|Biomechanics of Soccer Equipment|
The equipment used has a major effect on the way the game is played. The ball itself is of a certain size, construction, weight and pressure, all of which affect the way it responds in play. The ground on which the game is played also affects the nature of the game. Surfaces for soccer have evolved in response to both performance and economic needs. The controversy aroused by the introduction of the synthetic surface for soccer has led to biomechanical investigations into the performance and protection characteristics of all types of surfaces. The boot is an important piece of players' equipment and well fitting boots can not only aid comfort but also provide a positive interaction between player and surface to create traction for stopping, starting and turning. Boots aid player-ball interaction for passing and shooting and they must also protect the player and be resistant to the stresses of the game. Shin guards are essential in the modern soccer for protecting against kicks and blows and are mandatory in organized play, but have until recently been neglected in biomechanical investigations.
The soccer ball is characterized by its mass, diameter, internal pressure and surface texture. The full size ball is required to have a mass between 396 g and 453 g, a circumference between 68.5 cm and 71.1 cm, and an internal pressure of 0.6 to 1.1 atmospheres. The outer casing or cover should be of leather or another approved material which does not prove dangerous to players. It may be constructed in two main ways either with panels of material sewn together, or moulded from rubber or plastic with cover panels bonded or painted to the surface.
The mass of the ball is restricted by the rules of the game but small variations in mass can occur due to the absorption of water through the seams of the ball, or by absorption through the material. It has been found that both moulded and stitched balls absorbed moisture after a prolonged period of soaking, and that the moulded ball increased its mass by 1.3 % while the stitched ball increased its mass by 7.5 %. The increase in mass yielded an increase in impact force of 2.8 % for the moulded ball and 8.5 % for the stitched ball. The increase in force was thought to be directly related to the increase in ball mass (Armstrong et al. 1988).
In comparison of the impact force for moulded and stitched balls dropped from 18 m (providing an impact velocity of 18 m/s ) was found that the mean impact forces ranged from 851 N for the moulded ball to 912 N for the stitched ball and the rise time of the moulded ball was 27% lower than for the stitched ball (Lewendusky et al. 1988).
The performance of a ball is described by the way it flies through the air, bounces, slides and rolls on the ground. It is known from mechanical theory that a heavier ball will retain more of its initial velocity during flight, and that the aerodynamic forces of drag and lift acting on the ball will cause it to alter its flight path, particularly when the ball spins. Spin affects ball flight markedly, to the extent that the use of ball spin becomes a game tactic and as such, one might expect that characteristics of ball construction on this aspect of performance would have been scientifically investigated. There is published research on the flight characteristics of the soccer ball as a function of ball variables including spin. If the release angle, release velocity and spin have been constant, the projectile of the ball has been more stable than with the without spin (Luhtanen et al. 1993). When dropping a soccer ball from a height of 18 m and hitting the target force platform below. The success rate hitting the target was no better than 7% (Lewendusky et al. 1988). This was explained by the aerodynamic Magnus force acting on the ball as it slowly rotated during its descent. With regard to bouncing, sliding and rolling it has been shown that the resilience of the ball on the surface is determined more by the characteristics of the surface than the ball. This is also likely to be true of the frictional properties between a ball and the surface which determine its skid and roll behavior.
The performance of a natural turf soccer pitch is dependent on its constructional characteristics, weather conditions and its response to wear. Attempts have been made to quantify the playing quality of soccer surfaces and the merits of natural and artificial surfaces. The playing characteristics are thought to be determined by ball-surface interaction, player movement, and player-surface interaction. Several tests have been designed to quantify these characteristics and some have been adopted as a measurement standard. Ball-surface interaction is evaluated by tests of ball rebound resilience, rolling distance and velocity change. Player movement is evaluated by the frictional characteristics between a studded boot and a test surface. This takes two forms, a torsional traction test which uses a loaded studded plate rotated on the surface and sliding traction test which measures the distance a weighted studded boot travels on a trolly when supported such that the studs just contact the turf surface (Canaway & Bell 1986, Baker 1986). Player-surface interaction is evaluated from measuring the peak deceleration of a sphere dropped onto the surface, and by measuring the ground reaction force during a simulated running action using a constant man model (Kolitzus 1984).
It has been found that the playing quality of natural turf surfaces varied greatly according to quality of construction, maintenance, amount of wear, position on the pitch and the weather. In general, artificial surfaces tended to be more consistent in their playing quality and less dependent on the weather. There has been found considerable overlap with natural turf surfaces for ball rebound resilience, torsional traction, sliding distance and force reduction, it was also found that artificial surfaces tended to have a greater ball roll distance and peak deceleration for high energy impacts. These results have led to the latter prohibiting the use of artificial surfaces for high level play. This finding has been endorsed by the FIFA and UEFA and all competitive international matches are currently played on natural turf.
