| Biomechanical Aspects of Soccer Performances |
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IntroductionBiomechanical techniques can be used within any sport, and soccer in particular, to define the characteristics of skills, to gain an understanding of the mechanical effectiveness of their execution and to identify the factors underlying their successful performance. This knowledge and understanding can help to enhance the learning and performance of those skills. There is a wide range of skills which form the foundation of soccer performance but only one has been the real subject of detailed biomechanical analysis. Kicking is without doubt the most widely studied skill in soccer. Although there are many variations of this skill due to ball speed, ball position, nature and intent of kick, the variant which has been most widely reported in the literature is the maximum velocity instep kick of a stationary ball. In contrast, some skills such as throwing in and goal keeping skills have received little attention, while a vast range of others, for example, passing and receiving the ball, tackling, jumping, running, sprinting, starting, stopping and changing direction have not been the subject of detailed biomechanical analysis at all. There are also many factors which interact to affect the response of the equipment used within the soccer. The ball, boot and shin guards itself have mechanical characteristics which are subject to variation, but which can be reasonably well quantified. The interaction between the player and the equipment is also a source of variation, but it is difficult to quantify and makes the efficacy of equipment more difficult to predict. Although equipment manufacturers will undoubtedly have conducted extensive research, little of this is in the public domain. Injuries in soccer arise due to many inter-relating factors. Some of these factors are to do the effects of equipment and environment and can be isolated. The soccer boot has a poor protective function. Careful design of the boot can have a minor influence on the severity of inversion injuries. The inadequacy of the boot is indicated by the need, and success, of alternative methods for providing ankle stability. As compared to the developments in running shoe technology, little attention has been paid to shock reduction or rear foot control characteristics of the boot. Artificial surfaces produce different injury profiles than natural turf pitches. It would seem that the type of surface may be responsible for a change in injury profile by changing the nature of the game. This change requires an adaptation period, and players are more likely to be at risk if they change frequently from one surface type to another. Obtaining clear evidence of specific pitch constructional characteristics on injury is complicated by the interacting influences of a variety of factors. Careful instruction and skill development along with correct equipment would seem to be necessary for young players.
It is evident that coaches, physical trainers, physiotherapists and medical doctors in soccer have their own point of view as to the importance of these questions. This presentation will focus mainly on the suggestions relating to the better understanding and improved performance of the players and teams with safety methods. Individual Skills in SoccerThe aim of this part is to review from a biomechanical point of view individual skills, basic movements in football and to discuss the development of individual skills and basic movements in players and to draw conclusions about the methodology of teaching and coaching individual skills and basic movements to players. Individual skills range from the fundamental basis for possession of the ball, for keeping it under control in difficult match situations and for using it to good advantage. Good technical skill adapted to any particular situation enables the player to avoid losing the ball too frequently and then having to expend more energy in trying to regain it. Individual skill is not a singular element which can be explained in conclusive terms; in fact, it is constantly developing ( Figure 1 ). Figure 1. Different factors influencing individual skills in football  There is no form of individual skill which is universally valid for everybody. There are, however, a few basic rules for the trainers to follow. The important thing is for the trainer to perceive each player's own individual technical qualities and the ways in which these skills maybe developed further. Technically gifted young players are able to learn more skills and faster than the ordinary player. The trainer must ensure that a training session works on all the interdependent motor faculties of his players, such as speed and strength together or speed and agility. The physiological effects of the training are thus bound to complement each other and do not in any way cancel each other out. Additionally, individual skills depend upon perceptual, and maybe intellectual, abilities of the players. Motivation in the individual skill training depends on how complex or simple and how real or artificial the training is. Control of skillful movementThe movements in soccer are monitored by players internally. Sense organs within the muscles, joints and tendons provide information to the central processing system about their movements. This is commonly called muscle sense or kinesthetic sense. As movements are made, the information sent to the central processing system is used to monitor the movement for any necessary modifications. While this is occurring other information from external sources will also be used in the monitoring process. The player in football matches and practices makes decisions relative to his overall plan, assisted by perceptual stimuli coming from various sources. According to his past experiences the stimuli may not have any meaning to him. A process of selection occurs also so that all irrelevant information is disregarded. He is receptive only to the important perceptual stimuli which come from his immediate vicinity. All the perceptual elements which might provide signs for his decision whether to kick the ball or not, or in which direction etc. are accepted as information. The