Introduction

The field of biomechanics in soccer involves the study of the different movement patterns during game and practice. Coaches, whether they recognize biomechanics or not, are likewise concerned with the effects which the forces produce or the forces which act on a player. Their ability to teach the basic techniques of soccer depends largely on their appreciation of the effects they are trying to produce and the forces which cause them. Video and film recordings facilitate a more detailed skill and running analysis in soccer.

The aim of this presentation is to do a review consisting of the biomechanics of running and the development of running skills in the junior soccer players and to indicate how this information could be applied to soccer training and coaching. Individual running skill is not a singular element which can be defined in conclusive terms, in fact, it is constantly developing. There is no form of individual running skill which is universally valid for everyone. There are, however, a few basic rules for the trainer to follow. The importance for the trainer is to perceive each player's own individual technical qualities and the ways in which these skills may be developed further. Technically gifted young players are able to learn more skills, and faster than the ordinary players.

Coaches have to understand biomechanical structure of players concerning the total body segments and basic characteristics of their structure: mass, height, center of gravity and moment of inertia and additional biomechanical function of players concerning the acting forces in players and environment; weight or gravitational force, ground reaction force, friction, muscle force produced, elasticity forces and air resistance. Additionally the coach can gain an advantage, if he is able to evaluate or measure the basic structure and functions of his players.

Speed in Game Situation

Velocity in game situations in soccer depends on the foreshadowing of the game, reaction, choice reaction and movement time, starting position and velocities in start, maximal speed and final phases. The experienced high caliber players are able to read the game well.

Figure 11. Factors influencing the velocity in game situation in soccer

Running and maximal velocities in soccer depend on the muscular and reaction force production and mass of the player. In the different phases of running it is important to analyze the changes of velocity - decrease and / or increase - and to think about the reasons for these changes from the biomechanical point of view. Stride length and frequency in running can be divided into different parts as seen in Figure 12 .

Figure 12. An analytical model to evaluate running velocity in soccer

Contact time can be divided into single and double support time and according to muscular function into impact (eccentric), static and pushing (concentric) phases.

The primary factors influencing a high velocity in running are the high stride frequency and stride length. In most groups stride frequency provided the dominant role, especially in the start. In practice the stride frequency means short support phase especially short double support phase and fast movement of the recovery leg under the trunk but not too much forward movement respect of the center of gravity of the total body. All decelerating and resisting forces must be minimized.

The start in football sprints includes both the reaction and movement time. To the player it is important to find the valid stimulus for action in his visual field. This process from input to output consists of both the reaction and choice reaction time, which are important factors for high class football players. Fast reaction and reaction times are important factors for high caliber soccer player.

For economic running in soccer, it is important to check factors as follows:

• Horizontal distance of the center of gravity from the contact point of the foot during impact should be as short as possible.
• The angle of the shank relative to the ground level upon impact should be about 90 degree as compared to the ground level.
• Ball contact rather than heel contact.
• Vertical oscillation of the center of gravity should be small.
• Action of the recovery leg in impact phase should be accelerated.
• Active push of the ankle should exist already in impact phase.
• Co-operation of legs and arms harmonious.

Starting, Changing Direction and Turning

For fast starting in the good starting position, the vertical line of the center of gravity must be as close as possible to the edge of the support area in the direction of intended motion. In stopping the body lean is causing deceleration. Then the gravity line is taken as far as support friction permits from the edge in the direction of the motion.

A balance principle appropriate for running type linear acceleration and deceleration situations is as follows: For maximum stability in motion the base of support must be enlarged in the direction of momentum and in the direction of intended acceleration or deceleration. The greater the momentum, the larger the base of support should be in that direction. The greater the acceleration or deceleration, the larger the base of support should be in that direction.

Any change in the direction of running and in linear momentum is caused by an external impulse in the direction of that change. The greater the direction change desired, the greater must be the push and ground reaction force. In the abrupt direction changes the ground-foot friction need to be high enough to prevent slipping when the foot push is made. In wet conditions because of lowered friction the massive players are not so quick in chancing direction, accelerating or decelerating running and maintaining balance.

In turning around 180 degree the longitudinal axis of the player it is important to keep the mass distribution as close to the turning axis as possible so that the moment of inertia is small and the turning fast. The extra steps during turning need to be avoided.