Attempts have been made to quantify the effects of specific characteristics of artificial surfaces. The age of a surface affects its behavior. It has been reported that the impact absorption characteristics of a 5 year old artificial surface had significantly deteriorated compared to a newly laid surface and that the coefficient of sliding friction was affected by surface wetness. Differences have been reported also between wet and dry conditions for the coefficient of sliding friction, with wet generally being lower than dry. The direction of the pile of a surface affects its frictional properties (Bowers & Martin 1975). The coefficient of friction was found to be a function of the normal force applied, and as the normal force increased, the coefficient of friction decreased. Under realistic loading forces the coefficient of friction ranged from about 0.8 for a foam rubber backed polypropylene surface to 1.15 for a nylon pile surface. The friction force increased by about 8% when sliding against the pile. It has been also reported that no significant differences were found in torsional traction coefficient between wet and dry conditions for a variety of natural and artificial surfaces, but it was found that the longer pile and softer shock pad surfaces of artificial surfaces tended to have a higher torsional friction while sand filled artificial surfaces tended to have lower torsional friction. Torsional friction was found to be slightly higher on artificial compared to natural turf surfaces.
Measurements involving players running over instrumented surfaces have been reported for artificial surfaces (Valiant 1987). The presented data was applied on sliding frictional coefficients and rotational friction torques for a variety of movements (run, cut or sidestep, pivot). Subjects wore indoor soccer boots and performed on an artificial surface. The peak sliding frictional coefficient of 0.8 was found during the braking phase of each movement. The rotational friction torque produced during pivoting was 17 Nm. When a physical traction testing device was used to measure the maximum frictional characteristics of the shoe/surface combination, a frictional coefficient of 1.6 and a rotational friction torque of 30-48 Nm were obtained. It would appear that the shoe/surface combination was over-specified, in that it generated a higher friction force and torque than required in typical soccer movements.
The typical football boot is one which is still based on a leather construction, generally cut below the ankles, and with a hard outsole to which studs are attached. The thinness of the outsole provides the boot with its flexibility, while its hardness provides a firm surface for the attachment of studs. The studs may be either moulded as a part of the boot or detachable, and great variety is seen in sole stud patterns. Boots have a firm heel cup but do not usually include a heel counter as found in running shoes. Some boots have a raised heel to provide both heel lift and a midsole for shock absorption. Most boots will have a foam insock to aid comfort and fit.
Although manufacturers take a systematic approach to boot design, there have been very few published scientific reports. One notable exceptions is the work where the study has taken an ergonomic approach to the problem of setting design criteria for the manufacture of a soccer boot (Rodano et al. 1988). One part involved a clinical examination of the foot and lower limb to define normal morphology. Another involved an analysis of foot ground reaction forces. A third part used a kinematic analysis of various games skills such as kicking, dribbling and tackling. The data was used to provide a better understanding of the many functions of soccer footwear and they used this to make some specific recommendations regarding boot design.
It has been also identified that the soccer boot has an ergonomic function (Lees & Kewley 1993). It has been stated that it must be comfortable to wear and not be an encumbrance to the player or the play required of an individual. It must (a) perform in relation to the demands of the game (b) provide protection for the foot and (c) enable the foot to perform the functions demanded of it. In order to assess the demands of the game it has been investigated the physical demand which was placed on the boot during soccer playing and training. There has been identified the major categories of playing movements made during a soccer, and recording their frequency of occurrence during both training and playing. These actions were then repeated under laboratory conditions where the ground reaction forces recorded. The force in each sector was accumulated for each playing action taking into account the number of occurrences of each action, and was related to problems experienced by players. Boot splitting was the most common boot problem and was reported by 27% of the professional players and 15% of the amateur players questioned. The location of the splits were in the front lateral region of the boot and this correspond very well with the main directions of the stress on the boot. By accumulating the force acting on the boot over all actions, it was estimated that over a period of 90 minutes playing or training the stress on the boot was three time greater in training than in playing. This has consequences for the type of boot that is used for both types of play.
When the foot contacts the ground during a typical running stride the ground reaction force exceeds about 2.5 times body weight (Cavanagh 1990). The boot should have built into it materials designed to reduce the effect of these forces, but they often do not. The shock force experienced by the player can increase as a result of running speed or type of landing action used and will be higher on hard as opposed to soft surfaces. It can be assessed by the use of accelerometers placed on the lower tibia measuring peak shank deceleration. This characteristic has been investigated for running in soccer boots compared to running shoes at a speed of 3.5 m/s on a variety of surfaces (Lees & Jones 1994). The results indicated that the mean peak shank deceleration when subjects used soccer boots and ran on grass was 25.6 m/s2 while when they used running shoes and ran grass it was 23.3 m/s2 and on concrete it was 26.5 m/s2 . Running on grass significantly reduced the peak shank deceleration but wearing the less well protected soccer boots increased the peak shank deceleration by about 10%. The benefits of the softer surface were lost when using a boot which had no constructional midsole. Their view was that boot construction which included a midsole element could yield a beneficial reduction in impact severity.
