas laterales bajas del portero de fútbol. Incidencia, biomecánica y entrenamiento. Lecturas: Educación Física y Deportes, Vol. 24, Núm. 261.