relevant information is then processed in the central nervous system. All skills in football include all the domains which are dependent upon the control system of man. For learning skills, the actions of external and internal feedback loops are important. In the internal loop 1) the nerve endings in skin tell the footballer about the touch of the ball, 2) kinesthetic receptors in joints control the joint angle, 3) muscle spindles relate to the length change in the muscle, and 4) the Golgi apparatus the tension in tendon, respectively. The quality of this mechanism is obviously hereditary. Additionally the action of this mechanism picks out the talented players. In the external feedback system the visual and auditive systems play the most important role. Individual Skills - Basic Movements in SoccerLittle attention has been focused on the total skill and basic movement analysis of soccer players. Permanent records by means of video-tape and film-recording facilitate a more detailed total skill analysis in football concerning the individual skill analysis, the locomotion and tactical behavior of players in matches, training and studies. It has been therefore possible to obtain information about technique or skill frequencies (ball contacts, passes, dribbling, tackles, jumps, turns etc.), the discrete distances and times for high intensity work (striding and sprinting; without or with ball) and the times for the intervening periods of low intensity work (walking, jogging, forwards, backwards, sideways etc.). One of the studies concerning basic movements in twenty professional level footballers has demonstrated that the average distance covered in a match has been about 8.6 km and is increasing up to 14 km (e.g. Reilly & Thomas 1976, Bangsbo 1994) The distribution of different work rates in meters has been as follows:
Table 1 explains a summary of work rate of a top class player divided separately into the work rate due to the matches and training sessions with his/her team (individual training has not been taken into account in this summary). There is a limited number of training analysis available in the literature. Therefore the content of an average training session has been estimated according to the opinion of the author. The basic difference between the match and training has been that a training session includes more higher intensity small sided games. This means automatically more ball contacts and less walking when training. In Table 1 has been assumed that the number of matches for a season is 60 and the number of training sessions is 220 which means 5 practices for 44 weeks a year. According to this evaluation the total distance covered by a player in a season is as high as over 3000 km including in total more than 2.5 million steps a year. Resulting form the figures it can be concluded how important the relevant playing and training surface and boots are for the players. Table 1. An evaluation of work rate in match and training conditions for a season  At the top level soccer match 900-1000 actions with ball will be executed, 350 passes with one touch, 150 with two touches and the rest with more several touches and after dribbling the ball. The successful top teams need on the average 16-30 attacks and 7-10 shots to score one goal. The attacks which produce a goal take less than 25 seconds. Two-six players take part in these attacks and one-six passes will be needed to score one goal. The distance covered and the type of movement by players in soccer depends on the players position and role in the game (Luhtanen 1994). Table 2 explains a summary of action rate (technical and physical executions) of a top class player divided again separately into the action rate due to the matches and training sessions (individual training has not been taken into account in this summary). There is also a limited number of training analysis available in the literature. Therefore the content of an average training session has been estimated according to the opinion of the author. The basic difference between the match and training has been that a training session includes more higher intensity small sided games with higher number of ball contacts, passes and runs with ball, starts, turns, jumps and tackles. In Table 2 has also been assumed that the number of matches for a season is 60 and the number of training sessions is 220 which means 5 practices for 44 weeks a year. Table 2. An evaluation of technical action rates in match and training conditions for a season  Higher level skills in soccerGenerally and practically speaking, the content of skills could be defined to be a product of four different biomechanical elements as follows: In skillful performance as well as in kicking and jumping this means that all four biomechanical variables are existing at the same time in exactly the right combination. In general, the total force is a sum of several forces produced by internal ( muscular force ) and external ( reaction force, impact force, air resistance, etc. ) forces. In the human body, the velocity for the distal body parts ( foot, hand, head ) is produced through the human lever system with the joints. The linear velocity of the distal body part is dependent on the length and angular velocity of the respective lever ( shank, thigh, etc. ). The relative angular velocities for each body part will be produced through the respective muscle group ( knee extensors, dorsi flexors, etc. ). Accuracy means a certain space which can be time dependent because of the moving players on the field. Purposefulness means the final output of an execution relevant to the match situation. Most of the actions and maneuvers in match situations are executed with submaximal force and velocity, but highly precisioned accuracy and purposefulness. Fewer maneuvers will be executed with maximal force and speed. The most successful actions in games are seen when the purposefulness of an action is unique and the accuracy, speed and utilization of force are at a maximum. The principles of accuracy, speed and force associated with the performance has been explained in Table 3. Table 3. Goals of movements and principles associated with the performance
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