The studs and cleats are important for providing traction on a variety of surfaces. They have evolved from a simple ridge on the sole to leather cleats to the modern plugs and spikes of various lengths (Segesser & Pforringer 1989). The grip provided is a function of the depth of penetration of the stud and the firmness of the turf. Very wet turf will mean that short studs fail to penetrate into the firmer ground underneath and lead to slipping. On the other hand very hard turf will not allow good penetration, and lead to pressure areas on the foot at the heel or fore foot of the boot. Studs of varying length help to overcome some of these problems. The amount of grip provided by a surface is an important component of playing quality and has been dealt with from the point of view of the surface above, but stud configuration and type have an influence. It has been found that the sliding resistance was little affected by stud configuration he did report large differences in the coefficient of sliding friction between different stud types. The greatest difference between stud types was found in the torsional traction coefficients (Winterbottom 1985).
The boot should be able to distribute the force so that it is not concentrated in certain areas, such as for example under the heel, or more particularly under the head of the first metatarsal. The positioning of studs is particularly critical in this regard, as well as the method of attachment of stud to the boot. It has been demonstrated that wide screw studs lead to lower foot temperature than the conventional narrow screw stud, and this promotes foot comfort and reduces the likelihood of blisters. The foot is susceptible to knocking and treading by the feet of other players, and so the material of the boot should be able to provide protection to the foot from this. The use of padded leather is necessary. All of these methods have been used by manufacturers but no scientific data has been reported.
Boot designers have acknowledged the need to provide adequate for foot flexibility. This is achieved by providing a crease in the sole of the boot along a line where the metatarsal heads would sit in the boot. While this provides a suitable construction for flexion of the joints, it has been noted that a major factor affecting the success of a kick is the stiffness of the dorsal surface foot which has to sustain forces in excess of 1000 N. This stiffness is increased by the presence of the boot, but reduced by the flexibility crease created to promote metatarsal phalageal flexion. It has been suggested that these two requirements are incompatible, and that it might be possible for the sole of the boot to be designed with a hinge locking mechanism providing flexion in one direction but stiffness in the other. Such a design has yet to be made commercially available (Lees 1993).
Shin guards are used by soccer players to protect their shins and ankles from the effects of direct contact by an opponent's boot. Their primary function is to protect the skin, underlying soft tissues and bones of the lower leg from external impacts. They prevent injury by means of shock absorption, load spreading, and by modifying the energy absorption characteristics of the leg system. The wearing of these shin guards is mandatory in soccer and yet their effectiveness in reducing the severity of impact has only recently been evaluated, and no standard methods for their evaluation exist.
The shin guard is constructed with a hard outer casing and a softer inner layer. The material used for the outer casing is usually thermoplastic moulded to the curvature of the leg, with a shock absorbing inner material made of ethylene vinyl acetate or other foam type material. It has been attempted to quantify shin guard performance (Lees & Cooper 1995). In the tests, the ability of five types of contemporary shin guards of different types to reduce the impact from a direct blow. The methodology used followed that for the testing of cricket pads. This involved dropping a 5 kg mass directly onto the pad from a set height of 40 cm and monitoring its deceleration on impact with the shin guard placed on an wooden leg form supported on a rigid surface. From this the delay time and energy return were computed.
The results showed that shin guards were effective in providing a substantial reduction in impact deceleration of between 40 - 60% when compared to striking the wooden leg form without shin guard protection (Nigg & Yeadon 1987). They appeared to do this by increasing the delay time of the striker on the shin guard by 30 - 40 %. The harder outer shell of the shin guards acted as an area elastic surface and as such was effective in spreading the load. Thus it would seem likely that a combination of peak force reduction and an increase in impact area will reduce the localized pressure, and therefore the likelihood for skin abrasion and penetration from boots or studs. Although over 70 % of the impact energy was absorbed, the shin guards themselves do not contain sufficient material to absorb large quantities of energy, and so it is unlikely that shin guards are capable of preventing fractures from high energy blows. The results also showed that there were significant differences between shin guards with respect to peak deceleration which ranged from 28 to 56 % relative to the impact deceleration obtained without a shin guard. The poorer shin guard was constructed of a thermoplastic outer casing with a foam inner layer, while the better shin guard was of a similar thermoplastic outer shell but with a 5 mm ethylene vinyl acetate inner layer.